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12-10-2019, 03:04 PM   #1
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Another AF adjustment question!

I'm having some trouble understanding what's causing AF errors with my DFA 150-450 lens.
I've posted about this previously, but my question now is a bit different, hence a new post.
When my lens was new I thought the resulting images with my K-3 II were acceptably sharp.
After some time I decided to check front/back focussing and found it was back focussing and required about +5 AF fine adjustment.
Some time later again it was looking like there was a problem with sharpness, and more exhaustive testing resulted in a +7 setting. If I recall correctly a similar adjustment (about +7) was also required for my K-1.
Over the past year I have not been satisfied with the focussing accuracy and have been through the AF fine adjustment exercise a number of times and find that I can't correct the back focus on the K-3 II with a +10 setting, and come a tad closer to being correct with +10 on the K-1.
As both camera bodies exhibit very similar changing AF behaviour over time with this lens, it does suggest to me that there is something changing in the lens which is causing this.
What I am having trouble understanding is how this could be the case when live view AF (contrast detect) with both cameras will produce an image that doesn't exhibit front or back focussing.
I intend to take the lens and camera bodies to the local distributor/service centre soon to seek their advice, but in the meantime I would really like to better understand what seems to me to be a confusing situation.
Both cameras exhibit correct AF with live view, but both cameras exhibit incorrect PDAF, that has been varying over time.
Can anyone explain why this could be so?

Cheers,
Terry

12-10-2019, 03:30 PM   #2
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It could be that both your camera's mirrors need adjustment. Temperature, vibration e.g. use can misalign them.

12-10-2019, 03:34 PM   #3
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QuoteOriginally posted by tduell Quote
I'm having some trouble understanding what's causing AF errors with my DFA 150-450 lens.
I've posted about this previously, but my question now is a bit different, hence a new post.
When my lens was new I thought the resulting images with my K-3 II were acceptably sharp.
After some time I decided to check front/back focussing and found it was back focussing and required about +5 AF fine adjustment.
Some time later again it was looking like there was a problem with sharpness, and more exhaustive testing resulted in a +7 setting. If I recall correctly a similar adjustment (about +7) was also required for my K-1.
Over the past year I have not been satisfied with the focussing accuracy and have been through the AF fine adjustment exercise a number of times and find that I can't correct the back focus on the K-3 II with a +10 setting, and come a tad closer to being correct with +10 on the K-1.
As both camera bodies exhibit very similar changing AF behaviour over time with this lens, it does suggest to me that there is something changing in the lens which is causing this.
What I am having trouble understanding is how this could be the case when live view AF (contrast detect) with both cameras will produce an image that doesn't exhibit front or back focussing.
I intend to take the lens and camera bodies to the local distributor/service centre soon to seek their advice, but in the meantime I would really like to better understand what seems to me to be a confusing situation.
Both cameras exhibit correct AF with live view, but both cameras exhibit incorrect PDAF, that has been varying over time.
Can anyone explain why this could be so?

Cheers,
Terry
How dirty is the PDAF sensor? (under the mirror).
Have you ever cleaned it?
Try a rocket blower but don't touch anything with the tip of the blower.
PDAF has it's own focus sensor, CDAF is on the sensor which is why there's a difference.
12-10-2019, 03:41 PM   #4
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QuoteOriginally posted by tduell Quote
I'm having some trouble understanding what's causing AF errors with my DFA 150-450 lens
First and most importantly, to assess and front or back focus you need a rigorous testing regime. So...camera on tripod/natural light/proper test chart/ remote or timer trigger. Equally important is you need to do 10 tests each time making the camera work to focus from close or infinity setting. PDAF is just not accurate enough for you to rely on a single AF test. If you get 8/10 identical and can adjust for that you have done well.

QuoteOriginally posted by tduell Quote
What I am having trouble understanding is how this could be the case when live view AF (contrast detect) with both cameras will produce an image that doesn't exhibit front or back focussing.
Live view focusses on the sensor itself using CDAF. It does not use the PDAF system at all. You cannot , and have have no need to adjust the AF when using LV.

If after a proper testing regime you find that PDAF requires AF/FA in excess of the +10/-10 allowed in the menu , you can send camera and lens back to Pentax for free adjustment back to zero.

12-10-2019, 05:20 PM   #5
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If your testing and calibration regime is beyond reproach and consistent, and you've noted equivalent changes in AF fine adjustment for the same lens with more than one body, then theoretically it's possible one or more of the optical elements in the lens has shifted, which might be a result of various scenarios. Definitely something to get checked by your service centre - and be sure to give them the same information you've provided here.
12-11-2019, 01:44 AM   #6
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QuoteOriginally posted by Kobie Quote
How dirty is the PDAF sensor? (under the mirror).
Have you ever cleaned it?
Try a rocket blower but don't touch anything with the tip of the blower.
PDAF has it's own focus sensor, CDAF is on the sensor which is why there's a difference.
Yes, they have been cleaned, many times.
I have a clear understanding of the differences between CDAF and PDAF, which was really the whole point of my original message...if it's the lens that's at fault how can CDAF not be back focussing as well?

Cheers,
Terry

---------- Post added 12-11-19 at 01:52 AM ----------

QuoteOriginally posted by pschlute Quote
First and most importantly, to assess and front or back focus you need a rigorous testing regime. So...camera on tripod/natural light/proper test chart/ remote or timer trigger. Equally important is you need to do 10 tests each time making the camera work to focus from close or infinity setting. PDAF is just not accurate enough for you to rely on a single AF test. If you get 8/10 identical and can adjust for that you have done well.
I have a very rigorous testing regime, I do understand the need for that, but of course one can never rule out a testing error, which is why I have repeated the tests over an over again, 10 is a small number!

QuoteQuote:
Live view focusses on the sensor itself using CDAF. It does not use the PDAF system at all. You cannot , and have have no need to adjust the AF when using LV.

If after a proper testing regime you find that PDAF requires AF/FA in excess of the +10/-10 allowed in the menu , you can send camera and lens back to Pentax for free adjustment back to zero.
Everyone seems to have overlooked the fact that both camera bodies have progressed from moderate back focus to a state which is unable to be corrected, and THAT is what has led me to think it must be the lens at fault...hence if that is so, how can the CDAF be OK and PDAF not?
The corollary of course is that both camera bodies have progressed in back focussing at a similar rate, which may be the case, but a low probability in my view.

Cheers,
Terry

---------- Post added 12-11-19 at 01:56 AM ----------

QuoteOriginally posted by BigMackCam Quote
If your testing and calibration regime is beyond reproach and consistent, and you've noted equivalent changes in AF fine adjustment for the same lens with more than one body, then theoretically it's possible one or more of the optical elements in the lens has shifted, which might be a result of various scenarios. Definitely something to get checked by your service centre - and be sure to give them the same information you've provided here.
OK, that's an interesting observation.
If that is the case, would CDAF still be able to achieve sharp focus while PDAF can't?

Cheers,
Terry
12-11-2019, 03:44 AM   #7
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QuoteOriginally posted by tduell Quote
OK, that's an interesting observation.
If that is the case, would CDAF still be able to achieve sharp focus while PDAF can't?
Yes, it would. AF fine adjustment is only relevant to PDAF, as you know, and it has a quite limited range of +/- adjustment. CDAF isn't constrained by those adjustment limits - it just looks for the best contrast it can achieve.
12-11-2019, 02:43 PM   #8
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QuoteOriginally posted by BigMackCam Quote
Yes, it would. AF fine adjustment is only relevant to PDAF, as you know, and it has a quite limited range of +/- adjustment. CDAF isn't constrained by those adjustment limits - it just looks for the best contrast it can achieve.
OK, that helps, thanks.
It doesn't nail down any conclusion as to whether it's camera body or lens that is the real culprit here, but a bit of additional historical info might.
I also shoot with Sigma 85 f/1.4 and 24-70 f/2.8 on both bodies, although nowhere near as much as with the 150-450.
It occurred to me that I had completely overlooked any history of AF fine adjustment with those lenses.
I've been back through all my notes, which are a bit sparse on this, and my recollections are that I haven't had to revisit AF fine adjustment for these lenses for some time, as the resulting images have been acceptably sharp. That suggests to me that the camera bodies are probably not the culprits here.
My intention is to take back to the 150-450 to the distributor after all the xmas shutdowns, and lay out all the history for them.
Not sure what I'll do without the 150-450 if has to be sent away for repair/adjustment. The last time that happened, to replace the zoom lock button, it took 8 weeks

Cheers,
Terry

12-11-2019, 03:42 PM   #9
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QuoteOriginally posted by tduell Quote
OK, that helps, thanks.
It doesn't nail down any conclusion as to whether it's camera body or lens that is the real culprit here, but a bit of additional historical info might.
I also shoot with Sigma 85 f/1.4 and 24-70 f/2.8 on both bodies, although nowhere near as much as with the 150-450.
It occurred to me that I had completely overlooked any history of AF fine adjustment with those lenses.
I've been back through all my notes, which are a bit sparse on this, and my recollections are that I haven't had to revisit AF fine adjustment for these lenses for some time, as the resulting images have been acceptably sharp. That suggests to me that the camera bodies are probably not the culprits here.
My intention is to take back to the 150-450 to the distributor after all the xmas shutdowns, and lay out all the history for them.
Not sure what I'll do without the 150-450 if has to be sent away for repair/adjustment. The last time that happened, to replace the zoom lock button, it took 8 weeks
Given that your other lenses don't seem to need further AF fine adjustment, and assuming the direction and amount of new AF fine adjustment for the 150-450 is relatively consistent on more than one body, it would seem more likely to be the lens. Best to get it checked out in the New Year, as you suggest.

Good luck, and let us know how you get on?
12-28-2019, 10:05 AM   #10
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Check your alignment strategy. Low light, artificial light, correct settings make a difference. Different zoom settings may need different adjustments.
In my K1 the screw of the quick release plate pushed the af sensor out of place, resulting in serious shift w/wo plate in place... typically my lenses show the same spread of offset on all cameras. Not the same adjustment as all positions are relative.
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