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03-18-2015, 01:24 PM   #1
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Pentax DA* 16-50 PDAF front focus when stopping down.

My DA* 16-50 (Converted to screwdrive AF) has a back focus issue, so I dialed +8 in AF micro adjustment on my K-50 to correct it at f 2.8.
I notice that if I perform PDAF with viewfinder from f 2.8 to f 5.0, the focus plane would stay the same. However if I stop down to f 5.6 and smaller, the focus plane moves to a closer distance and the intended focus point is out of focus.
I know that a DSLR camera performs focus with the aperture wide open. So in either settings at f 2.8 or f 5.6 the camera always perform focus at f 2.8, then it stops down to the desired aperture to take the picture.
My workaround is that I focus with view finder at f 2.8 or f 5.0 with the back AF button, then stop down to take picture (e.g. f 8.0), but it's kind of slow when taking pictures of people.

It has the same behavior on my K-3 too.

Is anybody having the same issue?

03-18-2015, 01:40 PM   #2

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I have the same issue with Tamron 28-75 I think; never understood why because the AF is done wide open.
03-18-2015, 04:44 PM   #3
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QuoteOriginally posted by Huy Quote
It has the same behavior on my K-3 too.
There has been a little discussion regarding a change in focus with the K-3 depending on the set aperture, but not with the K-50. The thinking on the K-3 was that Pentax was fudging a little and defaulting to the f/5.6 sensor functionality at narrower aperture settings. (Each of the f/2.8 center column sensors operates in tandem with a f/5.6 sensor at the same location. This is needed with slower maximum aperture lenses due to prism "black-out".) If the K-50 is also doing it, the answer must be something else.

I first noticed it with my K-3 when checking the PDAF against my split-image focus screen.

03-20-2015, 02:56 PM   #4
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Hi, have you tested the focus using manual focus on the camera body? Doe's the focus move in the same way? Changing the subject, you mentioned that the lens has been converted to screw drive, Iv'e not found any info on disabling the SDM that worked, perhaps you could enlighten me. Meanwhile, I will see if I can test the focus on my 16-50 to see if it also has this problem.

03-23-2015, 10:09 AM   #5
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QuoteOriginally posted by pentasonic49 Quote
Hi, have you tested the focus using manual focus on the camera body
This is how I check the lens with manual focus.
1) Mount the camera on a tripod and use a focus test chart as the target.
2) Set the camera to use the back AF to focus.
3) Switch the AF/MF switch on the lens to MF
4) Set aperture to f 5.0
5) While pressing the back AF, manual focus with viewfinder until the focus confirmation shows up.
This is what I found:
6) When change the aperture to f 5.6, the focus confirmation is gone.
7) Change the aperture back to f 5.0, the focus confirmation shows up again.

QuoteOriginally posted by pentasonic49 Quote
Iv'e not found any info on disabling the SDM that worked, perhaps you could enlighten me
I used an old K100D and followed the instruction from this thread:

Hope it helps. Thanks.

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