This is a question for those of you who test your camera body and lenses for front and back focusing issues using a LensAlign or other method.
Say for example the scale you are using has 1-2-3-4-5 and you are focusing on the 3. If 2-3-4 are in focus with 3 in the center then it is perfect. But suppose that 3-4-5 is in focus, with 3 still in focus but just not in the center of the focus area. Do you use a fine tuning correction factor or just leave it alone?
In other words, should the focus point be in the dead center of the focus area, or is it considered OK for the focus area to be slightly shifted to the back just as long as the focus point is still within that area?
#1. "RE: AF fine tuning" | In response to Reply # 0jckingca Registered since 16th Dec 2010Sun 21-Apr-13 07:09 AM
If you are doing these tests with the lens wide open at largest aperture I would say there is a small amount of AF adjustment that could be applied.
I would run the test multiple times and at different distances to confirm the findings.
#2. "RE: AF fine tuning" | In response to Reply # 1Tue 23-Apr-13 02:03 PM
Thanks John. So you think the focus point should definitely be in the center of the focus area? I have seen on some websites that slight back focusing is intentional and perhaps even desirable, but I have yet to find a really definitive statement about it.
#3. "RE: AF fine tuning" | In response to Reply # 2archivue Registered since 26th Mar 2002Tue 23-Apr-13 03:00 PM
Well, if you're in a real life situation, as trying a portrait in low light, focusing on the nearest eye and the resulting picture is focused on the distant eye (or the ears) you might find it not so acceptable (Never saw that comment on intentional BF before !?)
If I want to "misfocus" on purpose, I'll do it, but even then I must know were the focus really is !
I do fine-tune my lenses (on a D3x) every year as I've discovered that for some lenses (mostly AF-Ds), time and wear, I guess, allows it to change a bit... But I just do simple tests with real life objects and don't change the value in a hurry if it worked well till then
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#4. "RE: AF fine tuning" | In response to Reply # 3Tue 23-Apr-13 10:49 PM
I think the origin of the idea that a little back focusing is "ok" goes back to older cameras that had very large AF sensor areas that often went far beyond the focus reticle in the viewfinder.
On those cameras it was never clear to me how I was supposed to actually focus on the eye without grabbing the eyebrow ridge or upper cheek, both of which are slightly in front of the actual eye we are trying to focus on.
I'm not saying I agree with that idea, or that it is even viable, but my own experience with my D2h made me understand why some people thought it would be a nifty idea .
Just suggesting the likely origin of that idea, and I don't think it was ever a very popular one.
I've also heard the idea that if your camera is going to mis-focus it is better to back focus a little than front focus a little. The reasoning being that when a camera front focuses it is very difficult to ascertain that it did, assuming you focused on the front of your subject. The actual focus is floating in front of the subject.
In other words, if you are going to have a problem it is better to have it in your face and easily measurable than "hidden". Or something like that . I actually agree with that although it is certainly preferable to just have the camera focus correctly!
I don't ever recall it being seriously suggested that a camera maker would intentionally back focus the camera to help people focus on eyes.
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#5. "RE: AF fine tuning" | In response to Reply # 4Wed 24-Apr-13 04:28 PM
Thanks for the replies. I guess I was kind of hoping somebody would say there is no need to mess with it as long as the focus point falls somewhere in the focus area, but I will go ahead and use AF fine tuning to adjust the focus point to be right in the middle of the target.
#6. "RE: AF fine tuning" | In response to Reply # 5Wed 24-Apr-13 05:28 PM
I think it's a personal decision as to how accurate you want the focus. If it were me, and I had spent enough time on it to come to a firm conclusion, I would adjust it. And thereafter I would watch it carefully to make sure that your tuning is right in the real world.
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#7. "RE: AF fine tuning" | In response to Reply # 0
I heard of different strategies when it comes to focusing: The rule of thirds, the second third and the front third.
The one I use is the rule of thirds. My reason is that I have been using that system for almost 40 years now and I do not see
the need to experiment with other strategies. Call it habit, laziness… or, more adequately, predictability.
Which ever tool you use, the best case scenario is that as you do, you are pretty sure that what is expected will be. In all,
you will take a strategy you feel good with and the rest is literature. I know why some colleagues are using others, but like I
said: "I do not see the need to experiment other strategies." I have been working with the D3s and the D3x for some years
now, and I just completed the same "marriage body-lens" with my new D800E and all my 17 lenses (the camera will recognize a
maximum of twenty lenses). You may use other lenses but the camera will not recognize them. Say you buy a new #21 lens, a
previously recorded "marriage" will have to be deleted to get the new one recognized every time it is mounted on the body.
Any lens used will work normally… but the fine tuning will no be there.
So here is the one I use: the rule of thirds. Let's use the scale from your example: 1,2,3,4,5 where all digits are equidistant.
By the way:
> whichever the adopted strategy, it should be applied to ALL your lenses in order to assure predictability,
> this adjustment is best performed with the lens wide opened and at the shortest focusing distance.
The rule of thirds stipulates that any lens/camera combination, at any aperture, focusing on any given point will render a depth
of field that will always have the same ratio: 1/3, 2/3. NOT EQUAL depth of field (DoF) but the SAME RATIO. That's cool because
the predictability is right there.
Alright, now let's use your scale. Set up camera, lens and scale. With the lens wide open and at the shortest focusing distance,
move the scale back and forth until the 3 is dead sharp. You should see (in the case of the rule of thirds) that the scale is sharp
between say the 2 and 5, 3 being the sharpest, the focus point. That is one third in the front of the focus point and two thirds behind
the focus point. In the case of the rule of thirds, this is what you want to see. If not, you will have to fiddle with the AF Fine Tuning
controls through the menu of your camera. After each adjustment make sure that the 3 is dead sharp before further tuning. The
adjustment scale goes from +20 to -20. Don't be shy, if you are not sure or start to sweat (like the first time I cleaned my sensors),
bring it back to the starting point and no dammage done!
This rule of third comes from portraitists (well more then a century ago) who wanted to focus on the eye, having the eye-nose area
(1st third) sharp (including moustache, beard and monocle!), as well as the eye-ear area (the other thirds) including the hair line and
the eventual jewellery. I think that this made sense then and still does for me today.
Other strategies will shift the focus point sometimes a bit forward or a tweak backward. The only thing that counts is predictability.
Adopt the strategy that fits you approach and stick to it.
#8. "RE: AF fine tuning" | In response to Reply # 7briantilley Nikonian since 26th Jan 2003Wed 24-Apr-13 06:49 PM
I like the way you've analysed this, but...
>The rule of thirds stipulates that any lens/camera
>combination, at any aperture, focusing on any given point
>will render a depth of field that will always have the
>same ratio: 1/3, 2/3.
...isn't actually a rule - it's a useful approximation but is only accurate in certain circumstances.
The 1/3 - 2/3 split for DoF is only EXACTLY correct when the lens is focused at one-third of the hyperfocal distance for the chosen aperture, focal length and sensor size. The approximation is fairly close for a small range of distances around that, but as you move the focus point nearer the split gets close to equality. Conversely, as you move the focus point further away the proportion behind the focus point grows - and is infinite when focused at the hyperfocal distance.
I'm sorry if this complicates things when you were trying to simplify them...
#11. "RE: AF fine tuning" | In response to Reply # 10km6xz Nikonian since 22nd Jan 2009Thu 25-Apr-13 04:36 AM
My own view is that AF focus is a non-static state that depends on many variables that the camera designers have tried to build algorithms to calculate compensation for the grossest of influencing variables. Such variables that impact the way a lens and AF servo system works include light source color temperature, distortion, CA, and diffraction.
Adjusting for exact center creates a probability, meaning out of 100 AF actions with the same conditions, an acceptable majority will have the focus point on the point you desire. Changes to the conditions and those odds shift, and sometimes result in an unacceptable number of desired result. The shift is due to the accuracy of the algorithms to model the conditions or due to a misadjustment or defect. A good example was the algorithm for light color temperature compensation was tweaked in alignment procedures of the D800 which some were complaining that there was an unacceptable shift under low level of tungsten source light.
All this is to suggest that whatever you adjust to, do not be surprised if the adjustment is appropriate for other distances, light or color. If it close, and you do not notice a problem with real world photos, you are not guaranteed to get the desired results under all conditions. You may find that the good real world shots in light that you are shooting actual images in, are adjusted just right for those conditions and tweaking with a test setup using different light or distances or ?? might adversely impact the odds of getting ratio of well focused images when using your typical conditions.
Which boils down to....if real world shots are good, don't mess with it;>)
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#12. "RE: AF fine tuning" | In response to Reply # 10briantilley Nikonian since 26th Jan 2003Thu 25-Apr-13 05:54 AM
My principle is that, if I'm getting focus accuracy that is good enough for the situations in which I shoot, then I don't worry about it. Fortunately perhaps, all my lenses and cameras have always focused well enough for my purposes out of the box, so I have never needed to test any of them.
#13. "RE: AF fine tuning" | In response to Reply # 10Thu 25-Apr-13 08:26 AM | edited Thu 25-Apr-13 08:32 AM by nrothschild
Since Stan and Brian have weighed in on the "if I should" question, I thought I would do the same, and I am in fundamental agreement with them. That may not have been clear in my previous replies.
I have occasionally "tested" my lenses, more out of curiosity than any real world concerns. I have used a variety of methods, including the LensAlign system.
I do not have any lenses that have obvious focus problems, such that I am very confident I need to adjust them.
I am concerned about my 500/4, and only with TC's. The problem I have is more a matter of focus consistency, and that might be expected to some degree when using TC's at f/5.6 and f/6.7.
In every case where I tested a lens, I did not achieve the reproducible consistency that I felt would be needed to adjust the lens. I think that is the biggest problem with "focus testing" and I have seen others express the same concerns. Other people seem to be more confident in the consistency achieved. It's a bit of a mystery to me.
I previously said "If it were me, and I had spent enough time on it to come to a firm conclusion, I would adjust it. ".
I said that, taking at face value that you had a problem worth correcting and you are absolutely certain where precise focus lies. Personally I'm not sure I could pin down the focus on any specific lens to the point where I knew it was at a very certain point within the DOF. Mainly due to the consistency issue.
I think that if you feel confident in your determination that an adjustment is warranted, and you decide to proceed with it, then you should split the difference, 50/50, and tune to dead center of DOF. Assuming you are testing at the near end of focus, as I think most of us do. If you are testing at distance than you have to look at DOF tables or use Brian's general advice on that matter, which summed it up nicely.
That's a big if, though, and if your real world images look ok and you are on a "fishing expedition" it is probably best to leave it alone.
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#14. "RE: AF fine tuning" | In response to Reply # 13Thu 25-Apr-13 11:15 AM
Thanks guys. I will proceed with your well-informed comments in mind. My photographs look fine. It is only when I do some testing that I see what appears to be some very slight back focusing with a lens or two.
#16. "RE: AF fine tuning" | In response to Reply # 15Thu 25-Apr-13 11:43 AM | edited Thu 25-Apr-13 11:46 AM by RockyIII
I use a LensAlign MkII and test per their procedure. I have tried some other methods too, such as staggered targets, flat target, and brick wall. The LensAlign seems to give the most consistent and repeatable results.
#17. "RE: AF fine tuning" | In response to Reply # 16walkerr Nikonian since 05th May 2002Thu 25-Apr-13 01:03 PM
Their FocusTune software product is a nice complement to the LensAlign tool. It automates the recognition of the best adjustment point and does it via the use of multiple samples to reduce error. I've found the latter particularly helpful. It's just $29.95, with a $10 discount for owners of the LensAlign.
#18. "RE: AF fine tuning" | In response to Reply # 17Thu 25-Apr-13 07:35 PM
I just bought FocusTune last night. Have not played with it yet, but I was hoping it would help in the ways you mention.
That $10 discount is said to be good only to the end of this month, which is what prompted me to buy it now.
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#20. "RE: AF fine tuning" | In response to Reply # 19lajolla Nikonian since 03rd Nov 2005Fri 14-Jun-13 09:16 PM
Back Focus issues with professional cine zoom lenses are still resolved by zooming out to the widest focal length, focusing at infinity, then adjusting the back focus knob on the lens until the wide angle image is in focus at the film plane or the image sensor. The focus problems discussed here are really not classical lens back focus issues, since Nikon F mount lenses do not have back focus adjustments.
With all Nikon cameras, SLR and DSLR, the distance from the lens mounting flange to the focal plane sensor or film surface has always been 46.5mm. All Nikon F mount lenses have been and still are designed and manufactured to hit that 46.5mm sensor/film plane dimensional sweet spot.
There are no manual back focus adjustments available to users. Forget about using charts or using Live View focus spots to change parameters. If your image is soft at your lens wide angle setting while focused on infinity, and your viewfinder diopter is set appropriately, then you should send in the DSLR/SLR camera body and lens in question in order to have a lens alignment check done. There is also a reasonable probability that the camera mirror-box needs to be slightly adjusted relative to the sensor/film plane and the camera lens flange.
A misadjusted mirror-box position relative to the sensor/film plane will deceive one into thinking there are lens back focus problems. Likewise the metal lens flange on the camera can be adjusted relative to the sensor/film plane. But the only way for Nikon techs to solve back focus issues are for both the camera body and the lens (or lenses) to be sent in for evaluation.
#21. "RE: AF fine tuning" | In response to Reply # 19cwils02 Nikonian since 18th Apr 2012Fri 14-Jun-13 10:39 PM | edited Fri 14-Jun-13 10:41 PM by cwils02
This is a subject near & dear to my heart. I spent a very frustrating year with my D7000 & its backfocusing problems. I spent a very long time studying on methods for testing and how to adjust for problems. I ended up opting for a less expensive Focus Pyramid solution. focuspyramid.com
Understand that to be really scientifically accurate, shooting at a slanting target ain't the best way. I read an article that talked about how Nikon tested. It was something like a square in the center, with a square to the left at a longer distance, and one on the right at closer distance. The distances on the left & right are variable; I assume depending on the lens used. I'm guessing that the reason is to come as close as possible to getting identical light on each square. I understand the problem with the slanted method even if I can't explain it.
As others have suggested, AF is not static. So much depends on the environment that you are shooting in, and your technique. But, as others have instructed me, it is good to know where your camera lens focuses under ideal conditions, so you will have a clue if it's your technique, etc.
I tested my camera and all my lenses over a period of two weeks in all sorts of outside lighting conditions, where I normally shoot. When the sun was out, it was important to re-orient the apparatus to keep the lighting as even as possible. I did this by lining up the shadows of the front legs of the table with the shadows of the back legs of the table where I had the target. I tested each lens wide open & at f/8, and at several focal lenghts if a zoom lens. I would estimate that this took me about 60 hours. After shooting I would view the images on ViewNX2 where I could see where the focus point actually was when I shot.
Once you determine if there is a problem, then you have to determine how much correction is needed. I ended up with each lens shooting with a -20, -15, -10, -5, 0, +5, +10, +15, & +20. Then I would just view them on the monitor, and narrow down the range. Then I would shoot varying the correction in that range. All of my lenses but one needed from -9 to -12, except my 85 f/1.8 which need +6 (go figure).
I was rewarded shortly afterwards by fine tuning, then shooting some squirrels, which I posted on here and got accolades for sharpness at 270mm on my 70-300 VR lens. I then sent the camera in (under warranty) & Nikon fixed the camera. No fine adjustment needed except for the 85 f/1.8.
That software may well speed up the process. Good Luck!
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