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Subject: "Fine Tune Calibration and Wide Angle focal length" Previous topic | Next topic
elec164 Silver Member Nikonian since 15th Jan 2009Tue 12-Apr-11 04:46 PM
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"Fine Tune Calibration and Wide Angle focal length"


US
          

Somewhere in all the information I read in the various threads discussing focus issues I seem to remember reading a reference to a caution with calibrating wide angle lenses. I have not been able to find that source again to link to it, so I can only rely upon the fact that I seem to remember reading some sort of reference to wide angle lenses.

Having tested my 17-55 f/2.8 with the angled chart at 55mm and determining it focused pretty much dead on I stopped there. This image shows the area of the angled chart were the focus point was centered upon.



What I particularly liked about this image (and hope everyone else can also see it) is that in the paper pulp detail shows the DOF surrounding the focal plane. It appears the focal plane is the top side (would have been the back edge being that an angled chart provides depth to the target) of the line because at the focus distance the distribution of the DOF would have been 49% front and 51% back. I could show you the graduated rulers on each side indicating a possible back focus issue, but to me this clearly shows the camera focused pretty much where I expected it to within .5mm or less. And a flat chart mounted on the wall using 55mm focal length had pretty much the same result.

But then I recalled reading a mention of the wide angle caution, and decided to try the flat parallel chart test shooting at the extreme wide end. What I discovered was quite disconcerting. At about 2.5 feet and 17mm I found Phase Detection AF quite unreliable with live view getting better focus and more consistent results. At best it was a 50-50 shot. I could half-press and achieve a lock; release the shutter then half press again and get a totally different distance. In fact at close distance I would liken it to a jumping bean on a hot plate at certain times. Wondering if it was a defect in my 17-55 I mounted the kit 18-105 with pretty much the same results. Taking the camera outside with a real world wide-angle scene and the jumping around was not as drastic, but still occurred. Now I know in actual application the DOF would more than cover the slight focus error under normal enlargement. But it makes me wonder if all this hand wringing and mental gymnastics about fine tuning for the average enthusiast is really worth it in the long run (especially with wide angle focal lengths).

I tried to research this a bit but have come up empty so far. I know that wide angle lenses tend to show more aberrations especially at the outer edges of the lens. And being that it appears the Phase Detection system takes a sample from opposite sides this could be a major contributing factor I would think, coupled with the fact that shorter focal lengths will provide a greater DOF at a given distance because of the small aperture at a given f-number. But after that experience, if I am shooting wide-angle and the scene is static, I am more likely to use live view or manual focus than Phase Detection AF (especially if the DOF is narrow).

This brings up my next point, 100% view. It seems it was possible that this same issue occurred with 35mm film AF cameras. After all how many times did you make a 40x enlargement of one of your captures and view the results at an 18 inch viewing distance? I would guess the answer would be no one. At best most only analyzed 8x10 prints with a few analyzing larger ones. So at best a few might be judging maybe 20x enlargements maximum whereas most only viewed about 7x enlargements. But now D7000 users are routinely viewing captures at about a 60x enlargement and claiming a softness issue. I would think if you viewed your favorite sharpest 35mm film image at 40x enlargement (a similar enlargement factor for the given format size) you would see a similar issue, am I wrong in assuming this??

Hopefully this post makes sense to everyone; for I have viewed others resultant images, read and pondered so much about all that has been posted about this (plus all the reference links) that it sometime seems to be becoming a foggy mess in my head.

Any comments about my thoughts would be greatly appreciated.

Pete

Pete

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Replies to this topic
Subject Author Message Date ID
Reply message RE: Fine Tune Calibration and Wide Angle focal length
four eighty sparky Silver Member
12th Apr 2011
1
Reply message RE: Fine Tune Calibration and Wide Angle focal length
elec164 Silver Member
12th Apr 2011
2
Reply message RE: Fine Tune Calibration and Wide Angle focal length a...
Len Shepherd Gold Member
14th Apr 2011
3
Reply message RE: Fine Tune Calibration and Wide Angle focal length a...
elec164 Silver Member
14th Apr 2011
4
     Reply message RE: Fine Tune Calibration and Wide Angle focal length a...
JohnE Nikon Silver Member
14th Apr 2011
5
     Reply message RE: Fine Tune Calibration and Wide Angle focal length a...
Len Shepherd Gold Member
14th Apr 2011
6
          Reply message RE: Fine Tune Calibration and Wide Angle focal length a...
JohnE Nikon Silver Member
14th Apr 2011
8
               Reply message RE: Fine Tune Calibration and Wide Angle focal length a...
Len Shepherd Gold Member
15th Apr 2011
9
     Reply message RE: Fine Tune Calibration and Wide Angle focal length a...
Len Shepherd Gold Member
14th Apr 2011
7

four eighty sparky Silver Member Nikonian since 08th Apr 2011Tue 12-Apr-11 05:30 PM
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#1. "RE: Fine Tune Calibration and Wide Angle focal length"
In response to Reply # 0


US
          

Are you referring to this link?

http://focustestchart.com/chart.html

  

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elec164 Silver Member Nikonian since 15th Jan 2009Tue 12-Apr-11 06:31 PM
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#2. "RE: Fine Tune Calibration and Wide Angle focal length"
In response to Reply # 1


US
          

That is the chart I used for the image supplied in this post, but it’s only relevance in the main gist of this thread is that at 55mm focal length at about 2.5 feet, the camera was able to reliable focus at the same point each time and was as accurate as I would need it to be (within .5mm of my expectation) . The links,as I referred to them in my OP, was all the references which totaled quite a few links on various site’s including official Nikon and Canon information.

I believe that result I got to be reliable enough for my purpose and more reliable then perhaps a chart like this example chart. Even though the author of that other chart suggests the camera cannot see the gray detail, it’s my experience that it possible can and therefore provides a more unreliable target. And it was probably a mistake on my part for including that image cause it is going to dilute the intent of this post, but to me it clearly shows the 17-55 AF perfectly (or darn near perfectly) with relatively no back or front focus.

The wide angle testing was done with an appropriate target parallel to the sensor (or a close as I could eye up and chosen to avoid the angled versus parallel argument) as many here suggest being the better target. So the jumping around of the focus has nothing to do with an angled chart issue and should be left out of the discussion. But I do suspect that lens aberrations could very well be the major culprit. And when setting the lens to 35mm or greater the jumping around of the focus point would subside for the most part with no apparent change once 55mm was again reached.

Pete

Pete

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Len Shepherd Gold Member Nikonian since 09th Mar 2003Thu 14-Apr-11 08:53 AM
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#3. "RE: Fine Tune Calibration and Wide Angle focal length as the only gear to go to the shops. My camera has an AF"
In response to Reply # 0


Yorkshire, GB
          

Adding "Focus here" in white on the black horizontal line has the advantage of giving the AF detector some reasonable horizontal readable detail to focus on.
A solid black horizontal line may provide little or no readable detail for the horizontal detection line, and very little vertical readable detail for high accuracy with the vertical detection line.
The "Focus here" lettering quality is not particularly sharp - by any standards
It is interesting to note how it is suddenly "fashionable" to try to do things with something intended for a different purpose.
My car has a reverse gear - but I would not use reverse gear as the only gear to go to the shops.
My Nikon cameras have an AF detection system which is most accurate with good readable detail parallel to the sensor so I would not expect to be able to consistently check focus or calibrate to the highest accuracy with a 45 degree 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 2009Thu 14-Apr-11 01:10 PM
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#4. "RE: Fine Tune Calibration and Wide Angle focal length as the only gear to go to the shops. My camera has an AF"
In response to Reply # 3


US
          

If you read my post objectively not subjectively you would have noticed that I did not use that chart when checking the issue with wide angle focus and the target parallel to the sensor. You might have noticed also that when I went to 55mm I did not have the difference issue with the parallel target I had with 17mm and I also took the camera outside to a real world scene.

But then according to some opinions even the Lens Align Target would not be the best target to use either, even though it’s parallel to the sensor and therefore could potentially fool the AF .

But thanks for your comment just the same.

Pete

Pete

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JohnE Nikon Silver Member Nikonian since 15th Jun 2010Thu 14-Apr-11 03:35 PM
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#5. "RE: Fine Tune Calibration and Wide Angle focal length as the only gear to go to the shops. My camera has an AF"
In response to Reply # 4
Thu 14-Apr-11 03:35 PM by JohnE Nikon

New HArtford, US
          

Pete,
Interesting observation and analysis.

Perhaps the wide angle uncovers the minor tolerance variances in manufacturing which affects phase detection differently than contrast detection.
When I did my initial tests I deemed my 10-24mm lens to need no correction. I believe I tried at both 10 and 24mm. I tested a few months ago. (BTW I think it is recommended to test at the mid point in a zoom lens... I'm not retesting.)
I never use this lens for portraits where I am the most picky about perfect focus. As you mention the DOF with this lens on the wide side makes precise focus nearly irrelevant for many images likely to be shot at a wide focal length. You will have a more limited DOF with 2.8 than I do and precise focusing may be more important for you.

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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Len Shepherd Gold Member Nikonian since 09th Mar 2003Thu 14-Apr-11 07:13 PM
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#6. "RE: Fine Tune Calibration and Wide Angle focal length as the only gear to go to the shops. My camera has an AF"
In response to Reply # 5


Yorkshire, GB
          

>Perhaps the wide angle uncovers the minor tolerance variances in manufacturing which affects phase detection differently
>than contrast detection.
Mathematically it should not - because phase detect is based on depth of focus (not the same as depth of field) which is primarily affected by lens aperture, not focal length.
Getting the sensor parallel to the target is difficult with wide angles, and as a multiplier of focal length to focus distance wide angles have more depth of field than telephotos.

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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JohnE Nikon Silver Member Nikonian since 15th Jun 2010Thu 14-Apr-11 07:36 PM
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#8. "RE: Fine Tune Calibration and Wide Angle focal length as the only gear to go to the shops. My camera has an AF"
In response to Reply # 6


New HArtford, US
          

>>Perhaps the wide angle uncovers the minor tolerance
>variances in manufacturing which affects phase detection
>differently
>>than contrast detection.
>Mathematically it should not - because phase detect is based
>on depth of focus (not the same as depth of field) which is
>primarily affected by lens aperture, not focal length.
>Getting the sensor parallel to the target is difficult with
>wide angles, and as a multiplier of focal length to focus
>distance wide angles have more depth of field than
>telephotos.
>
>
Len,
This is interesting. I only tested my 10-24 with the much maligned angled focus method where getting the sensor parallel is less of an issue and wonder if I would now have problems if I used a flat focus chart.

JohnE Nikon
https://plus.google.com/photos/104310967428146619677/albums?hl=en

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

Visit my Nikonians gallery.

  

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Len Shepherd Gold Member Nikonian since 09th Mar 2003Fri 15-Apr-11 02:20 PM
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#9. "RE: Fine Tune Calibration and Wide Angle focal length as the only gear to go to the shops. My camera has an AF"
In response to Reply # 8


Yorkshire, GB
          

> I only tested my 10-24 with the much maligned angled focus method where getting the sensor parallel
>is less of an issue and wonder if I would now have problems if I used a flat focus chart.
The only way to be sure is to to a further test with a flat target.
I am fortunate in that years ago I got a copy camera adapter for my Durst enlarger - helpful for getting the camera sensor parallel to anything on the baseboard.
For those in the UK this weeks Amateur Photographer includes a quite good lens testing chart for checking centre and corner resolution, colour reproduction accuracy and distortion.
It is relatively small in size at 16 x 10.5 inches target area.
It neatly avoids the focus target dilemma of using concentric circles by putting a tone diagonal through the middle and varying the circle boundary detail between bottom left and top right.

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 2003Thu 14-Apr-11 07:18 PM
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#7. "RE: Fine Tune Calibration and Wide Angle focal length as the only gear to go to the shops. My camera has an AF"
In response to Reply # 4


Yorkshire, GB
          

>If you read my post objectively not subjectively you would have noticed that I did not use that chart when checking the
>issue with wide angle focus and the target parallel to the sensor.
Sorry I mis-understood - the only target you illustrated is the one with the text
>This image shows the area of the angled chart were the focus point was centered upon.

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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