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12-04-2011, 08:08 PM   #1
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Front- and Back-Focus overview and tips

Wrote this over the weekend, hope you enjoy it!

Fixing Front and Back Focus - Introduction


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12-04-2011, 09:32 PM   #2
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Adam, If I read your article correctly you state that the K-x has autofocus finetune, I don't think this is the case is it? Maybe the same for the K-r too.
12-04-2011, 09:48 PM   #3
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It does actually, but that wasn't clarified. Here's how you do it:
https://www.pentaxforums.com/forums/forum-suggestions-news-help/167556-questi...ml#post1733892

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12-05-2011, 04:07 AM   #4
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I think the article could have put more emphasis on the fact that the Spyder product does not facilitate alignment of the camera with the target and hence is not really worth a recommendation.

There are more hints, and pointers to DIY solutions in my AF adjustment hints.

12-05-2011, 05:23 AM   #5
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Good article! It overlooks one aspect of focus error - mainly focus shift as a lens is stopped down. Most lenses exhibit some amount of focus shift, where the optimum plane of focus moves farther away as the lens aperture gets smaller, but usually it is small enough that the increasing depth of field hides the shift. It is also more of a problem with high-speed (large aperture) lenses. With some lenses it is significant, and results in the sharpest part of the picture being farther back than was intended. People complained enough about the Leica 35mm f1.4 Asph (~$3500) that Leica redesigned it to add a floating element to solve the issue (now ~$5000).
Testing for it is quite simple. Mount the camera on a tripod aimed at an alignment target (either of the tools you discussed, or a ruler at 45 deg. to the lens axis). In manual focus mode set the focus accurately wide open and take a shot, Then without touching the focus stop the lens down a stop at a time and take a shot at each stop. Compare the images and look for the sharpest center of the "in focus" band. The band increases in width as the lens is stopped down, but the area that was sharpest wide open should remain in the well-focused area regardless of f-stop.
A modern autofocus camera could in theory compensate, if the focus shift of a particular lens is "understood" by the camera, but I haven't heard of any camera bothering with it. Most lenses for dSLRs are slow enough that it isn't an issue, and when the focus error is noticed people assume it was some other problem.
I've tested a number of fast rangefinder lenses, and most exhibit some shift, and a few are quite bad. I've been meaning to test my manual Pentax-M f1.4 and f2.0 lenses - so I'll comment if I find they have the issue also.
12-05-2011, 05:28 AM   #6
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There's another method that gives pretty good focus calibration without special equipment -- the Live View Method.
The article have already concluded that the contrast detection method of finding focus is not susceptible to front or back focus issues. So we can use this for comparison and a way to calibrate the phase detection focus. There are some limitations; you need to be able to see differences in focus distances on the lens, either with a scale or with the focus ring if it's fixed relative the focus distance.

Place the camera on a fixed position in front of something flat with a high contrast parallel to the sensor. Not too far away, the "resolution" is better at close distances where a small difference in distance gives a larger change on the lens. Go to live view and press focus button and take note on how the focus ring or scale is positioned when focus is acquired. Try this a couple of times and move focus manually to both ends between tries. Use own judgement to determine how the lens is supposed to aligned when your subject is in focus and make a non permanent mark as a reference if you need. Now turn off live view and try to focus and compare with marking. If it's misaligned, make a small adjustment in the menu and try again (it could be good to manually move focus between tries). Repeat until you are satisfied.

While this method hardly will satisfy the pixelpeepers and perfectionists out there, it's a quick and easy way to get an often good enough auto focus calibration.
12-05-2011, 09:35 AM   #7
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Focus shift illustration

I tested the Pentax-m 50 f1.4 to illustrate how f-stop can affect back focus, even with manual focus. This lens does show some shift, but the increasing depth-of-field as you stop down is enough that the initial plane of focus stays acceptably sharp - yet you can see the effect.
At f1.4 the sharp band is centered on the focus line, extending about -6mm to +6mm:
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Stopping down to f4.0, the sharp band now extends from about -10mm to +30mm, so while the focus line is still sharp, the center of focus has shifted about 10mm farther away
Name:  Pentax 50M f4.0.jpg
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By f5.6 the sharp band extends from about -10mm to +45mm, so the sharpest plane continues to shift farther away from the intended focus
Name:  Pentax 50M f5.6.jpg
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The intended focus plane ramains sharp at all these settings, so this shift isn't enough to worry about, but I have tested other (non-Pentax) 50mm lenses where the intended focus point got progressively unsharp all the way to f5.6, where the depth of field was finally enough mask any further shift.
12-05-2011, 10:00 AM   #8
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Fixed Sensor

FWIW my *istDS with a fixed sensor never (well almost never) had any FF/BF issues, which has always let me to believe the problem has something to do with the movable sensor in the newer Pentax models more than anything else. If the subject was in focus in the viewfinder on the *stDS, it was also in focus in the produced image, which was not the case with any of the newer Pentax DSLR's with SR that I have used.

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12-05-2011, 10:19 AM   #9
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It doesn't appear that I can adjust camera focus with K100D or I haven't found the info. Thanks
12-05-2011, 04:28 PM   #10
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QuoteOriginally posted by hman Quote
It doesn't appear that I can adjust camera focus with K100D or I haven't found the info.
Check my K100D Tips & Tricks article. It contains information about how to do the AF fine adjustment with the K100D (only one setting for all lenses, which you can change dynamically though, if you are so inclined).
12-05-2011, 06:00 PM   #11
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Can someone clarify what type of lighting should be used for focus testing (daylight, artificial, etc.)? The article doesn't specifically say. Also, is it necessary to set a specific white balance versus using AWB? I'm just thinking back to the K-5 issues and everyone doing a myriad of tests for front focusing under tungsten light. I was just thinking, if you adjust focus under tungsten, wouldn't that screw up focus for other lighting conditions?
12-05-2011, 07:55 PM   #12
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Shoot under the conditions you normally experience FF/BF under. Ideally, start by testing in bright light, make corrections there, and see if that solves the problem in artificial lighting as well.

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12-05-2011, 08:51 PM   #13
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Thanks Adam. How about white balance? Should that be adjusted or is it fine to just leave it on AWB?
12-05-2011, 09:02 PM   #14
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It shouldn't affect the focus since it's applied afterwards.

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