Mark III AF micro adjustment question

When making micro adjustments on a zoom lens, should you do your tests at a particular focal length or does it not matter? For example, when tuning a 100-400L, is there a preferred focal you should use?

Well, at what mm do you use it the most? I'll bet 400 for that zoom. Then, tune it for 400.

I believe it does matter. And agree with gasrocks, fine-tune it at the FL you use most often.

Wait... How are you doing micro AF adjustments? Is that a 1D only thing?

i've found it doesn't work well for zooms. It's been an exercise in frustration for me. adjusting for one focal length can throw it off for the rest of the range. And then even varying the subject distance also messes it up. I think it's kinda useless for zooms. I think it would only work if we were able to define a table of offset values depending on subject distance and focal length.

picturecrazy wrote in post #5244735
i've found it doesn't work well for zooms. It's been an exercise in frustration for me. adjusting for one focal length can throw it off for the rest of the range. And then even varying the subject distance also messes it up. I think it's kinda useless for zooms. I think it would only work if we were able to define a table of offset values depending on subject distance and focal length.

agreed.

not sure how the offest values are calculated or if they're constant. But when I calibrated the 70-200 at 200mm at a distance of about 15 ft, it was off when the distance was greater (and vice versa). This makes me think it's "constant"

will have to play with it more

ben_r_ wrote in post #5244653
Wait... How are you doing micro AF adjustments? Is that a 1D only thing?

In short, yes.

gasrocks wrote in post #5244446
Well, at what mm do you use it the most? I'll bet 400 for that zoom. Then, tune it for 400.

JC4 wrote in post #5244596
I believe it does matter. And agree with gasrocks, fine-tune it at the FL you use most often.

picturecrazy wrote in post #5244735
i've found it doesn't work well for zooms. It's been an exercise in frustration for me. adjusting for one focal length can throw it off for the rest of the range. And then even varying the subject distance also messes it up. I think it's kinda useless for zooms. I think it would only work if we were able to define a table of offset values depending on subject distance and focal length.

Thanks guys. You are all correct. Not the answer I was hoping for but you are all correct. I just had a chance to adjust the 24-70L. At 70mm, +6 was right on and 24mm was backfocused. 24mm liked an adjustment of -3 but of course 70mm was then all whacked out.

I want a firmware update that allows adjustments by focal length (24mm,50mm,70mm) and the camera takes an average for the tweener focal lengths.

Or maybe I should switch to all primes. Hmmm...