Digital Rebel XT Focus Problem

[venatici]

I purchased a Canon Digital Rebel XT several years ago and have been mildly pleased with it.  Until recently, i haven't been getting the picture quality I want.  Well, I did some research about the RAW format and began using my digital darkroom software to a fuller extent and have got great results since.  However, there is one problem i'm having that I can't seem to fix.  Focus at long distances.

When i take a picture up close, either macro or within 20 feet, i get a good rich crisp picture.  However, when i shoot long distances, say 200 yds and up, i never get a good crisp image.  It doesn't matter what lens i am using (18-55, 28-90, or 100-300), it doesn't matter my shot settings, or WB settings.  I can get a good sharp focus using Manual Focus, but i can't for the life of me figure out why i can't get a good AF shot at longer distances.  Could it by my sensor needs cleaning?  AF mode (I've tried all three modes)?  Or could it be defective?  Or, heaven forbid, is that just the way it is?

Many thanks.

[yayas]

Firstly, correct me if I'm wrong.
My understanding, with 200yards or more the depth of field is so deep and the target should have great probability to get in focus range. But if you get the sharper result when you're manually focusing the target, it could be focus problem.

Very likely you have a focus shift problem, either front or back focus. It's not uncommon on Rebel cameras.
The shift is so little it's not noticed in short distance, but magnified at longer distance.
If all the lenses have consistent characteristic, i.e. front focused, then the problem is the camera.

There's a adjustment in the camera (with hexagon screw) but I don't encourage you to do it if you're not very sure about it.
Believe me, I did it with my old Rebel XT (350D) and it worked.
I did it because the shift was severe I couldn't get the focus within 2 yards.

[venatici]

The focus problem exists when shooting F/4.5 or F/15.. I don't think it is a depth of field issue.

Perhaps I should research the adjustment screw as all my lenses are front focus...  Thank you for the idea

I'm curious to know what else could be the cause.

[Bob Atkins]

If you can get sharp images with manual focus but not autofocus then that rules out external causes such as the lens not being able to focus to infinity, distortion caused by filters or internal misalignment. If you can get sharp images at the same distance with manual focus but not autofocus, then it's an autofocus issue.

Autofocus problems can be related to both the lens and the camera body. If all lenses show the same effect, then it's likely to be the body. Unless you have experience in camera repair and adjustment I'd strongly suggest you let Canon take a look at it. There is a focus adjustment screw on some early Digital Rebel models (There's one on the 300D I think), but I don't know if there's one on the XT (350D). Even if there is it's a tricky adjustment and you could potentially do more harm than good. It's not meant for user adjustment.