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04-21-2016, 05:59 AM   #1
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Lens focus calibration guidelines

Hello,

I just ordered a Focusing Calibration Ruler Chart whatever thingie. I've never calibrated for front/back focus before. But with wide open faster lenses that is important to check. Plus I'm curious
I have a basic idea on how this works. Tripod, properly aligned to the target, same height etc.
But, are there some guidelines in terms of distance? Does it makes a difference the distance to the target?
How about zooms? Should those be tested at widest? longest? middle? the focal length I think I'll use the most? Does it make any difference?
I'll play with it as soon as I get it, but I just want to have a heads up.

Thanks,
Ismael

04-21-2016, 07:52 AM - 1 Like   #2
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QuoteOriginally posted by ismaelg Quote
I have a basic idea on how this works. Tripod, properly aligned to the target, same height etc.
This is really critical. We are talking millimeters or less here so having the lens even slightly misaligned will give you false results
QuoteOriginally posted by ismaelg Quote
But, are there some guidelines in terms of distance? Does it makes a difference the distance to the target?
I've read 40 to 50 times the focal length. So for a 50mm lens you should be about 2.5 meters. Personally I use much less distance than that more like 10 to 20 times focal length but the official recommendation from Canon and the lens align makers is about 50x.
QuoteOriginally posted by ismaelg Quote
How about zooms? Should those be tested at widest? longest? middle? the focal length I think I'll use the most? Does it make any difference?
It can make a huge difference depending on the lens. I test at widest, middle and longest to see what the pattern is then decide how to handle it. Usually I take an average and do the best I can but in some cases I've used the focal length I most often use.

Also, do not be surprised if your lens needs no adjustment. On my k-5 I had to adjust all of the lenses. On the k-3 only one showed any need and it was so small I just left it alone.

Best advice: go slow, repeat the test multiple times. Do not be in a hurry to make changes. Make sure everything is very precisely aligned. You can also use Liveview as a check since it uses a different focusing system.
04-21-2016, 09:23 AM   #3
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QuoteOriginally posted by jatrax Quote
I've read 40 to 50 times the focal length. So for a 50mm lens you should be about 2.5 meters. Personally I use much less distance than that more like 10 to 20 times focal length but the official recommendation from Canon and the lens align makers is about 50x.
I use 20x unless the lens cannot focus that close. The intent is to provide adequate target for the AF system to function at optimum while having the lens focused at a reasonable real-world subject distance. That being said, 50x seems a little on the long side to me.


Steve
04-21-2016, 09:54 AM   #4
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QuoteOriginally posted by stevebrot Quote
That being said, 50x seems a little on the long side to me.
Yes, I've tried it at 50x but the DOF is broad enough that I cannot really make a good determination with the target I use. At 20x things are much clearer.

04-21-2016, 02:57 PM   #5
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QuoteOriginally posted by stevebrot Quote
That being said, 50x seems a little on the long side to me.
Infinity works fine for me. It depends on what you are trying to do - whether simply to compensate for any flange thickness tolerances, as intended, or engage in a futile attempt to fudge the built-in lensrom settings.
04-21-2016, 03:45 PM   #6
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QuoteOriginally posted by kh1234567890 Quote
Infinity works fine for me. It depends on what you are trying to do - whether simply to compensate for any flange thickness tolerances, as intended, or engage in a futile attempt to fudge the built-in lensrom settings.
I think the intent for the OP is to do fine adjustment of the PDAF system to correct for front/back focus. For that sort of adjustment as well as focus screen calibration, the assessment is generally done at moderate distance. Setting his "Focusing Calibration Ruler Chart" at infinity would prove cumbersome for that task.


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04-21-2016, 07:05 PM   #7
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One very simple and accurate (albeit tedious) AF adjustment procedure is to let the AF fine adjustment menu do the work from the beginning:

- set up a perfectly flat black-and-white contrasty focus target (newspaper page can work) at a reasonable distance (20x or 50x etc, according to what you usually shoot), shine a bright continuous light on the focus target;
- mount lens on camera, put camera on a tripod, set up camera/lens perpendicular to the focus target, frame the target in the viewfinder so that it gets some good detail or feature (not much point shooting an empty scene);
- turn off SR, set to JPEG, turn off optical corrections, set camera to AF.S, set centre-spot AF, set metering to centre weighted or spot, using TAV, set aperture wide-open, set shutter speed to something moderately fast, try and keep ISO low - you want to keep detail;

Then:
- go into the AF adjust menu, set the AF adjust to -10, save setting, carefully take photo.
- go into the AF adjust menu, set the AF adjust to -9, save setting, carefully take photo.
- go into the AF adjust menu, set the AF adjust to -8, save setting, carefully take photo.
- Repeat the above all the the way through to +10.

Eventually you will have 20 images showing AF adjusted from -10 to +10, all as sequential file names too.

Choose whichever image looks sharpest, and use the setting that image was shot at as the AF adjust value for your lens. The AF adjust value will also be stored in the EXIF, if you forget what the adjustment value was.

Last edited by rawr; 04-21-2016 at 07:11 PM.
04-22-2016, 01:44 AM   #8
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QuoteOriginally posted by rawr Quote
Then:
- go into the AF adjust menu, set the AF adjust to -10, save setting, carefully take photo.
- go into the AF adjust menu, set the AF adjust to -9, save setting, carefully take photo.
- go into the AF adjust menu, set the AF adjust to -8, save setting, carefully take photo.
- Repeat the above all the the way through to +10.

Eventually you will have 20 images showing AF adjusted from -10 to +10, all as sequential file names too.
You can save yourself some effort by using a bigger step size (2 or 3) first to get a rough adjustment and then go to 1 steps. Also remember to defocus before each shot.

As for the target distance - note which focal distance zone you are working at (read this from the EXIF using ExifTool or similar). This will vary for each lens type. Mis-focus at infinity will annoy you the most, for critical focus at macro distances you are better off using liveview anyhow.

04-22-2016, 04:52 AM   #9
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QuoteOriginally posted by kh1234567890 Quote
You can save yourself some effort by using a bigger step size (2 or 3) first to get a rough adjustment and then go to 1 steps. Also remember to defocus before each shot.
Good points. Bigger step sizes will certainly get you into the zone quicker. And defocusing between each shot is indeed very important and should not be overlooked.
04-22-2016, 05:02 AM   #10
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Described on the link below is a quick and reliable method for Canon & Nikon cameras, I can't see any reason why it shouldn't also work with Pentax.

Dot-Tune: Autofocus Fine Tuning in under 5 minutes - FM Forums

Let us know if you try it.
04-22-2016, 05:09 AM   #11
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QuoteOriginally posted by Ray-uk Quote
I can't see any reason why it shouldn't also work with Pentax.
That technique should work fine for Pentax too.

It's similar to the approach adopted by Nikon in the D5 & D500 to automate AF fine tuning.
04-22-2016, 07:27 AM   #12
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Thank you all for the comments. Very good information here.
I watched the video on Dot-Tune process and it makes a lot of sense. Will try that when I get a chance.

Thanks!
04-22-2016, 11:15 AM   #13
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QuoteOriginally posted by rawr Quote
One very simple and accurate (albeit tedious) AF adjustment procedure is to let the AF fine adjustment menu do the work from the beginning...
Confirm the final adjustment 10 times to account for scatter and you will have something.

I am not saying this won't work, only that my experience has been that the variance of the PDAF system with some lenses may be +/- several fine adjustment steps and that a thorough final check is worth the effort.

Over the years, I have noticed that a significant number (most?) of the "AF sucks" rants on this site involve cases where fine adjustment was done with no improvement and where it was not clear whether the system calibration was out in the first place.*


Steve

* Typically the user would be shooting 45 degrees to a meter stick with apparent "backfocus" being due to focus target ambiguity.
04-22-2016, 11:16 AM   #14
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QuoteOriginally posted by rawr Quote
That technique should work fine for Pentax too.

It's similar to the approach adopted by Nikon in the D5 & D500 to automate AF fine tuning.
I have used the Dot-Tune on my K-3 and it does work.


Steve
04-22-2016, 11:18 AM   #15
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QuoteOriginally posted by stevebrot Quote
ypically the user would be shooting 45 degrees to a meter stick with apparent "backfocus" being due to focus target ambiguity.
And hand held
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