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Archive 2012 · Odd 7D/35L AF Behavior

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p.1 #1 · p.1 #1 · Odd 7D/35L AF Behavior

I've consistently had trouble getting my 35L to focus correctly on my 7D. I've sent them to Canon for calibration, only to have them tell me that they were both within specs. After noticing a pretty consistent pattern of front-focus, I did the whole MA process, and settled on a +10 setting. I did the MA in my house, in somewhat crummy light (my house is pretty dark inside, even during the day). This setting always seemed to work reasonably well indoors, but I literally couldn't hit the broad side of a barn outside in good light.

Anyways, fast forward to this week, when I took the plunge on the Reikan FoCal manual adjust software (which is pretty awesome, by the way). I set it up outside in open shade on a clear day and had it do the auto-adjust process on the 35L. What it found was that the ideal MA setting was a +1, and that image sharpness would fall off dramatically by +10. This was very much at odds with my experience with the lens, so I did a quick test...

And sure enough, in the dim innards of my house, a +1 setting just stinks. It results in front-focused pictures. +10 is much closer to the mark. But outside, or in a reasonably bright room, +1 is right on.

Has anyone else experienced a phenomenon like this, where the correct MA setting varies by the brightness of the scene?

Feb 05, 2012 at 12:31 AM
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p.1 #2 · p.1 #2 · Odd 7D/35L AF Behavior

Have you read this: http://www.fredmiranda.com/forum/topic/1007359

Feb 05, 2012 at 01:33 AM

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p.1 #3 · p.1 #3 · Odd 7D/35L AF Behavior

Thanks for the link. I tried it but it didn't seem to change anything.

Feb 05, 2012 at 02:58 AM
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p.1 #4 · p.1 #4 · Odd 7D/35L AF Behavior

Distance to the target will also have a big affect. The closer you are, the more front focusing you tend to see.

Software like FoCal is showing us just how poor the AF can be with some lenses, or body lens combinations, as well as how distance and amount of light affects AF.

FoCal recommends good light, 10 ev or more to get reliable and accurate AF settings, if you shoot in low light or low contrast, all bets are off.

It will be interesting to see how different bodies / lenses compare as the software is expanded to cover other models and brands.

Feb 05, 2012 at 03:18 AM

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