Auto Focus Fine Tuning

Art Scott · April 23, 2012

Could not find any discussions on this but I am sure they are here somewhere...anyway... ... ...

1 - Does it matter what one uses for a subject for doing this?

2 - Does it matter the distance from subject? should one do this at several feet or is min working distance fine?

3 - what aperture should one be using for this?

I had totally forgotten that this could be done, until I was looking for something in my David Busch book and ran across it and just thought it might be worth while looking into especially with my 50-500 ... ...never worried about things like this with film cameras....:D

Thanx in advance.

Matthew Saville · April 23, 2012

* Shoot wide open on your aperture, and turn your in-camera sharpening all the way up if you plan on doing this using the back of the camera for quick focus / sharpness confirmation.

* Get on a tripod, preferably also with a cable release. Use Single Shot autofocus, unless you're performing a specific test.

* Shoot an immobile subject, a flat surface that fills the entire focus point with no chance of missing, and plenty of texture. It might be a good idea to test both in bright sunlight and in very dim indoor light.

* Unless you're having a specific close / infinity issue with your lens, perform the test at a normal distance of 10-30 feet depending on the focal length of the lens.

* Compose the shot so that you can see a clear and gradual transition from foreground, to subject, to background. Indoors I just put a cardboard box on the carpet with a newspaper taped to the side, or outdoors a tree trunk with the grass composed in the shot works fine. Try not to have too steep of an angle between the grass / carpet and your camera angle, obviously this will decrease your accuracy if your plane of focus is at an angle. Neither does it need to be laser-perfect, as some home calibration systems will have you believe.

* Speaking of which, ...send me some $$ because I just saved you from buying one of these closet-space-waster gizmos: http://michaeltapesdesign.com/lensalign.html

Good luck!
=Matt=

Pupator · April 23, 2012

Or buy this, do it automatically, and be done.

http://dgrin.com/showthread.php?t=218413

Dan7312 · April 23, 2012

I think that Ziggy has mentioned that 3 times the min working distance.

Art Scott wrote: »

2 - Does it matter the distance from subject? should one do this at several feet or is min working distance fine?

Dan7312 · April 23, 2012

This one http://regex.info/blog/photo-tech/focus-chart is free. You can decide for yourself if it is worth closet space.:D

[QUOTE=Matthew Saville;1768411

* Speaking of which, ...send me some $$ because I just saved you from buying one of these closet-space-waster gizmos: http://michaeltapesdesign.com/lensalign.html

Good luck!
=Matt=[/QUOTE]

zoomer · April 23, 2012

I set three small objects on the carpet, close medium far. Then I lay on the floor, shoot at wide open, one shot at each distance.....this makes it really easy to see the line of focus and then I dial it in to where the three objects are in the focus plane.
Not very scientific...but that is how I did it.
Every lens I have needed a bit of tuning.

ziggy53 · April 23, 2012

1) Basic AF accuracy is best accomplished using a flat target, i.e. a brick wall or other large textured surface. If a camera/lens cannot accomplish this task, it has no chance of passing any subsequent AF tests including real world use. Make sure that the target is at the typical distance for your normal use of the lens, meaning that a long focal length lens that you would normally use for wildlife at a distance should be tested at that same distance for AF accuracy. Always test a lens at least twice Minimum-Focus-Distance (MFD). Lenses rarely do their best at MFD (the exception being true "macro" lenses.)

If a lens shows consistent front or back focusing with the wall test, then use micro-focus adjust technique until you get consistent accuracy. If the particular camera body does not have a micro-focus option, it should be serviced, or the lens should be serviced. If you have consistent results with all lenses then send in the camera. If the problem only exists with a particular lens, it's the lens that needs service.

If the lens passes the wall test, proceed to the next test. If the lens fails the wall test and you cannot gain accurate AF with micro-adjust, send the lens back to where it came from or explore camera issues.

2) A fence line or similar, shot at an angle to the subject. Put a singular strong-contrast target on the top of the middle post and use a single focus point in the camera to focus against the target. This shows focus accuracy and/or how easily distracting for/aft objects influence AF accuracy (compared to the above wall shot). It also shows bokeh tendencies at different aperture settings.

3) A focus target/chart like in the following:

http://regex.info/blog/photo-tech/focus-chart

I do recommend testing these charts at twice minimum focus distance or greater. Most lenses, especially most zoom lenses, do not do their best at MFD. True macro lenses are the major exception.

4) A US stop sign, or similar very high contrast signage, both centered and off center and to the side and/or corner. This can show chromatic errors and off-axis errors.

5) A spectral highlight at night. (Point source ideally, but a street light at some distance will do.) Try to avoid "blooming" of the source but allow some white clipping. This shows internal reflections and dispersion problems. Again, test both in the center and with the subject at the sides or corners.

6) Now just shoot "typical" subjects for your use of the lens(es). It's important that the lens perform correctly for your intended uses.

Art Scott · April 23, 2012

I have been looking at the FOCAL ... ...

Thank you everyone ... now time to play and get my already acceptable lenses better....

Auto Focus Fine Tuning

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