PENTAX DSLR AF-Fine Adjustment (AF-FA) - Tolerance Problem

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Kerusker
Kerusker Senior Member • Posts: 1,409
PENTAX DSLR AF-Fine Adjustment (AF-FA) - Tolerance Problem
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PENTAX DSLR AF-Fine Adjustment (AF-FA)

For the PENTAX procedure see:

How to optimize focusing accuracy with large-aperture lenses
https://www.ricoh-imaging.co.jp/english/explore/technic/001/
Especially
(2) Making minute focus adjustment in AF mode for the optimum performance of camera and lens
https://www.ricoh-imaging.co.jp/english/explore/technic/001/#part2

According to the above mentioned 'PENTAX_AF-FA_Procedure' (P_AF-FA_P) it's a rather simple method to achieve a 'fine' result. But that's not the case even with fully functional body and lens(es).

Let's assume the setup is according to the above P_AF/FA_P.
Then we may apply different focusing methods:
⦁ Manual focus in Live View (LV) using Contrast Detection (CD) = MF/CD with display magnification x16 (hit OK-button)
⦁ Auto focus in Live View (LV) using Contrast Detection (CD) = AF/CD with display magnification x16 (hit OK-button)
⦁ Auto focus (with View Finder (VF) using Phase Detection (PD) = AF/PD. You may take a shot and review with 100% using the monitor or do a half press of the shutter button and watch the AF-indicator (hexagon) in the VF.
⦁ Manual focus (with View Finder (VF) using Phase Detection (PD) = MF/PD. You may take a shot and review with 100% using the monitor or do a half press of the shutter button and watch the AF-indicator (hexagon) in the VF.

The last i.e. MF/CD doesn't make much sense but will be very interesting with the following addition.
⦁ Manual focus (with View Finder (VF) using Phase Detection (PD) = MF/PD.
⦁ Set the AF-FA value for the lens,
⦁ do a half press of the shutter button and
⦁ watch the AF-indicator (hexagon) in the VF - does it show up or not?
MF/PD references this procedure in the text below.

Now lets find out more about the AF/PD behavior of a body/lens combination - let's call it AF/PD-Tolerance AF/PD-Tol.

Use MF/CD if the lens doesn't focus accurately with AF/CD - you can detect this by controlling the AF/CD and a follow up MF/CD. For simplicity I will use MF/CD in this article.

AF/PD-Tol measurement procedure:

Set up the combo (body/lens) for AF-FA as described by PENTAX (above; camera on tripod).
1. setup distance and align target to combo
2. turn off SR, use ISO 100
3. 'calibrate' with MF/CD
4. set AF-FA value for the lens (one) to Vn (menu C4/25,...,OK)
5. press the shutter button half way and observe the AF-hexagon in VF
6. note Vn and hexagon on resp. off
7. repeat steps 4. to 6. for Vn = -10,-9,-8,....,0,+1,+2,...,+10

Then usually you will get
⦁ -10,-9-,-8,-7,-6,-5,-4,-3,-2,-1,0,+1,+2,+3,+4,+5,+6,+7,+8,+9.+10
where the bold numbers are the Vn with hexagon on (i.e. AF/PD in focus).
Result: The combo has AF/PD-Tol (-5,...,+3).

Usually a faster procedure is to find the -6,-5 and +3,+4 transitions.

Problem of the PENTAX DSLR AF-Fine Adjustment (AF-FA)
If there is a big AF/PD-Tol for a lens, then the PENTAX procedure is
1. starting with AF/PD
2. switch to MF and LV
3. manually focus to highest contrast on screen (MF/CD) - note direction towards 0 (closer, BF) or oo (further away, FF)
4. adjust AF-FA value accordingly (BF or FF).

Problem: AF/PD in step 1 may focus anywhere in the range of the AF/PD-Tol (-5,...,+3)!
The result of the PENTAX procedure may not be as good as expected since the AF/PD Tol isn't even known usually.
If the lens has a very wide AF/PD-Tol, then you should not wonder about shots being out of focus very often.
For fast lenses and for zooms the situation is even worse.
For a zoom you may have a AF/PD-Tol (-10,..,+2) at the short end and (-7,...,+9) at the long end (UW-Zoom F:4).

In this case other factors have to be considered like:
⦁ tolerate FF at the wide end (but yield infinity focus)
⦁ use a special AF-FA value for close focus resp. far focus
⦁ is the AF/PD-Tol special for the lens or common?

One of the latest AF discussions was:
Focus Oscillation in K3 iii? https://www.dpreview.com/forums/post/66391046

The K3 III has the latest PD-System (13) - what's about AF/PD-Tol on this body?

Any critic and help or idea is highly welcome.
If of interest, I could present the AF/PD-Tol of several lenses on my K-1 (primes and zooms).

Thanks for reading.

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][.Kerusker
we don't see that we don't see (eye's blind spot)

 Kerusker's gear list:Kerusker's gear list
Sony RX10 IV Pentax K-1 Pentax smc DA 15mm F4 ED AL Limited Pentax K-5 IIs Pentax smc FA 50mm F1.4
Fujifilm FinePix IS-1 Pentax K-1
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