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01-22-2019, 11:20 AM   #1
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Definitively assessing back-focus with K10D

I've come to the conclusion that I have back-focus problems with my K10D. If someone could evaluate what I've done so far and offer any suggestions, I'd appreciate it. (This stems from a thread started in the travel sub-forum: Italy-France kit thoughts and the inevitable upgrade question - PentaxForums.com)

Camera: K10D
Problem: Growing evidence of back-focus problems, initially discovered by a large number of photos in which the sharpest focus is just behind subject
Tests done (all using the largest aperture for the lens used):
+ sample shots with a known focus point, manual focus lens
+ tripod mounted shots taken of a row of AA batteries, as suggested here: Checking Your Camera for Front Focus or Back Focus. | Camera Light & Lens Forum News
+ calibration patterns, as used here Sharp lens - PentaxForums.com

I concluded (or at least convinced myself) that I have back-focus issues. So, I upgraded to firmware v1.31 and entered Debug Mode. With some tests, I settled initially on a correction of +0.60um. I'm in the process of testing this. The results seem somewhat promising, but I may need to go further.

Two questions:
1. Am I better off sticking with the M50 f1.7 lens which allows me to most easily spot problems (with its shallow DOF at 1.7), or should I use my 16-45 f4?
2. Should I move to the vertical brick-wall test at this point?

01-22-2019, 11:52 AM   #2
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Use M50 f1.7 for shallow depth of view & this...

Jeffrey Friedl's Blog Jeffrey’s Autofocus Test Chart


good luck
01-22-2019, 11:54 AM   #3
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QuoteOriginally posted by Lambic Quote
2. Should I move to the vertical brick-wall test at this point?
That will not show if you getting BF/FF
01-22-2019, 12:35 PM   #4
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QuoteOriginally posted by pschlute Quote
That will not show if you getting BF/FF
The brick-wall test would be a good verification of the correction, though. Yes?

01-22-2019, 04:34 PM - 1 Like   #5
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QuoteOriginally posted by Lambic Quote
The brick-wall test would be a good verification of the correction, though. Yes?
Only if it is correct, it won't help if it's not.

The wall gives you a yes/no but the chart gives you how much it's off and which direction.

But you need to take care with the setup using the chart, square-on and 45. If it's not 45 it won't be accurate, if it's not square-on you will get different results each side.
01-23-2019, 02:56 PM   #6
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I'm not sure how a manual focus lens would solve your problem? The BF/FF adjustments are for auto-focus lenses.

When shooting with MF, the focus confirmation only indicates you've hit a range where you are likely in focus. The BF and FF settings don't impact that at all, that I am aware of.

As for a brick wall test, it won't necessarily help you much unless the depth of field is narrower than the relief in the wall. The autofocus test chart is probably a better idea. I like just creating a dummy subject that represents my typical focal length, aperture, and distance to test things out. In my case, I just shoot my dog over and over because he is willing, and I can test focus on his eye while seeing what is actually in focus. I don't shoot Macro or so close that a razor thin depth of field is a problem, and the corrections can usually work well for most shooting.
01-23-2019, 05:19 PM   #7
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More tests last night using Jeffery Friedl's chart. Good quality chart and thorough instructions. Thanks for the suggestion, blind-bat.

I set the chart at 45 degrees instead of the camera - it was much easier to measure the angle that way. The results appear to suggest that +60um is the correct adjustment. It's certainly much better than it was, with all the lenses. Interestingly, there's a lot of play in the focus lock 'width'. Using the 50mm f1.7 from 1m, if I set the focus coming from short to long, or long to short the results are very different. You can coax back-focus OR front-focus out of it. I suppose with such a shallow DOF, you might need to go a little past the initial focus lock point (from either direction) to really centre it.

emalvick: the focus confirmation (and the manual lens) approach seems to correct whatever is happening. I suppose the calibration is correcting the AF confirmation? Also, keep in mind this is a K10D - only one adjustment can be made and thus it applies to all lenses.
01-24-2019, 10:49 AM   #8
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Yes. I had a K10d. The calibration is basically correcting the AF calibration. I believe that it just takes whatever the confirmation is and then adjusts it by the setting you set. For the K10d, it is just one setting for every AF lens. For other cameras it may be lens specific (or they allow a certain amount of lens specific combinations).

As for the MF, my point was just what you discovered, the focus confirmation is over a range, which results in BF or FF depending on what direction you are focusing from (if you go from the initial focus indication). The solution to that issue is to either go a little past the focus point (like you suggested), try to see the focus for what you are focusing on, or get a focus screen that can give you a tighter indication.

With regard to a focus screen, I installed a split-prism screen in my K10d. I needed shims to get it to work otherwise I ended up with the equivalent of BF for my manual lenses (i.e. when I saw focus confirmation, it wasn't quite inline with what the lens was focused on). Since that camera, I've just learned to see the focus and "center" myself in the range. Another note, is that if you shoot where you have a wider depth of field, centering in the range indicated is less critical. I only mention that because I tend to shoot landscapes at an aperture of 5.6 to 11 or of people where my depth of field, while narrow, is not so narrow that the slight difference in centering the manual focus is noticeable. I think it is a bit more accurate than how the AF works only because I've had to make a lot more adjustments over the years for AF than I've had problems with MF. I'm only not fast enough with MF to have the patience to use it as often as I could (although I do it almost exclusively when I am on a tripod).

By the way, I love that 50mm. It's probably my second favorite lens to the 31mm.

01-24-2019, 12:25 PM   #9
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I agree the manual 50mm is great, but I still rely on the AF lenses for 90% of my shots. I've found evidence of BF with those as well, but mine are not quite as suited to focus checks as the 50mm as they go no lower than f4.

Since we have established that focus confirmation is achieved at the start of a range with the manual lens, is it the case that sensor centres the AF lenses in the middle of that range?
01-24-2019, 07:51 PM - 1 Like   #10
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I should have probably chimed in earlier but never the less...
Here's the skinny on focus adjustments straight from Pentax itself. I find it weird that there's no focus chart though.
How to optimize focusing accuracy with large-aperture lenses / Beautiful Photo-life | RICOH IMAGING
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