I was talking to Damian last night about my pixel peeping post and I thought I should explain one of the problems that people look for when purchasing a new lens, “front focusing” and “back focusing”.
Now first of all this can be a problem with the Camera. When the 10D first came out there were many who complained that it had focus problems. More often though from what I have seen in forums it usually appears in new lens purchases or trials on cameras that have never exhibited the problem before. While it is the camera that calculates where to focus using its sensors it is the lens that actually focuses, only by swapping lenses can you tell if it is faulty AF or faulty lens.
If you auto-focus on an object the camera will attempt to fix the focus at the correct distance between the camera and the object. Front focusing is when this calculation goes wrong and it focuses before the object, back focusing is where it incorrectly focuses behind the object. Either way what you achieve is a photograph where the focus is in the wrong place making your object blurred or soft. Manually focusing still works but in effect something is wrong with either the lens or the camera.
How is this related to pixel peeping? One of the problems people complain about is third-party lenses having this problem as a common thread. The rumour has it that because third-party manufacturers have to reverse engineer and such to develop compatibility there will be issues when talking to the camera. To compound the noise on the problem some lenses have had re-chipping to address focusing problems. This causes a lot of anxiety for customers who want to buy these lenses but hear all the horror stories and when trying out lenses have the doubt “is it me, the camera, the lense, the chip or AF?”.
As with a lot of this kind of thing it can be user error and it can be just a problem caused by bad light. AF needs good contrast to focus properly but can be thrown off by glaring light too. Also consumer level cameras like the D70/D50/350D are not designed to be super-perfect focusing it should only be within tolerance of the focus is within depth of field. When working at really tight DOF don’t expect it to be perfect every time with shaky hands!
If you find your setup focusing off to any degree and want to test it there are good tests and guides around, Bob Atkins has what seems a great guide. Personally I haven’t tried it and hope to never have to!
front, back, focusing, focussing, lens, camera, test, photography, tips
