BIF question

[Bill Boehme]

I tried some BIF shots today (ducks and gulls) and am wondering if the results shown here would be considered reasonable quality for the XTi with the EF 70-200 f/2.8 lens. Considering the cluttered background, I thought that it did a pretty decent job of tracking in this situation. My keeper rate was a bit less than 50%, but most of the rejects were due to operator error. The shooting data are: ISO = 400, shutter speed = 0.001 second, aperture = f/5.6, focal length = 200 mm, crop size approximately 35%, and shooting distance approximately 40 - 50 yards. I used AI Servo focusing with only the center point active and maintaining the focus point on the lead duck -- also using the * button to focus. I have been wondering whether tracking would be more effective with all focus points active, but was hesitant to try it because I suspect that the XTi doesn't have a sophisticated tracking system such as the ones found on more sophisticated models. Any suggestions and tips would be greatly appreciated.

http://www.pbase.com/bill_boehme/image/105595174/original.jpg

[BradM]

I would not use all AF points with that type of background, it is too easy for the camera to find an area of higher contrast and pick it up as the focus point. A cleaner background, like blue sky or a much greater distance between it and the subject all AF points can make it a bit easier to keep the subject in focus.

The shot shown is rather good for such a significant crop and the the distance from the subjects, I would be very happy to see those kinds of results even 50% of the time. I did shoot with the XTi for a while and while it's fps is a bit slow and the AF is not the best out there it is still a more than adequate tool for the job.

[Bill Boehme]

Thank you very much, Brad, for the helpful information. You confirmed my thoughts about the autofocus. I figured that even in AF that it is simply seeking the points of greatest contrast without any type of predictive tracking estimation being applied.

One of these days, I may get a camera that has the more sensitive double-cross type focus sensor in the center that will provide more accurate focusing when the light is low. Except for bright sunny conditions, I find that many shots miss the correct focus by a small but objectionable amount either in front or back of the aim point. Also, it seems that when the light is less than very bright, the focus tends to stick until the focus error is great enough to make it refocus.

[ebann]

XTi has the same AF system of a 30D... but only at 3 fps.

[gymell]

Looks like you got the bird in pretty decent focus, but BIF against that kind of background is difficult with any type of AF. I can't really speak to the abilities of the XTi specifically, but I've always just used center point AF. Not saying that is the best way, I just haven't had a lot of luck with all AF points.