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Old Jul 18, 2005, 12:23 PM   #1
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The 35mm F2was to be my"normal" lens on the Rebel XT, but it has a strangebackfocus ONLY on the RXT. When I put it on the 20D, it functions perfectly. And my other lenses function perfectly on both cameras, however, this is the only lens I have that is faster than F3.5.

To confirm my field findings, I set up a test with three clothespins standing about 3 inches apart on a black surface and a black background. The left was about 2 inches behind and 3 inches to the left of the center one, and the right about 2 inches ahead and 3 inches to the right of the center. I put the cam on a tripod with cable release. Center focus point and aperture priority with the aperture set to F2, ISO400, 1/100th shutter.

Using the remote cord, I half-pressed the remote shutter button to lock focus (focus point right on the center clothespin), then fully pressed to take the picture. Here is a full size center crop of the first image. The center clothespin is sharpest, with the other two still within the DOF of that lens at 5ft/F2 (+/- 3 inches):


Released the button then repeated the procedure without touching the camera. Here is the second image. Notice the focus has shifted, and how the back clothespin is sharp and the others not:


Once more. Same as above. The focus seemed to shift a bit more:


When I conduct the exact same test with the 20D, every picture is the same and perfect. What's more, I did the test again except with my 17-85mm on the RXT set to 35mm, and its results were consistent and perfect. Same with the 20D.

Canon telephone tech support says send the camera and lens to Irvine. I wonder if I'm going to wait six weeks then get it back with a letter saying "No problem found. Cleaned, lubricated and adjusted to factory specifications." And then it still doesn't work--the phone guy just wanted to get me off the phone. Is maybe this lens--or fast lenses in general--not compatible with the RXT autofocus?

Any observations and in particular experience with this camera and lens combination will be most appreciated.

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