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Subject: "AF fine tuning - again" Previous topic | Next topic
jadiniz Registered since 25th Dec 2010Thu 31-Mar-11 01:32 AM
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"AF fine tuning - again"


Estoril, PT
          

After reading so many posts regarding Af tuning, I made my own experiment: an A4 sheet covered with multiple lines of the letter "X", with a solitary center bigger X for aiming. Set it on a table, and took a few shots from my chair.

The repeating patterns of the letters allows a great view of where the focus plane lays, and I found that my lenses benefited from AF adjustment:

35mm f1.8 (-13)
18-105mm VR (-5)(@50mm)

Testing was done wide open to allow for a small DOF, and the angle from the camera to the sheet was quite shallow.

Tomorrow I'll try to repeat the test using a tripod and longer distances, as this was done pretty close to minimum. But the difference before and after was quite noticeable. If the results are consistent with what I got today, I believe my IQ from now on will trult benefir from this.

http://egozarolho.blogspot.com
1. Good content, good aesthetics and good tecnique. On that order.
2. Light is more important than glass and pixels.
3. In the digital photography process, software is as important as gear.

  

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Reply message RE: AF fine tuning - again
briantilley Moderator
31st Mar 2011
1
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Len Shepherd Gold Member
31st Mar 2011
2
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jadiniz
31st Mar 2011
3
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elec164 Silver Member
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chris_platt Silver Member
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visionguru
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01st Apr 2011
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elec164 Silver Member
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Len Shepherd Gold Member
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     Reply message RE: Not necessarily so
Len Shepherd Gold Member
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briantilley Moderator Deep knowledge of bodies and lens; high level photography skills Nikonian since 26th Jan 2003Thu 31-Mar-11 08:13 AM
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#1. "RE: AF fine tuning - again"
In response to Reply # 0


Paignton, GB
          

If your sheet of paper was at an angle to the camera, that's a pretty unreliable way of doing fine-tuning, because the AF system will see targets at slightly different distances.

Brian
Welsh Nikonian

  

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Len Shepherd Gold Member Nikonian since 09th Mar 2003Thu 31-Mar-11 08:30 AM
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#2. "RE: AF fine tuning - again"
In response to Reply # 0


Yorkshire, GB
          

Even if the sheet is parallel to the sensor a row of X's can be an unreliable basis for fine tune - because the row of x's is likely to be a symmetrical pattern - see page 93 of the D7000/lens kit instructions or example 5 (geometric patterns) at
https://nikoneurope-en.custhelp.com/app/answers/detail/a_id/4585
With a fine detail subject if the 35mm needs -13 (highly unlikely) the viewfinder at nil will look noticeably unsharp when AF is good.

Photography is a bit like archery. A technically better camera, lens or arrow may not hit the target as often as it could if the photographer or archer does not practice enough.

Len Shepherd

  

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jadiniz Registered since 25th Dec 2010Thu 31-Mar-11 12:23 PM
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#3. "RE: AF fine tuning - again"
In response to Reply # 2


Estoril, PT
          

Those are quite reasonable comments, but truth is I consistently got the same row properly focused when I took multiple shots at the same AF tuning. O would give two or three rows focused beyond the target X, +20 would almost focus on the outer edge of the sheet, -20 would consistently focus the first few rows. I am pretty confident that -13 was the correct adjustment. One (big) caveat: short distance. Will this replicate in normal shooting distances???

The way I set up the test was to ensure absolute minimal DOF with a fixed focal distance, wide open at short distance. The total acceptable DOF was around three lines, clearly visible on the back screen and on the computer. The sheet was laid horizontal and the shots were pointing to the center from a position around 30 degrees higher. Tonight I can perform the test again and post the comparison pics (yesterday I erased the files)

For clarity, here's a sample of my test target:

xxxxxxxxxx xxxxxxxxxx
xxxxxxxxxx xxxxxxxxxx
xxxxxxxxxx xxxxxxxxxx
xxxxxxxxxx X xxxxxxxxxx
xxxxxxxxxx xxxxxxxxxx
xxxxxxxxxx xxxxxxxxxx
xxxxxxxxxx xxxxxxxxxx

http://egozarolho.blogspot.com
1. Good content, good aesthetics and good tecnique. On that order.
2. Light is more important than glass and pixels.
3. In the digital photography process, software is as important as gear.

  

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elec164 Silver Member Nikonian since 15th Jan 2009Thu 31-Mar-11 01:44 PM
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#4. "RE: AF fine tuning - again"
In response to Reply # 0
Thu 31-Mar-11 01:46 PM by elec164

US
          

OK!!!

After reading this thread I finally succumbed to this insanity.

I am not very knowledgeable on the intricacy of how Phase Detection AF works but have a rough idea. And while I can see the argument Brian and Len bring forth, there are alternate points of view.

For instance I found this persons article which seems to have been developed during the D70 AF debacle, which he has since revised. It appears that June 2004 was the last revision. He puts forth a rather compelling argument why the 45 degree angle target is a myth, and why his test target chart is a valid method. He mentions the DOF affect that I posted about in another thread, and also about how the alignment of the sensor within the viewfinder marker may not be precise. He also designed the chart with varied line spacing so that when properly aligned at 45 degrees, the resultant image could be used as a mm measurement directly without a calculation or translation chart.

So I downloaded the PDF and printed it out. Set my camera up as best I could (alignment is critical for accurate results) and viewed the images in PS.

My experience, setting up and aligning the camera to get perfect consistent results was difficult at best. My 17-55 f/2.8 seemed to focus dead on at 55mm. But I only determined that by actually looking at where the camera focused. If I judged by reading the graduated lines on either side then lens aberration or misalignment would have given me a false reading. I then put on the kit 18-105 f/3.5-5.6 VR and feel that it indeed back focused by about 6mm.

But these assessments were done at 100% view which as we know is like viewing a print the size of the average door with your nose pressed to the glass. Am I really going to notice a 6mm miss-focus when viewing an 8x12 print from the same viewing distance? I seriously doubt it, but then I am age appropriate visually challenged!!

Now I am not saying that it's not possible that some people might have indeed received defective or miss-calibrated bodies or lenses, which indeed can happen. But for the most part I think this obsession with AF fine tune and 100% viewing on a monitor is detrimental in most cases.

Just my two cents worth.

Pete

Pete

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chris_platt Silver Member Nikonian since 04th Apr 2009Thu 31-Mar-11 03:45 PM
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#5. "RE: AF fine tuning - again"
In response to Reply # 4


Newburg, US
          

I've landed on that sight a number of times. I don't know if his chart works or not, but I'm a little put off in the way he calls his detractors ignoramuses and then proceeds to inaccurately describe the auto-focus system on the D70. What he describes appears to be a contrast detect focus system rather than the phased detect system actually employed by the camera.

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visionguru Registered since 03rd Nov 2008Thu 31-Mar-11 04:40 PM
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#7. "RE: AF fine tuning - again"
In response to Reply # 5


Chicago, US
          

>I've landed on that sight a number of times. I don't know if
>his chart works or not, but I'm a little put off in the way he
>calls his detractors ignoramuses and then proceeds to
>inaccurately describe the auto-focus system on the D70. What
>he describes appears to be a contrast detect focus system
>rather than the phased detect system actually employed by the
>camera.

His explanations are technically sound. As a DYI method to calibrate AF system, 100% precision is not important and not necessary. The idea is very intuitive: if you focus on a feature, it should be "sharper" than features at different depths. As for those detractors, unless they can offer some contructive suggestions other than nit-picking his work, they are indeed good for nothing ignoramuses.

I've used the chart many times. It worked quite well. As in OP's case, -13 adjustment is night and day, like a nearsighted person see things with and without glass.


Jay
- Chicago Nikonian

  

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briantilley Moderator Deep knowledge of bodies and lens; high level photography skills Nikonian since 26th Jan 2003Thu 31-Mar-11 05:06 PM
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#8. "RE: AF fine tuning - again"
In response to Reply # 7


Paignton, GB
          


The ONLY way to be sure that a camera's AF system has chosen to focus on the part of the target that you want it to focus on is to have the entire area under the AF sensors parallel to the camera's imaging sensor. Simple as that.

But I don't want to get thought of as an ignoramus, so I will offer a constructive suggestion rather than just criticise the linked article and chart...

To check the AF performance of a lens/camera combination - without incurring the cost of something like the LensAlign system, there is a simple and reliable procedure. Set up the camera on a solid tripod. Find a good flat AF target and set it up on a vertical surface parallel to the camera sensor plane. Using a small mirror fixed temporarily to the centre of the target can help to get correct alignment. Using AF, focus on the target and take one image. Now move the tripod carefully a little closer to the target (whilst keeping everything parallel) and without re-focusing take a second image. Move the tripod back a little way past the starting point and repeat. If the first image is the sharpest, you're good to go. If the second image is sharper you have front-focus and if the third is sharper you have back-focus. In those cases, apply an appropriate amount of Fine-Tune and repeat the test.

Brian
Welsh Nikonian

  

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visionguru Registered since 03rd Nov 2008Fri 01-Apr-11 12:01 AM
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#17. "RE: AF fine tuning - again"
In response to Reply # 8


Chicago, US
          

>
>The ONLY way to be sure that a camera's AF system has chosen
>to focus on the part of the target that you want it to focus
>on is to have the entire area under the AF sensors parallel to
>the camera's imaging sensor. Simple as that.
>
Yes. Even slanted surface with distinctive features, it can get quite close.

>To check the AF performance of a lens/camera combination -
>without incurring the cost of something like the LensAlign
>system, there is a simple and reliable procedure. Set up the
>camera on a solid tripod. Find a good flat AF target and set
>it up on a vertical surface parallel to the camera sensor
>plane. Using a small mirror fixed temporarily to the centre
>of the target can help to get correct alignment. Using AF,
>focus on the target and take one image. Now move the tripod
>carefully a little closer to the target (whilst keeping
>everything parallel) and without re-focusing take a
>second image. Move the tripod back a little way past the
>starting point and repeat. If the first image is the
>sharpest, you're good to go. If the second image is sharper
>you have front-focus and if the third is sharper you have
>back-focus. In those cases, apply an appropriate amount of
>Fine-Tune and repeat the test.

Another way is just shoot with AF adjustments -20~+20 and pick the sharpest.

I don't understand who would invest that kind of money on stuff like LensAlign. For most lenses it's pointless trying to achieve 100% accuracy because there is none. -1 vs -2, who is the sharpest? different people might call differently. At different distance or focal lens, there can be slight changes too.

That being said, if a lens is obviously front/back focusing (that's when the chart in the mentioned document is most useful), the AF adjustment feature is critical.

Jay
- Chicago Nikonian

  

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JPJ Silver Member Nikonian since 20th Aug 2009Fri 01-Apr-11 12:22 AM
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#18. "RE: AF fine tuning - again"
In response to Reply # 17


Toronto, CA
          

>
>Yes. Even slanted surface with distinctive features, it can
>get quite close.
>

Or it can fool the AF system, producing a false positive for a focusing problem, which when adjusted will cause further headaches.

In order to ensure the the AF system is focusing as designed as Brian stated the plane of focus needs to be 100% parallel to the sensor.

Further, the focusing target needs to be significantly larger then the focus point itself as the actual focus sensor is often larger than the brackets and not centered.

Also focus should be measured at the center of the image as possible to eliminate the potential of field curvature errors.

Also I don't know if this was mentioned yet or not, but focus should also be measured at your normal working distance. Most people tend to end up measuring their AF system at some unnaturally close distance (especially if you are using one of the slant charts). I don't know how many thread sI have read over the years where people have done it this way and after adjustment and a return to normal shooting the perceived focus problem is worse than ever.

Let us not forget the near impossibility of fine tuning a zoom lens.

In short, if a properly tested lens needs calibration you are generally better off to send it in to be calibrated imo.

Jason

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visionguru Registered since 03rd Nov 2008Fri 01-Apr-11 03:25 AM
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#20. "RE: AF fine tuning - again"
In response to Reply # 18


Chicago, US
          

>
>Or it can fool the AF system, producing a false positive for a
>focusing problem, which when adjusted will cause further
>headaches.
>
First of all, on a high contrast chart like that, it's hard to fool the AF system, otherwise, none of real world shots would be in focus.

You suspect focus problem before shooting charts to correct it, not the other way around. I don't see how it can cause headaches, it's just an offset value added by the firmware. Turning on and off AF adjustment takes a couple of button pushes.

>In order to ensure the the AF system is focusing as designed
>as Brian stated the plane of focus needs to be 100% parallel
>to the sensor.
>
100% is not needed, it's far from scientific experiment, hardly anyone can notice a couple of mm off in AF, it takes more that to start bothering.

>Further, the focusing target needs to be significantly larger
>then the focus point itself as the actual focus sensor is
>often larger than the brackets and not centered.
All you need is a distinctive feature in comparable size to the measuring area.

>
>Let us not forget the near impossibility of fine tuning a zoom
>lens.
>
Again, perfect adjustment does not exist. Unless it's a poorly designed lens, it's a reasonable assumption that if at one focal length that a zoom has front or rear focus, at other focal lengths it will show similar amount of front/rear focus. The AF adjustment feature can turn a softy into a decent useable lens.


>In short, if a properly tested lens needs calibration you are
>generally better off to send it in to be calibrated imo.
It's not that simple. Camera makers often ask you to send in body and lens for adjustment. It could affect all your other lenses and future lenses. That makes this kind "soft" adjustment feature very important.

I think the reason people have different opinions on the AF adjustment feature is many people never had a lens that really need it. In my experience, none of my Nikon lenses needed AF adjustment, but I did have quite a few 3rd party lenses needed big AF adjustments. Again, it's not scientific experiment, making a lens from bad to useable, we don't need 100% accuracy.

Jay
- Chicago Nikonian

  

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JPJ Silver Member Nikonian since 20th Aug 2009Fri 01-Apr-11 12:53 PM
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#28. "RE: AF fine tuning - again"
In response to Reply # 20


Toronto, CA
          

I base my 'headache' comments on countless threads on the internet of people shooting angled test charts only to complain later that they are 'still' having problems when they go shoot real world shots. I suspect most of this has to do with testing the lens way too close to their target (no one shoots like this normally) and at times due to AF error from using an improper angled target that consists of multiple lines that don't fill the sensor. At least in some of these cases there was nothing wrong with their AF to begin with, resulting in non-stop fine tuning and much frustration.

Ultimately unless we hit a lab and test this 1000 times or whatever it would take to get statistical significance, we have no way of disproving the others theory. You may in fact be correct in your statements, I respect your opinion on this. Enough of these tests run afoul of what Nikon states could fool the AF system that I personally can't believe they are reliable. We will agree to disagree on the validity of these tests. Honestly, if people get good results using them they will ignore my babbling. The faction here that don't believe in these tests have a point, people can consider it and decide if they agree.

As for calibration not being as easy as I state, most pros I know have their lenses calibrated yearly, but seldom send their body in unless there is a problem. Maybe they are doing it wrong, but they are not having problems.

Jason

Visit my Nikonians gallery.

Visit my Nikonians gallery.

  

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elec164 Silver Member Nikonian since 15th Jan 2009Fri 01-Apr-11 05:28 AM
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#23. "RE: AF fine tuning - again"
In response to Reply # 18


US
          

>In order to ensure the the AF system is focusing as designed
>as Brian stated the plane of focus needs to be 100% parallel
>to the sensor.

Well it does not appear that it does with all the testing I have done since starting to participate in this thread. It seems that you just needs to provide a clear and concise target. The problem using Jose’s target or a fine graduation ruler at an angle is that multiple targets can be covered under the one AF sensor. The PDF I linked to uses an angled target, but only provides one line 1/8th of an inch high on a white background which provides a clear and concise target when used as described in the instructions. The fact the paper is angled to the sensor seems to be transparent to the AF sensor.


>Further, the focusing target needs to be significantly larger
>then the focus point itself as the actual focus sensor is
>often larger than the brackets and not centered.

As you say when I measured the vertical part of the center AF sensor on my D7000, I discovered it is MUCH larger than the area marked in the viewfinder (although it seemed fairly centered extending equally on each side). It was about the height of three boxes (one on each side of the center box). In fact I was able to use the test chart focus target to track the AF sensor by inching the target along. The 17-55 has a distance scale and I was able to reliable repeat the same results inching the target along. I moved the line down till the camera couldn’t focus. Then inched it up until it could and recorded the distance. Then I moved it up until I couldn’t focus again then inched it back down till it did. The closest marking was the metric scale, which went from being centered under the 5 of the 0.5 and to just left of the 0 of 0.5 as I tracked the target along the vertical portion of the center cross type AF sensor. So clearly the 1/8th of an inch was enough for the AF system to function.

>Most people tend to end up measuring their AF system at some
>unnaturally close distance (especially if you are using one of
>the slant charts).

This is a very interesting and valid point. The Lens Align web site recommends a minimum distance of 4.5 feet for their system, which is probably the furthest distance the chart I was using could probably be used at. But that limit was because of the text and box that was surrounding the white area where the focus line was. Any further away and the those other areas would fall under the AF sensor providing multiple targets at different distances which would be problematic. But I imagine if you provided a larger area, then the chart should still be usable if you could provide sufficient calibration ruler for the DOF.

Truth is until this thread I would have never bothered trying it, and I probably spent more time testing then I would have wanted to. But I did learn some new things, and the next time I aim using single point AF on a target I want to focus upon, I now realize that a lot more of the scene is under the AF sensor then I thought there was.

Pete

Pete

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elec164 Silver Member Nikonian since 15th Jan 2009Thu 31-Mar-11 05:15 PM
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#9. "RE: AF fine tuning - again"
In response to Reply # 5


US
          

>What he describes appears to be a contrast detect focus system
>rather than the phased detect system actually employed by the
>camera.

I agree that name calling is never a good thing. And yes his description of the AF system is very simplistic.

But that aside, as I understand it, all auto focus systems use contrast. It is just that with Phase Detection the image is split and projected onto two sensors and a phase angle can be determined by comparing the two images for similar light intensity patterns when deciding which direction to move the focus. And that is what makes Phase Detection auto focus faster than just contrast detection alone. Simple contrast detection just knows that the image is out of focus, but does not know which direction to move in. So it moves one way and if the contrast lowers it stops and moves the other. With Phase Angle Contrast Detection the logic can determine whether the focus is in front of or behind the subject and move appropriately.

Pete

Pete

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chris_platt Silver Member Nikonian since 04th Apr 2009Thu 31-Mar-11 06:55 PM
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#10. "RE: AF fine tuning - again"
In response to Reply # 9


Newburg, US
          

Yes, but my understanding is that a phase detection system doesn't determine focus by maximizing contrast between adjacent pixels as suggested on that site (though that is the result). It does it as you've said by calculating the amount of lens movement required to bring a split image into phase - i.e., by determining where those light intensity patterns impinge on each of those CCDs and calculating lens shift to move the brightest point to the same (center) position on each CCD. One sample from one side of the aperture and another from the opposite of side the aperture looking at the same small sample of the scene. To me, it's akin to adjusting the lens to center the peak of two different histograms rather than trying to maximize the peak. I don't see that as doing the calculations based on contrast. The camera will never know if it has increased or maximized contrast, because it won't necessarily sample again.

Maybe I'm being overly anal about his comments (and acknowledged character flaw) and he is talking more about the results than the process - but it really doesn't work by "adjusting the focus to achieve the best contrast between adjacent pixels." It's like the difference between manually focusing on a normal screen and on a split screen. In one you search for the sharpest highest contrast image (back and forth), and it the other, you focus until the split image is aligned (but a high contrast edge helps you do that). It didn't give me confidence that he really understood how it worked.

Or maybe I just wasn't on his side because he so rudely handled his detractors.

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elec164 Silver Member Nikonian since 15th Jan 2009Thu 31-Mar-11 07:16 PM
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#12. "RE: AF fine tuning - again"
In response to Reply # 10


US
          

Thanks for taking the time to provide that description, for it furthers my understanding and knowledge of Phase Detection AF.

Pete

Pete

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visionguru Registered since 03rd Nov 2008Thu 31-Mar-11 11:42 PM
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#16. "RE: AF fine tuning - again"
In response to Reply # 10


Chicago, US
          

>Yes, but my understanding is that a phase detection system
>doesn't determine focus by maximizing contrast between
>adjacent pixels as suggested on that site (though that is the
>result). It does it as you've said by calculating the amount
>of lens movement required to bring a split image into phase -
>i.e., by determining where those light intensity patterns
>impinge on each of those CCDs and calculating lens shift to
>move the brightest point to the same (center) position on each
>CCD. One sample from one side of the aperture and another
>from the opposite of side the aperture looking at the same
>small sample of the scene. To me, it's akin to adjusting the
>lens to center the peak of two different histograms rather
>than trying to maximize the peak. I don't see that as doing
>the calculations based on contrast. The camera will never
>know if it has increased or maximized contrast, because it
>won't necessarily sample again.
>
Yeah, that's the basic idea of phase detection. The CCD captured intensity profiles indeed look like "histograms", which can have peaks and valleys, finding the shift (like "phase" difference as in sinusoidal waveforms) between the 2 profiles can help the camera to determine how much it needs to move the lens. Though implemented very differently from contrast detection AF, the end result is similar.

>Maybe I'm being overly anal about his comments (and
>acknowledged character flaw) and he is talking more about the
>results than the process - but it really doesn't work by
>"adjusting the focus to achieve the best contrast between
>adjacent pixels." It's like the difference between
>manually focusing on a normal screen and on a split screen.
>In one you search for the sharpest highest contrast image
>(back and forth), and it the other, you focus until the split
>image is aligned (but a high contrast edge helps you do that).
> It didn't give me confidence that he really understood how it
>worked.
>
I agree. He got the general idea, but not details. His chart is clean and easy to use, though including 2 diagonal direction rulers might help more.

>Or maybe I just wasn't on his side because he so rudely
>handled his detractors.

The name calling in his article is too much. I guess his detractors really annoyed him.

The article was written before AF adjustment feature was available on prosumer cameras, finding out back/front focus was important back then. Nowadays, one can setup the camera shooting several sets of 41 pictures (with AF adjustment value from -20 ~ +20), and pick the "sharpest", his chart is not as useful. Still, it's a good chart to get know your lenses.

Jay
- Chicago Nikonian

  

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benveniste Moderator Awarded for is high level skills in various areas, including Macro and Landscape Photography Nikonian since 25th Nov 2002Fri 01-Apr-11 04:07 AM
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#22. "RE: AF fine tuning - again"
In response to Reply # 10
Fri 01-Apr-11 04:07 AM by benveniste

Boston Area, US
          

If one is willing to do so fairly heavy slogging, you can review Minolta's patent for their Maxxum autofocus system and see the algorithm they describe. This patent was later held to infringe a prior Leica/Honeywell patent:

http://www.google.com/patents/about?id=5l0vAAAAEBAJ&dq=4445761

Since modern AF systems have more CCD's and more computing power, they tend to use more sophisticated algorithms. I don't know which algorithmic family Nikon uses, but good guesses include variations of Squared Gradient and Sum-Modified Laplacian.

Looking at the descriptions of these algorithms is a painful reminder of just how out of practice I am at serious math.

If you want to photograph a man spinning, give some thought to why he spins. Understanding for a photographer is as important as the equipment he uses. - Margaret Bourke-White

  

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chris_platt Silver Member Nikonian since 04th Apr 2009Fri 01-Apr-11 02:15 PM
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#29. "RE: AF fine tuning - again"
In response to Reply # 22


Newburg, US
          

Now that's cool (even though I said I wasn't going to comment anymore). So as I read that patent, even though the system calculates an initial defocus signal to move the lens a predetermined amount, it continues to refine that calculation with additional sampling and calculations coming up with a recursive mean value and adjusting the defocus signal if necessary while the lens is on it's way to the initially determined point - thus compensating for scattering (ambiguous wave forms?).

It would be fun to get a peek at Nikon's algorithm.

Visit my gallery.

  

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elec164 Silver Member Nikonian since 15th Jan 2009Fri 01-Apr-11 03:25 PM
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#31. "RE: AF fine tuning - again"
In response to Reply # 29


US
          

>It would be fun to get a peek at Nikon's algorithm.

Yes it would for it seems it may be similar.

In a reply I just posted to Len, I provide an experiment to try. If you’re interested you can read about it there instead of repeating it here.

But when doing it, one thing I noticed is that when the camera was trying to focus on the thin line it hesitated momentarily at one distance, but immediately shifted slightly further before locking focus. So it appears Nikon’s algorithm is doing something similar as described in the Minolta patent. It seems that when the image was a total blur it made one calculation, but as it neared focus it seems to have recalculated and adjusted that distance.

Pete

Pete

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JohnE Nikon Silver Member Nikonian since 15th Jun 2010Fri 01-Apr-11 06:22 PM
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#34. "RE: AF fine tuning - again"
In response to Reply # 31
Fri 01-Apr-11 06:27 PM by JohnE Nikon

New HArtford, US
          

Pete,
I posted regarding fine tuning a few weeks ago at
http://www.nikonians.org/forums/dcboard.php?az=show_topic&forum=329&topic_id=6672

I used 3 different angled focus tests and one flat focusing test on all my lenses.

One of the 3 was the one you link to.

Despite all the theoretic reasons why these tests "may" not work well or may not provide an accurate focus target I found that they did work well and gave consistent results.

My conclusion was that my D7000 had no focusing problems but some of the lens needed tweaking.

For the real world my len that "needed" -18 fine tuning now provides me with sharp images. > 1/2 of my lenses required no fine tuning and 2 required <5 units of tuning.

After all the debate I decided to test fine tuning on real subjects and found consistent results. Sharper images with fine tuning turned on as opposed to off.

I have learned a bunch along the way including info on AF tolerance , contrast vs phase focusing etc.

While some may not think that 6mm of back focusing matters. I find that if I want to crop significantly especially in my wildlife photos, maximizing sharpness allows me the largest crop. With the lens that needed -18 I just found that when I shot with it wide open images were a little soft. This corrects with fine tuning.


Just wanted to reassure you. You are not alone with your impression that this angled test provides an accurate reproducible focus target.

I have no doubt that the flat focusing tests described which includes an angled portion would also work.
If I were a pro, I would probably buy one to see if maybe -17 or -19 would be better for my 1 lens.

The one suggestion above which considers using a flat focus target and shooting with -20 to + 20, I think would be difficult. I found that it is very hard to determine on flat target which is sharper when dealing with only a couple units of fine tuning. I suspect I don't have a great flat target and will attempt with a target like Len's where line pairs are included. If I find any deviation from my results, I will post.

JohnE Nikon
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https://picasaweb.google.com/104310967428146619677


"Cameras and lenses are simply tools to place our unique vision on film. Concentrate on equipment and you'll take technically good photographs. Concentrate on seeing the light's magic colors and your images will stir the soul." Jack Dykinga

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JohnE Nikon Silver Member Nikonian since 15th Jun 2010Fri 01-Apr-11 07:45 PM
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#41. "RE: AF fine tuning - again"
In response to Reply # 34


New HArtford, US
          

I did a little searching online and found a focus test with line pairs.
The article also goes into modulation transfer function which is a better test of your system.
I have not experimented with this yet but thought it was worthwhile to share.
http://www.bobatkins.com/photography/technical/lens_sharpness.html


JohnE Nikon
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"Cameras and lenses are simply tools to place our unique vision on film. Concentrate on equipment and you'll take technically good photographs. Concentrate on seeing the light's magic colors and your images will stir the soul." Jack Dykinga

Visit my Nikonians gallery.

  

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jadiniz Registered since 25th Dec 2010Thu 31-Mar-11 09:04 PM
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#14. "RE: AF fine tuning - again"
In response to Reply # 4


Estoril, PT
          



> Am I really going to notice a 6mm
>miss-focus when viewing an 8x12 print from the same viewing
>distance?

All this micro-focusing fuss may be a little over the top, but I'm getting tired of capturing a beautifull smile from one of my kids on a close-up head shot, taking care to focus on the eye, only to later realise that the eyelashes are all mushy and the hair above the ears is razor sharp, one by one.

While Nikon's over-conservative literature advises against using AF micro-tune on a regular basis, truth is we have the tool at hand to guarantee that a 16MP 1200€ DLSR is capable of achieving focus accuracy. Not using the adjustment provided to precisely match body and lenses is a waste of money.

http://egozarolho.blogspot.com
1. Good content, good aesthetics and good tecnique. On that order.
2. Light is more important than glass and pixels.
3. In the digital photography process, software is as important as gear.

  

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Len Shepherd Gold Member Nikonian since 09th Mar 2003Fri 01-Apr-11 06:58 PM
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#37. "RE: AF fine tuning - again"
In response to Reply # 14


Yorkshire, GB
          

>I'm getting tired of capturing a beautifull smile from one of my kids on a close-up head shot, taking care to focus on the
>eye, only to later realise that the eyelashes are all mushy and the hair above the ears is razor sharp, one by one.
Unfortunately Nikon's AF is not good with some AF targets - sometimes including the area around the eye, especially using an outer AF point to get better composition.
Nikon advise manual focus can, with subjects, be more accurate than AF - which is part of the learning curve of getting good resulting AF
https://nikoneurope-en.custhelp.com/app/answers/detail/a_id/4585

Photography is a bit like archery. A technically better camera, lens or arrow may not hit the target as often as it could if the photographer or archer does not practice enough.

Len Shepherd

  

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igordb Silver Member Nikonian since 22nd Dec 2010Thu 31-Mar-11 03:45 PM
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#6. "RE: AF fine tuning - again"
In response to Reply # 0


CA
          

What I suggest is to try take a picture in live view mode and normal mode. The sharpness in both modes should be the same. I understand that in live mode the focus is slow, but this is not the point. In controlled light, it should be not difficult. If you see difference in sharpness, you need to calibrate AF. That is my experience.
The best way is to try it on a vertical target and camera should be parallel to the subject, no angles.
Igor

  

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chris_platt Silver Member Nikonian since 04th Apr 2009Thu 31-Mar-11 07:10 PM
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#11. "RE: AF fine tuning - again"
In response to Reply # 6


Newburg, US
          

That's a really interesting suggestion. I would suspect the contrast detection focus used in live view would not be as prone to calibration issues - it's more like manual focus - move it back and forth until it's good. Then compare that to the phase detect which would seem to me to have more potential points of failure - in the camera and the lens.

I'd expect one potential problem with this method would be ensuring the camera is really trying to focus on the same point.

Visit my gallery.

  

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chroaz Gold Member Nikonian since 26th Apr 2009Thu 31-Mar-11 07:23 PM
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#13. "RE: AF fine tuning - again"
In response to Reply # 11


Cave Creek, US
          

Has anyone tried Michael Tapes' LensAlign system? He has a new MkII version out I think.

Chris

When words become unclear, I shall focus with photographs. When images become inadequate, I shall be content with silence.
- Ansel Adams

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www.throughmeyelens.com

  

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Len Shepherd Gold Member Nikonian since 09th Mar 2003Fri 01-Apr-11 06:51 PM
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#36. "RE: AF fine tuning - again"
In response to Reply # 13


Yorkshire, GB
          

>Has anyone tried Michael Tapes' LensAlign system?
I have not - part because it costs a lot of money - part because Nikon say the type of AF target used can cause focus errors - and part because the left hand side of the target I showed in reply 27 shows front focus that is a target error with my 60mm G and 300 VR.
Back to the target in reply 27 it is only accurate if printed out at 5x7 inches and photographed at 26x focal length.

Photography is a bit like archery. A technically better camera, lens or arrow may not hit the target as often as it could if the photographer or archer does not practice enough.

Len Shepherd

  

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KnightPhoto Gold Member Nikonian since 18th Dec 2006Sat 02-Apr-11 02:40 PM
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#42. "RE: AF fine tuning - again"
In response to Reply # 13


Alberta, CA
          

>Has anyone tried Michael Tapes' LensAlign system? He has a
>new MkII version out I think.
>
>Chris

Hi Chris,

Yes I use the Lens Align - in my case I bought the Pro Plus version, so I actually have two different AF targets that come with the Pro kit (the normal one plus the larger one for use with super-telephotos). It might be an interesting exercise for me to test the two different AF targets and see if I get any different results (as Len alludes there is potential for) - yah that'll happen when I get some free time in 2012

The MKII looks interesting.

The reason I bought the Lens Align is consistency of build, structure, and plus of course that it combines a flat AF target with an angled ruler to judge the front-back focus. You can read in this thread (and John E Nikon's earlier thread) general concerns being expressed about consistency with various DIY methods. Yes I too started down the fine tune path using two different DIY rigs but the problem for me was getting the home-built rig I made into a consistent repeatable position and therefore a lot of doubt about my results. I decided to bag it and buy the consistency. Mind you I am talking about tuning f1.4, f2, f2.8 shot wide-open and extreme telephotos (e.g. my 500mm with TC17E at 850mm shot on a DX camera with a FOV equivalent to 1275mm!).

Once AF tune is completed, then follow up as John E Nikon (and the OP did here) with real-world shooting to back up your results to make sure they are working for you. I have no doubt John and Jose and Pete have obtained better results following their fine tuning because they are clearly careful photographers. It's just that the Brian and others are advocating "don't try this at home with these particular approaches" because a less careful and experienced photographer can easily get a wrong result.

Anyhow we shouldn't kid ourselves, even WITH Lens Align this is a difficult area to venture into because we are talking very small tolerances here!

As an aside, I recently subscribed to DigiLloyd and he basically tests photo equipment to make his living - very interesting guy and materials that he writes - his descriptions of how little of a bend or twist or unevenness on a body or lens mount due to original manufacturing flaw (or in subsequent use and twist/bend) easily showing up in his lens testing is a harrowing tale indeed. He talks about left/right resolving differences, curvature of field, all kinds of interesting and scary things that can and do go wrong. Differences IIRC of 10 or 20 microns easily show up in his test regime. It's a wonder we get anything in focus.

Best regards, SteveK

'A camera is an instrument that teaches people how to see without a camera.' -- Dorothea Lange
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chroaz Gold Member Nikonian since 26th Apr 2009Sat 02-Apr-11 05:39 PM
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#45. "RE: AF fine tuning - again"
In response to Reply # 42


Cave Creek, US
          

Thanks Steve for this.

I think I may take my first plunge into some viable fine tuning (I did attempt one of the 45 degree methods a little while ago and was not convinced!) and will probably try the LensAlign MKII as a start. Consistency and repeatability of set up and results is obviously key, and this looks like it will do the job reasonably economically.

But as you say, translating the results to success in the field is the objective - and each has his own set of individual criteria to measure that. Practice, Practice, Practice!!

Chris

When words become unclear, I shall focus with photographs. When images become inadequate, I shall be content with silence.
- Ansel Adams

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igordb Silver Member Nikonian since 22nd Dec 2010Thu 31-Mar-11 10:28 PM
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#15. "RE: AF fine tuning - again"
In response to Reply # 11


CA
          

"I'd expect one potential problem with this method would be ensuring the camera is really trying to focus on the same point. "
If you have enough light and contrast subject (which you need in any case to have reliable results)that should be not a problem for live view mode.

  

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elec164 Silver Member Nikonian since 15th Jan 2009Fri 01-Apr-11 03:01 AM
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#19. "RE: AF fine tuning - again"
In response to Reply # 15


US
          

>"I'd expect one potential problem with this method would
>be ensuring the camera is really trying to focus on the same
>point. "

Which method are you referring to, there were at least three that have been mentioned so far?

The Lens Align system provides a target that Brain advocates, a large target that ensure the whole sensor area is covered by it and aligned parallel to the sensor. There can be no mistake at what is being targeted. The measuring ruler is then placed at an angle for easy DOF perception and calibration reading.

The one Jose (the OP) created has a grouping of X’es that is put at an angle to the camera. But as Brian points out that does not provide a clear target for the AF sensor and you cannot ensure it is picking the spot you want (an explanation to follow).

And the PDF I linked to. A side from the crude name calling and error in describing the Phase Detection AF, if anyone put their biases aside and tested his methods you would have discovered some interesting facts you may have not know.

First is that the AF sensor covered a much larger area then the outline in the viewfinder. Sometimes near triple the size (eight more boxes surrounding the one shown in the center). Included in PDF along with the test chart was a page with just a black line. Using that page you can measure how much of an area surrounding the rectile is covered by the AF sensor, what type it is (horizontal, vertical or cross type) and you can verify this for yourself. That fact alone makes Jose’s test target unreliable because multiple X’es could be under one sensor and the camera will pick the one it feels provides the best contrast which might not be the one you wanted (the bigger X).

But you may ask why the target the PDF I linked to would be different and better then Jose’s? Because he took that into account when designing the chart and the main focus target is surround by an area of white paper large enough to encompass the whole AF sensor. And I agree with him that to the AF sensor it only sees a black line on white paper and does not even realize that it is at an angle. And anyone that has tried to focus a camera on a white sheet of paper knows that, well it can’t, simple as that. So while it is at an angle, the AF sensor does not know that nor does it care, it just sees one black line on a sheet of white paper.

I am certainly open to why the “Judge Judy litmus test” which suggest that the chart I linked to would not make this chart sufficient enough for micro adjusting the AF system. It seems to addresses Brian’s concern for providing a clear and precise target, as well as Len’s concern with a link to Nikons info on targets that can fool the AF system (which does not seem to apply to the PDF chart). Tell me what I am missing???


Pete

Pete

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chris_platt Silver Member Nikonian since 04th Apr 2009Fri 01-Apr-11 03:35 AM
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#21. "RE: AF fine tuning - again"
In response to Reply # 19
Fri 01-Apr-11 03:38 AM by chris_platt

Newburg, US
          

"Which method are you referring to, there were at least three that have been mentioned so far?"

He was replying to my post #11 which was replying to his post #6 about comparing live view focus with normal focus. That's what I get for not quoting - not everybody reads these things in threaded view the way I do. But thanks for the explanations of all those other systems

I'm just going to back off and watch now. I really don't have a dog in this fight and don't want one. Just interested in the discussion.

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jadiniz Registered since 25th Dec 2010Fri 01-Apr-11 06:42 AM
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#24. "RE: AF fine tuning - again"
In response to Reply # 19


Estoril, PT
          

Guys, I jus re-checked my 35mm f1.8, here's the shot. The jpg was only downsized, but you can clearly see my aiming point on the center, and where the true focus plane lays. This is my result with -13 dialed in, any other value clearly offsets the focus plane. Althought the target point is much smaller than the viewer aimpoint, the D7K never seems to have ay trouble acquiring it and giving me a solid green dot.

My 18-105 takes -5 and my 55-200 needs -4 to get the same results.


http://egozarolho.blogspot.com
1. Good content, good aesthetics and good tecnique. On that order.
2. Light is more important than glass and pixels.
3. In the digital photography process, software is as important as gear.

Attachment #1, (jpg file)

  

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briantilley Moderator Deep knowledge of bodies and lens; high level photography skills Nikonian since 26th Jan 2003Fri 01-Apr-11 07:27 AM
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#25. "RE: AF fine tuning - again"
In response to Reply # 24


Paignton, GB
          

I'm afraid the central "x" on your chart looks far too small, and the surrounding characters are too close, to provide an unambiguous target for the AF system

Brian
Welsh Nikonian

  

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jadiniz Registered since 25th Dec 2010Fri 01-Apr-11 08:04 AM
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#26. "RE: AF fine tuning - again"
In response to Reply # 25


Estoril, PT
          

Then why is that the camera consistently focuses the same way? I'm not trying to come over as stubborn or unpolite against a much more experienced photographer, but the camera delivers again and again the same results, even refocusing from infinity each time.

http://egozarolho.blogspot.com
1. Good content, good aesthetics and good tecnique. On that order.
2. Light is more important than glass and pixels.
3. In the digital photography process, software is as important as gear.

  

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Len Shepherd Gold Member Nikonian since 09th Mar 2003Fri 01-Apr-11 07:05 PM
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#39. "RE: AF fine tuning - again"
In response to Reply # 26


Yorkshire, GB
          

>Then why is that the camera consistently focuses the same way? I'm not trying to come over as stubborn or unpolite
It is likely because the AF can pick up the small x coming from infinity well before AF centers the x on the center of the viewfinder mark. It thinks it has found a target - but does not realise it is further away than intended.
There have been a few threads showing that when the test is repeated coming from minimum focus AF locks on in front of the equivalent of the x.
The reason for Nikon (and Canon) saying a parallel target is essential is to avoid the AF focusing closer or further away than expected with a small AF target.

Photography is a bit like archery. A technically better camera, lens or arrow may not hit the target as often as it could if the photographer or archer does not practice enough.

Len Shepherd

  

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elec164 Silver Member Nikonian since 15th Jan 2009Mon 04-Apr-11 01:01 AM
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#47. "RE: AF fine tuning - again"
In response to Reply # 39


US
          

>The reason for Nikon (and Canon) saying a parallel target is
>essential is to avoid the AF focusing closer or further away
>than expected with a small AF target.

Due to the new thread about fine tune AF I wanted to research and see if the +20 or –20 had any specific meaning.

Turns out, according to this article by Nikon, the numbers are arbitrary. Odd thing though is that it also seems that Nikon makes no mention of using a target parallel to the sensor in this article. And in fact they just recommend placing a subject with depth in the scene (an accurate ruler works well) and just focus on a pre-defined area. But they do suggest checking the manual for tips on AF point selection.

Pete

Pete

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Len Shepherd Gold Member Nikonian since 09th Mar 2003Fri 01-Apr-11 08:38 AM
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#27. "RE: AF fine tuning - again"
In response to Reply # 25


Yorkshire, GB
          

>I'm afraid the central "x" on your chart looks far too small, and the surrounding characters are too close, to
>provide an unambiguous target for the AF system
Adding some detail to Brian's comment one of Canon's technical comments is
"The nature of the AF sensors used by EOS digital SLR’s as well as those from other manufacturers is that they perform most reliably when the entire length of the focusing area sees readable detail. This condition is not satisfied by a thin line on a piece of paper"
Your small cross can be substituted for the single line being commented on.
Canon clarify the main target should be parallel to the sensor
"It's OK to include an angled chart in a test photo. In fact, Canon Factory Service Centers always do this. But the test target is always separate from the angled chart, and parallel to the camera's focal plane”
Back to Brian's comments - the D200 AF detection lines are up to 7 times as long as the viewfinder mark (depending on camera set up) so a single small cross does not meet the "most reliable" criteria along the entire length of the focus line.
Generally AF can detect a small cross or square of detail smaller than the viewfinder mark - but with variable accuracy You need 100% accuracy to decide if fine tune is needed.
If you send me a private message I can send a large file of the picture below - and instructions as to how to use it.
I have always found the bottom right a good AF target printed at 5x7 inch size in a 2 feet by 3 feet picture area.
If printed out to a good standard and used parallel to the sensor it is possible to read off results to 200 lpm - more good enough for checking focus accuracy



Photography is a bit like archery. A technically better camera, lens or arrow may not hit the target as often as it could if the photographer or archer does not practice enough.

Len Shepherd

Attachment #1, (jpg file)

  

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elec164 Silver Member Nikonian since 15th Jan 2009Fri 01-Apr-11 03:12 PM
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#30. "RE: AF fine tuning - again"
In response to Reply # 27


US
          

Len, I know you and Brian advocate for the method of using a large target parallel to the sensor. Nothing wrong with that and it surely is error proof. But have you actually tried Jose’s chart or the PDF I linked to. And as to Jose’s target, the picture he shows is significantly different then what he showed earlier in this thread. The first one as you and Brian say is not a good target. But the second one is better and provides a clearer target but the X shape can still cause mm type error for it could be focusing on the top part of the X or the bottom. A single solid line on the other hand cuts across the vertical line of the cross type sensor and provides a precise point.

I have played with and tested the PDF chart extensively, and quite frankly, it works and the results are repeatable. Now someone may argue that you have to be to close which makes the test invalid, but it can accurately focuses on the line first time every time. Now I know Brian and yourself are firmly entrenched in one method and are most likely not willing to waste your time to even experiment and try this, and that’s OK. Because as I stated your method is failsafe and fool proof. But quite frankly if you did experiment with that chart and where open to it, I think you would walk away with a different opinion.

As to the target having to fill the sensor area, first point is that the area is much larger then what’s represented by the marker in the viewfinder. So even if you filled the marked area in the viewfinder, it does not cover the whole sensor area. Second point is that a blank sheet of paper fits the “fill the sensor area with the target”, but as we know you cannot focus on a blank piece of paper. Put a black line on the paper a 1/8th inch high that falls somewhere on the sensor, and suddenly the camera can focus. Now angle the paper keeping that 1/8th inch line the same distance from the camera and it will still focus on the same line at the same distance. Yeah if you put multiple black lines on the paper at an angle that fall under the sensor, then the camera picks the one that provides the best contrast. But if there is only one line, there is nothing else to pick and the fact that the paper is at an angle is transparent to the sensor. As long as the line covers enough of the sensor and provides enough contrast for the system to detect, it will focus.

Still in doubt, try this experiment. Take a plain sheet of paper and mount it to a wall. Take your camera with a 50mm focal length and mount it to a tripod about 5 feet away. That fits the criteria of being parallel to the sensor, filling the sensor with the target, farther than Lens Align’s minimum focus distance but closer then Canon’s recommendation of 50 times the focal length mm. Now try and focus on the paper. You can’t! Why, the target has no contrast area. Now take a ball point pen and a ruler and place a horizontal line on the paper (trace two or three times to make a nice clear single nib line) in an area that will fall somewhere on the center AF sensor (no need to be exactly center). Try and focus again and it should have achieved focus. And it should do so reliably every time. Now back your camera up to 10 feet and try again. Can’t focus again for the line is now not covering sufficient area of the sensor and not providing sufficient contrast. Now take your ruler and pen and align it to the first line and triple the size of the line. Try and focus now and you should be able to. Twice the focus distance I would think would need twice the line width, but I went three times just to be sure.

Now try and repeat the experiment with the paper mounted at an angle. The results are pretty much the same. The sensor cannot tell the paper was at an angle, it just sees a black (or blue) line on a white background and as long as that line occupies the minimal area necessary and provides sufficient contrast, it will focus.

You can stick with the manufacturer recommendation if you wish, but after careful experimentation and constant repeatable results, I have to respectfully disagree with the assertion that the target needs to fill the sensor area and be parallel to the sensor. Is the manufacturer and Lens Align method fool proof and failsafe, yes it is. But that does not mean the other method is incorrect or will not work if properly done, and experimentation seems to prove it does. I know I am not going to change your mind or the mind of anyone whom agrees with you, but after careful experimentation and consistent results, the data speaks for itself.

Pete

Pete

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briantilley Moderator Deep knowledge of bodies and lens; high level photography skills Nikonian since 26th Jan 2003Fri 01-Apr-11 04:16 PM
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#32. "RE: AF fine tuning - again"
In response to Reply # 30


Paignton, GB
          

I suspect that few people would be as methodical as you, Pete

The problem with an angled target is that, while it might work pretty well some (or even quite a lot) of the time when designed and set up carefully, you still cannot be SURE what part of it the camera has chosen to focus on. A flat, parallel target completely eliminates that variable, which can be very important when what you need is a reliable, repeatable result. Why take the chance...?

Brian
Welsh Nikonian

  

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elec164 Silver Member Nikonian since 15th Jan 2009Fri 01-Apr-11 04:28 PM
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#33. "RE: AF fine tuning - again"
In response to Reply # 32


US
          

>Why take the chance...?

Well I certainly cannot argue against that point!

As an aside I was wondering about Jose’s X and decided to try a single diagonal line with the paper on the wall test. Turns out I need twice the single nib width at a 5 foot distance. So the line cutting diagonally across the vertical AF sensor needed to be thicker, which when you think about it, makes sense.

Pete

Pete

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Len Shepherd Gold Member Nikonian since 09th Mar 2003Fri 01-Apr-11 07:17 PM
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#40. "RE: AF fine tuning - again"
In response to Reply # 30


Yorkshire, GB
          

>But have you actually tried Jose’s chart or the PDF I linked to. And as to Jose’s target,
>the picture he shows is significantly different then what he showed earlier in this thread.
Nikon and Canon emphasize some targets may fool AF into poor results some of the time - my experience with a range of less than ideal AF targets is Nikon and Canon are right.
Whilst I have not tried out these specific targets I have tried similar "unsatisfactory" targets - with variable accuracy. Variable accuracy is no good for fine tune decisions.
Ideally up to 10 or more pieces of good but different readable detail is required for AF to perform at it's most accurate.
x symbols in particular provide no more than 3 pieces of information for AF to read, 2 of which (the white surfaces) show identical detail.
I see no point in re-inventing AF targets using detail confirmed by the manufacturers as capable of causing mis focus

Photography is a bit like archery. A technically better camera, lens or arrow may not hit the target as often as it could if the photographer or archer does not practice enough.

Len Shepherd

  

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Len Shepherd Gold Member Nikonian since 09th Mar 2003Fri 01-Apr-11 06:45 PM
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#35. "RE: Not necessarily so"
In response to Reply # 11


Yorkshire, GB
          

>I would suspect the contrast detection focus used in live view would not be as
>prone to calibration issues - it's more like manual focus - move it back and forth until it's good.
Nikon list 7 types of target (plus camera or subject movement) which may cause contrast detect AF to fail, compared to 6 types with conventional AF. What you "suspect" and reality are not the same.
It is "AF - because you have to be in LiveView to use it. It is still quick and easy to switch to LiveView to check phase detect AF accuracy.

Photography is a bit like archery. A technically better camera, lens or arrow may not hit the target as often as it could if the photographer or archer does not practice enough.

Len Shepherd

  

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chris_platt Silver Member Nikonian since 04th Apr 2009Fri 01-Apr-11 07:03 PM
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#38. "RE: Not necessarily so"
In response to Reply # 35


Newburg, US
          

Thanks Len - I was actually speculating about conditions within the system that could be subject to hardware or firmware calibration requirements - actual defects that might need to be addressed, not target conditions that might cause a focus error. I wasn't suggesting that a healthy contrast detect system is more reliable than a healthy phase detect system. But your point is well taken.

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igordb Silver Member Nikonian since 22nd Dec 2010Sat 02-Apr-11 04:25 PM
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#43. "RE: Not necessarily so"
In response to Reply # 38


CA
          

I think we missed the point of this conversation. My point was not to fool AF (you can find a lot of different ways to do so), an opposite. Live mode uses main sensor to detect focus, where normal mode AF uses separate sensors, mirrors etc..., and has to be very well aligned and calibrated. Where live mode uses main sensor signal to detect focus.
Igor

  

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chris_platt Silver Member Nikonian since 04th Apr 2009Sat 02-Apr-11 05:14 PM
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#44. "RE: Not necessarily so"
In response to Reply # 43


Newburg, US
          

Yes, your point was a good one. It got derailed a little bit by subsequent posters reading partial quotes and trying to respond to them.

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visionguru Registered since 03rd Nov 2008Sat 02-Apr-11 07:54 PM
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#46. "RE: Not necessarily so"
In response to Reply # 35


Chicago, US
          

Contrast detect AF only utilizes the signal from the image sensor. It's indeed immune to calibration issues.


Jay
- Chicago Nikonian

  

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