My current method to determine AF-Fine Adjust values

Started Mar 26, 2013 | Discussions thread
ForumParentFirstPreviousNextNext unread
Flat view
Kerusker
Contributing MemberPosts: 746Gear list
Like?
My current method to determine AF-Fine Adjust values
Mar 26, 2013

My approach is a more practical than scientific one.
Starting point is Live View (LV using contrast detection on sensor = CD-AF) beeing more accurate than Phase Detect AF (PD-AF or AF for short). Using manual focus in LV (LVm) with up to 10 times magnification may be even more accurate than auto focus in LV (LVa). For simplicity, I use LVa for this description.

procedure A:
Auto focus in LV (LVa), turn off LV and observe the lens barrel when auto focussing (PD-AF).
The barrel may not move, turn towards 0 or turn towards infinity (v, ->0, ->oo)
v:      the AF-FA value seems to be ok (at least close to best)
->0:   PD-AF does FF i.e. move the AF-FA value to negative direction (f.ex. +5 ->+1 or +2->-4)
->oo: PD-AF does BF i.e. move AF-FA value to positive direction (f.ex. -2->+3)
Repeat procedure A until the lens barrel does't move anymore.

In procedure A you will find that there may be several AF-FA values without barrel movement.
Therefore we will try to find the AF-FA limits where the barrel starts moving towards 0 (V->0) and towards infinity (V->oo). Usually we'll get a range V->oo,.....,V->0 in the AF-FA scale from -10 to +10, but some times V->oo or V->0 may be out of this range (and can't be determined).

procedure B:
B1) Auto focus in LV, turn off LV and observe the lens barrel when auto focusing (PD-AF).
R1) The barrel may not move, turn towards 0 or turn towards infinity (v, ->0, ->oo)
Repeat B1 with a different AF-FA value until the barrel does not move.

B2) Lower the AF-FA value and repeat B1 until the barrel starts to move towards infinity (V->oo)
B3) Increase the AF-FA value and repeat B1 until the barrel starts to move towards 0 (V->0)

B4) Pick an AF-FA value in the middle between V->oo and V->0 (if you prefer slight FF, take the one to the left of the middle value).

For a zoom lens you should do procedure B at least for the wide end and the long end of the zoom and then pick an AF-FA value which is approximately in the middle of both V->oo,....,V->0 ranges. In bad situations, there may not be a common AF-FA value at all in both ranges.

You can find a video for a similar procedure here:
'Dot-Tune' http://www.youtube.com/watch?v=7zE50jCUPhM

I've tried 'Dot-Tune' on my K-5 but since focus is set only once to determine the AF-FA range, it seems to depend a lot on the chosen focus setting. Also all mechanical variances are excluded since AF is turned off for the lens and only the AF indicator is used.

In procedure B above you should execute B1) several times for each AF-FA value to get good statistics with all AF elements included. (LVa does refocus each time!).

Procedure A and B can be executed without taking any picture (but you can of course).
Test setup is very similar to the one in the video.

I use the following settings on my K-5:

2-sec timer (turns shake reduction off)
Enable AF on AF Button and disable AF on Shutter Button Half Press (Exposure Menu 5 'Button Customization')
Menu C1/4 'Meter Operating Time' = 3sec (minimum)
AF Center Point only
Lens aperture wide open (this is usually the case for auto focusing, but aperture may be closed for LV if target is very bright)
distance from cam to target should be about 50 times the focal length of the lens (or at least 25 times)

If the AF-FA ranges are often outside the -10,...,+10 scale (at one end), you should use Debug to control (and change) the AF µ-Correction value.

If you've read all this, you sure have an interest in AF fine tune.

Thanks

-- hide signature --

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

 Kerusker's gear list:Kerusker's gear list
Pentax K-5 II Pentax K-5 Pentax smc DA 15mm F4 ED AL Limited Pentax smc DA* 16-50mm F2.8 ED AL (IF) SDM Pentax smc DA 55-300mm F4.0-5.8 ED
Pentax K-5
If you believe there are incorrect tags, please send us this post using our feedback form.
ForumParentFirstPreviousNextNext unread
Flat view
Post (hide subjects)Posted by
ForumParentFirstPreviousNextNext unread
Keyboard shortcuts:
FForum PPrevious NNext WNext unread UUpvote SSubscribe RReply QQuote BBookmark post MMy threads
Color scheme? Blue / Yellow