Let's put this to rest: 50 / 1.2L backfocus issue

Started Oct 7, 2007 | Discussions thread
ForumParentFirstPrevious
Flat view
joe mama Forum Pro • Posts: 12,623
Let's put this to rest: 50 / 1.2L backfocus issue

There are many misunderstandings about the backfocus issue of the 50 / 1.2L. Let me describe the situation: as an inherent part of its design (no floating rear element), the lens will backfocus when stopping down, focusing closely, and using the center AF point.

THIS ISSUE DOES NOT HAPPEN WHEN USING AN OFF-CENTER AF POINT!

Some people may have AF issues besides what is described above -- those 50 / 1.2Ls, and/or the cameras they are used on, are defective. But as for the rest of them, it is a design limitation that cannot be fixed with calibration.

Allow me to illustrate. The following pics are crops that were taken at the lens's minimum AF distance on a 5D -- maximum backfocus is about 1cm -- 1/2 an inch. In every case, AF was used (not manually focused), and the focal point was "21g":

f / 1.2 -- center AF -- dead on

http://www.pbase.com/joemama/image/81439719

f / 2 -- center AF -- backfocused

http://www.pbase.com/joemama/image/81439720

f / 2.8 -- center AF -- backfocused

http://www.pbase.com/joemama/image/81439722

f / 2.8 -- upper right AF point recomposed -- dead on

http://www.pbase.com/joemama/image/81439721

Here's a "real life" pic taken using an off-center AF point:

5D + 50 / 1.2L @ f / 2.8, 1/125, ISO 1600 -- resized

http://www.pbase.com/joemama/image/81299802

5D + 50 / 1.2L @ f / 2.8, 1/125, ISO 1600 -- 100% crop

http://www.pbase.com/joemama/image/81299803

The reason the backfocusing occurs is explained here:

http://www.vanwalree.com/optics/spherical.html

As for using manual focus: this option will not work unless you purposely front focus to compensate for the focus shift when the lens stops down and the camera takes the pic (as cameras always focus wide open), or you hold down the DOF preview button when manually focusing.

What I don't understand about all those who complain about the backfocusing issue is why center AF is so important for this type of shooting. I understand that it would be great to be able to use center AF for close focusing stopped down shooting, but I don't understand why having to use an off-center AF point for this singular situation is such a huge issue for so many.

Don't get me wrong -- I'm not passing judgement. The slow AF of the 85 / 1.2L has been a deal breaker for me, and many people, no doubt, cannot understand why fast AF on such a lens is important to me.

Also, for anyone having AF issues other than described in this post , then that is an entirely different issue, and those lenses (or lens/body combinations) are definitely in need of calibration. However, the situation, as I described and demonstrated above, is a "design limitation", not a "defect", as it cannot be corrected except with a new lens design and/or a more intelligent camera AF algorithm that recognizes the lens and the shooting situation (a firmware implementation, I might add, that I'd love to see in the 5DII).

By the way, if I am mistaken in any of this, I'd be pleased to be corrected.

ForumParentFirstPrevious
Flat view
Post (hide subjects) Posted by
tmr
tmr
tmr
tmr
tmr
tmr
tmr
tmr
tmr
tmr
tmr
tmr
tmr
tmr
tmr
tmr
tmr
tmr
tmr
tmr
tmr
tmr
tmr
tmr
tmr
tmr
tmr
tmr
ForumParentFirstPrevious
Keyboard shortcuts:
FForum PPrevious NNext WNext unread UUpvote SSubscribe RReply QQuote BBookmark MMy threads
Color scheme? Blue / Yellow