python2000 wrote:
Every time there is a poll here about AF on the 40D it gets very good marks. The occasional problems appear to be mostly user error, lemons, or Alan321.
TrojanHorse - please read the thread, or at least the recent bits. The major complaints (the problem that is being discussed) are not sports-type photography on big targets.
The problem has nothing to do with focus mode. Most of us have used earlier models with success, I've certainly been using servo and 1-shot modes with great success on the 20D. Yes some people may have troubles with this or other things, but this is not what is causing the problem being discussed here by the OP.
jrsubs wrote:
Specifically, there are significantly greater problems with the 40D than with earlier cameras (20D and 30D) in the following situations:
-snip-
In these situations the 40D is appalling compared to the 20D, and the 30D is somewhere in between.
I have tested the following:
- moving the central focusing point between two targets with similar contrasts but at different distances (using AF servo continously pressed while moving the camera around). Focusing was consistent and always selected the target that was under the focusing point (as marked in the viewfinder). The selectivity was quite good, with the actual active area being slightly larger than the marked focusing point, but less than twice the length/width. At least as good as the 30D if not slightly better.
- bird in flight against blue sky. Very high accuracy and clearly better than the 30D. The bird did take more than just a small area, because I used a suitable focal length. I would estimate that the bird was spanning 1/6 to 1/4 of the image width, wingtip to wingtip.
- bird in flight against a town silhouette. Some problems in keeping the camera focused on the bird instead of the background, but I felt it worked better than the 30D. When in focus, the bird was perfectly in focus, when not, the background was in focus. The bird typically took less than 1/5 of the image width, meaning less than 1/25 of the area.
I have always been very demanding for camera AF behaviour. In fact, one of the reasons why I hesitated to replace my 30D with a 40D was because I was afraid that I would lose in AF selectivity, even though people had been praising the 40D AF accuracy. I am quite sure that I would notice the problems you describe in my typical use.
However, so far I haven't noticed a single case where the 40D would be worse than the 30D was but several where it was clearly better.
Though, one 40D that I had earlier tested in a camera shop, constantly backfocused a 50f1.4 lens, wide open, inside the shop (with fluorescent tube lighting). My 30D also backfocused under similar conditions, while my 350D did not, under exactly the same conditions.
So, my guess is that some/many 40D's have a faulty AF unit, but the fault is only visible under specific conditions (as listed by you). Thinking how my 30D served me very well year around, except when twice a year I happened to take indoor photos under fluorescent lighting and using faster than f2.8 glass. If there was even a little light from other kind of sources, like the Sun or bulbs, focusing worked very well. Only when all the light was from fluorescent tubes and the lens was faster than f2.8, there were problems. The backfocusing was pretty consistent under those circumstances for a specific lens.
Thus, I would guess that most users will never encounter the conditions that you described and thus the actual problem has not been well-diagnosed and confirmed by Canon - like the problem I had with my 30D.
You could certainly be right, unfortunately that gives no joy to people experiencing problems that are not agreed by Canon. We end up using other cameras, not as good, losing money just because shops obviously have trouble taking back camera after camera, and everyone begins to doubt their sanity.
