Why I get bad AF with my 24-70 f2.8L

Started Mar 20, 2005 | Discussions thread
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Why I get bad AF with my 24-70 f2.8L
Mar 20, 2005

Since I got the 24-70 f2.8L for my 10D over a year ago I have been aware that I cannot rely on AF for landscapes (usually 24mm at infinity in good light). I could see that many of the shots were back focused because the distance mark on the lens was showing beyond infinity.

AF 100% crop taken on a slightly hazy day at f2.8

I get better results with manual parfocal focusing (zoom to 70mm, manually focus then zoom and frame the shot).

Manual Parfocal 100% crop

Although I would not normally shoot landscapes at f2.8 I do not think that you should have to use small apertures to mask focus errors. The AF errors can sometimes be big enough to show at smaller apertures. I was always very happy with AF for indoor shots. Now I think I may know where the problem lies. I took a series of AF test shots in bright light and in lower light. The results are variable but these are typical:

AF in bright light (1/4000 sec f2.8) ā€“ back focused

AF in lower light (1/90 sec f2.8) ā€“ accurate

MY THEORY: AF sensors do not have infinite dynamic range. To give good low light AF response, the sensors can get overloaded with bright light resulting in lower accuracy (remember that the focus sensors get the image at full aperture). The small apertures usually used with bright light can mask focus errors for smaller print sizes.

Iā€™d be very grateful if anyone else can confirm these results. I tried with other lenses and the effect of bright light is not so clear as with my 24-70 f2.8L at 24mm.

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