micro focus adjustment technique

[jodaco]

Bob,

Thanks for you balanced and informative review of the 50D. While just recently registering here, I have been lurking for years and your "real world" perspective has always been a welcome balance to the obsessed pixel peepers elsewhere. You led me to purchase both the Canon 10-22 and Tamron 28-75 both of which have given me endless enjoyment and beautiful shots.

One aspect of the 50D I would love to hear you elaborate on is the micro focus adjustment. What would you recommend as the best technique for making these adjustments? Also I have read about issues with zoom lenses since they may require different adjustments at different points in the range. Do you have any comment on this?

Thanks again for all your help over the years.

-Josh

[Bob Atkins]

What I'd do would be to set the camera up in front of a test target and take a series of shots at maximum aperture and settings of -20, -15, -10, -5, 0, 5, 10, 15 and 20, then see which one was sharpest. Then I'd do it again, perhaps with smaller steps around what looked like the best setting.

You'd have to be a little careful, since I'm sure there are small shot to shot variations in focus each time you shoot which may show up with really fast lenses with a very small DOF. You might have to average the results from several shots.

Then I'd do it all again at a different distance from the target, then I'd repeat the whole process again at a different focal length if the lens was a zoom.

Of course if I didn't suspect there was actually a focus issue with the lens, I wouldn't bother wasting my time on testing every lens I owned!

I didn't have the time to see if there was any effect of focus distance and/or zoom setting on the position of best focus, one reason being that I don't have any lenses that have any serious focus accuracy issues. It's certainly possible that there could be focal length related effects with a zoom. I guess at some point we'll start hearing from users who do have lenses with focus issues and how well the microfocus adjustment deals with it. I'll be keeping an eye open for such reports.

[KeithB]

Wild guess, but I would think that micro-focus is good for two kinds of people:
1.  Macro shooters - the depth of field is so narrow that small differences in focus can make a big difference in the shot.

2.  Pixel Peepers who care more about dinking around with the camera than getting good-enough shots.  (Now I sound like Ken Rockwell!)

[Bob Atkins]

I'm not so sure about #1 (Macro shooters). Generally for macro work, manual focus is used whenever possible so that the photographer can precisely place the narrow zone of focus exactly where they want it.

A lot of the focus complaints I see concern portraits ("the nose is in focus byt the eyes aren't..."). However I'm not sure that's always due to focus accuracy. It may be more a case of where the camera is choosing to focus at a different spot than the photographer wants.

There are certainly some cases though where lenses do have a problem, and this would be a quick and easy solution to the alternative of shipping things back and forth to Canon for permanent adjustment.