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08-14-2018, 07:39 AM   #1
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Pentax D 28-105 f3.5-5.6 WR back focus issues?

Not sure if it's the body (K1 mark II) or the lens as I've only had it a few days. Going to do a bit more testing but what I've discovered initially is that the 28-105 WR back focuses wide open at 28mm. I initially thought it was my AF points, so I tried center, and spot. Most image subjects were still slightly OOF with the background being sharper. At the long end, wide open at f5.6, I didn't really notice too much of an issue. Nevertheless, I adjusted the K1-II's AF micro adjustment to +8 this morning and tested 28mm and focus seems to be spot on. Not sure how that adjustment will affect anything in between up to 105mm hence the additional testing to be done.

Will also have to verify my other lenses such as the Pentax 50 1.4, and Tamron 70-200 and Tamron 90mm. If memory serves though, the test shots I made with those lenses did not show any AF errors. If on the other hand, all lenses require significant + adjustment, then chances are the body will need to go back to Ricoh for AF module calibration. Does anyone have experience sending anything for service to Precision Camera (apparently they're Ricoh's authorized service center in the USA)?

FWIW, I'm new to the K1. Was a Canon digital user for almost 15 years with my last being a 5D.

08-14-2018, 08:11 AM   #2
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I dont understand how you can have correct af at 105mm and back focus at 28mm.
08-14-2018, 08:45 AM   #3
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That's what I thought. I initially thought my lens was just soft wide open but then I started pixel peeping at 100% and noticed that the background in most shots was sharper. Moving to manual focus wide open at 28mm showed a significant improvement in performance.

I dunno. Wide open at the tele end is still 5.6 so perhaps DOF was just wide enough to mask any minor micro focus issues.
08-14-2018, 10:15 AM   #4
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QuoteOriginally posted by amstel78 Quote
Wide open at the tele end is still 5.6 so perhaps DOF was just wide enough to mask any minor micro focus issues.
What can happens if the camera-subject distance stays the same for 28mm and 105mm, the area where the camera focuses at 28mm is not the same as the area at 105mm, making the user believe of a focus error at 28mm.
Some people doing portraits complain that their Pentax camera cannot focus on the eye, when the subject distance is such that the AF point cover more than the eye. When using center point, the AF point area should remain smaller that the area we want to focus on, if not the focus will be taken on an average plane shifted from the wanted plane of focus.

08-14-2018, 10:27 AM   #5
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I have the 28-105 and the tamron 70-200 both needed AF adjustments. Once you get them dialed in they are great.
08-14-2018, 10:31 AM   #6
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QuoteOriginally posted by Ascencio Quote
I have the 28-105 and the tamron 70-200 both needed AF adjustments. Once you get them dialed in they are great.
Just out of curiosity, but can you please tell me how much adjustments were required for both? I don't mind adjusting, but +8 in my case is uncomfortably close to max travel.



---------- Post added 08-14-18 at 05:34 PM ----------

QuoteOriginally posted by biz-engineer Quote
What can happens if the camera-subject distance stays the same for 28mm and 105mm, the area where the camera focuses at 28mm is not the same as the area at 105mm, making the user believe of a focus error at 28mm.
Some people doing portraits complain that their Pentax camera cannot focus on the eye, when the subject distance is such that the AF point cover more than the eye. When using center point, the AF point area should remain smaller that the area we want to focus on, if not the focus will be taken on an average plane shifted from the wanted plane of focus.
Noted. I did try with subjects such as a round potted cactus where the overall subject was larger than the focus point. Same issue at 28mm whereas it appeared in focus at 105. Subject distance was approximately 50" away. When I get home, I'll post the jpegs with exif data.
08-14-2018, 11:10 AM   #7
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QuoteOriginally posted by biz-engineer Quote
I dont understand how you can have correct af at 105mm and back focus at 28mm.
The "how" and "why" have to do with how PDAF works and how focus is done on modern zooms. Without going into the details,* it may simply be enough to point out that adjustment for Sigma zooms using the USB Dock involves adjustment at multiple focal lengths and focus distances to account for this possibility.


Steve

* The summary explanation is that subtle misalignment of non-stationary optical elements used for both zoom and internal focus may result in an asymmetrical (ambiguously skewed) signal to the AF sensor at various points of their travel.

Last edited by stevebrot; 08-14-2018 at 11:17 AM.
08-14-2018, 11:15 AM   #8
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QuoteOriginally posted by amstel78 Quote
Noted. I did try with subjects such as a round potted cactus where the overall subject was larger than the focus point.
I would suggest a flat target at right angles to the lens axis having high contrast black/white boundary(ies) at center using center AF point. The intent is to provide a non-ambiguous point of focus so for sensitive evaluation of the need for adjustment.

BTW... Welcome to the Pentax Forums!


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08-14-2018, 11:21 AM   #9
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QuoteOriginally posted by stevebrot Quote
The "how" and "why" have to do with how PDAF works and how focus is done on modern zooms. Without going into the details,* it may simply be enough to point out that adjustment for Sigma zooms using the USB Dock involves adjustment at multiple focal lengths and focus distances to account for this possibility.
I would have thought that focus error comes from a difference between the AF optical path and the direct path leading to a focus error regardless of the focal length.
08-14-2018, 11:47 AM   #10
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QuoteOriginally posted by biz-engineer Quote
I would have thought that focus error comes from a difference between the AF optical path and the direct path leading to a focus error regardless of the focal length.
The subject is complex, but it enough to say that the issue with PDAF front/back focus is not related to physical distances on the optical path per se. I would provide a link, but have not found a reference that provides a direct and concise explanation. The best I might do is to make the comparison to a classic split-image rangefinder* where instead of a clean split of a clear line, one sees a fuzzy line darker on one side and where the fuzziness and asymmetry is due to optical aberration and/or alignment defect.


Steve

* PDAF is essentially the same, though on a very small scale and using a linear array of photodetectors rather than the eye to evaluate.
08-14-2018, 12:40 PM   #11
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QuoteOriginally posted by amstel78 Quote
I don't mind adjusting, but +8 in my case is uncomfortably close to max travel.
+8 is within the maximum +10 tolerance. I have a number of lenses that require +10. They have not changed over the nearly two years that I have had the K1 so I doubt they will do so now. you can always send the camera in for a warranty repair if you need more than +10 to fix a lens AF
08-14-2018, 01:04 PM   #12
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It's also possible, is it not, that the body's AF module is fine but the lens itself is out of optical alignment? I don't recall seeing this issue with the other lenses I own but will test again.
08-14-2018, 04:21 PM   #13
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OK, so I've managed to do some quick testing using a flat surface; basically my fridge with a test target stuck to it. All were shot at widest aperture settings. What I've discovered is the following:

1. Pentax 28-105 WR: +10
2. Pentax FA 50 1.4: +10
3. Tamron 90mm macro: +10
4. Tamron 70-200: +10

As all lenses need at minimum +10 back focus adjustment to achieve critical focus at 100%, is that indicative that there's something amiss with my camera's AF module?

Also, I'm within my 30 day return or exchange window at Adorama. Would you guys recommend an exchange or repair route? If sent for repair, is this something that's adjusted via software or would they have to open the camera?

Last edited by amstel78; 08-14-2018 at 05:47 PM.
08-15-2018, 10:08 AM   #14
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Since you can't crank it up to eleven, I would get an exchange.
08-15-2018, 04:30 PM   #15
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Took a walk over to Adorama after work. Was able to exchange the body. What a difference. 0 setting on AF micro adjust and the 28-105 is tack sharp wide open. Definitely something screwy with my original body. Anyway, problem solved! Thanks to all for your feedback.
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