My new lens might have a focusing problem

[ClemmonsHoo]

I just got a new Canon 70-200mm f4 L for my Rebel XT. I noticed that my bird pictures are not very sharp. I also have seen numerous reports of front or back focusing issues some people have had with this lens. I did a test tonight. I used a tripod and mirror lockup. I fosused on a white box with smallish black text from about 8 feet away at 200mm and f/4.0. I took one shot in autofocus and one shot manually focused. The manual focused shot was tack sharp and the auto one was slightly off. I repeated this several times with the same results.

I then went back, and in autofocus mode, I unfocused CCW and shot, then unfocused CW and shot. I did this several times. This time, even tho I was in autofocus for every shot, a few of the shots were very sharp. The rest looked slightly off like before. I am very certain there was no camera shake or anything like that involved, and the fuzziness did not look like camera shake.

Is this a bad lens or am I obsessing too much? If it is bad, should I just send it back, or go thru the hassle of getting the lens/camera recalibrated?

I'd buy one from a local store, but we only have Ritz which is overpriced IMO.

 

[dinodan]

Have you tried any other shots, or just this test? The AF system needs a certain amount of "stuff" to grab onto, or it will tend to "hunt". Certain things can be difficult for AF, such as a small light colored object against a large dark background or vice-versa. Of course low light also.

I would take it out and try to do some more "real world" shooting before taking it back. You might be testing the lens under difficult AF conditions...

Maybe you could post some of those shots? (Bird ones and test shots)

 

[Alex_B]

get (print out) a lens focus test chart and repeat the test with that.
since such a test chart is set up at a 45° angle, it allows you to actually see if it is front of back focussing. If you then repeat the test several times, and it does not just scattered focus deviation around where it should be, but if it has a preference (front or back), then either your camera's AF or the lens has a problem and you best let both be adjusted.

I handed my camera and 3 lenses in recently and got them back two days later with everything adjusted nicely. But this was in Europe and I do not live far from the service centre which did it.

oh, and when you do the AF testing, make sure there is enough light, and of course there are acceptable tolerances regarding slight front and back focus.

 

[ClemmonsHoo]

Here is the test I did this morning. 200mm, f/4.0, 1/200, from ~6 feet away. I repeated this test several times, sometimes unfocusing CW, sometimes unfocusing CCW, before autofocusing. The manual focus image is the best of several that I took as my eyes are not that good, and the viewfinder on the XT isn't any better, oh well.

http://www.mindspring.com/~glennrey/pics/lens_test/auto_focus.jpg

http://www.mindspring.com/~glennrey/pics/lens_test/manual_focus.jpg

It looks like it is backfocusing to me, although I am not sure how far out it needs to be before being deemed out of spec.

 

[Big Mike]

It's not unheard of for there to be small focus problems on cameras and/or lenses from time to time. Send or take it into a Canon service centre and they will diagnose and fix the problem for you. If you got the lens new, it should be under warranty.