Autofocus adjustment testing target

Started Mar 4, 2013 | Discussions thread
Julian Vrieslander
Contributing MemberPosts: 919
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Re: Autofocus adjustment testing target
In reply to coronawithlime, Mar 4, 2013

coronawithlime wrote:

I decided to do some lens AF adjustment retesting today. Bright sun. No Wind. Tripod. Mirror up. AF-S. Wide open and 1/6000. Regular stuff. Used a Nikon 60mm f2.8 macro and a Nikon 28-70 f2.8 @ 70mm, both on a D800. On a whim I decided to use the FoCal target outside. Printed three of them and lined them up on a wall. Noticed that my lenses would not focus on the target. Live View focused ok, but using the viewfinder was a no-go.

Very odd.  I have never had problems with the FoCal target - when I use it with my D800E, I get a rapid focus lock.  I can only guess at the possible causes for what you are seeing, most of which are probably obvious for you.  Camera to target distance inappropriate for the target size?  Camara optical axis perpendicular to target?  Reflections or flare?  Are you using matte paper?  AF sensor positioned over the right area of the target?  Don't trust the bracket markings in the view finder - you may need to do a bit of testing to see the real position and extent of the sensors.

And, of course, double-check all camera settings.  I can't even guess at the number of times I have made silly mistakes in setting up shots and tests.

On a whim, I downloaded and printed out one of the targets that are said to be similar to what Nikon shows in their service manual and uses in their shop. Looked a lot like the one Thom Hogan has in his article on FA testing.

Is this the type of target with a non-periodic black and white bar pattern, similar to the one I made and uploaded here?

http://julian.marmotandweasel.com/photos/AF_target.pdf

This particular target was designed to be printed on an 8.5x11" page and placed at a distance 50x focal length of the lens you are using.  For testing the outer (line shaped) AF sensors, the bars should be oriented horizontally.  For the center (cross shaped) sensors, the orientation can be vertical or horizontal.

I get good AF lock with this target, but I can't say that it works noticeably better than the FoCal target.

I'm interpreting this to mean that my previous testing was faulty due to inconsistent focusing on a more difficult target. Which is kinda distressing, because I had convinced myself that I did a pretty good job the first time around, but I believe I was mistaken.

It might be worthwhile to do at least one run with the bar target to see if you get the same AF Fine Tune results as you got with the FoCal target.  Perhaps you won't need to run all your tests again. Let us know what you find.

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