Do you have similar problems with your other lenses, if you have any ? The back-focus issue might be difficult to diagnose on kit lenses (like the XSi's EF-S 18-55mm f/3.5-5.6 IS) due to the narrow aperture. Try to test these lenses as close to their minimal focus distance (MFD) as possible, as the closer from the subject, the shallower the depth of field. If you don't have other lenses to start with, ask a friend or rent one from a reputable dealer.
To test your lenses, tape a ruler to a wall and shoot at a 45 deg. angle on a tripod (with cable release, mirror lockup, RAW format and self-timer) on the 6 in. mark, then preview your pictures at 100% magnification on your computer. Any focus problem will be apparent.
I have a front-focus issue with my XTi that has been exacerbated by the EF 85 f/1.8 I bought and I tested it with other lenses to make sure the problem did indeed come from the body. I guess I'm gonna send it to Canon soon, as now it's rather inconveniant to get soft pictures all the time with a good lens like this...
If the body is under warranty, the focus calibration will only cost you one month or so without your camera. I guess that your XSi is still under warranty, as it was launched one year ago and Canon warranty is also one year. Act fast ! /emoticons/emotion-5.gif
STL
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Download this focus test:
http://www.dphotojournal.com/download-free-focus-test-chart/ (http://www.dphotojournal.com/download-free-focus-test-chart/]<span style="color: #3366cc;)
Print, tape to the table, and use a tripod. Oh, and read the directions too!
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Thanks. I had previously shot an informaltest with the 85, then more formally used the ruler method (before I read the posts). I shot at the "MFD" and again at about 6 feet. With these lenses at 1.8, it is easy to seetheresults:
* Turns out I was mistaken about the 50, it is dead on, or perhaps SLIGHTLY front focusing, if Iunderstand correctly thatDOF should be 1/3 ahead of the focus point and 2/3 behind.
*The 85 focuses accurately at the MFD BUT back focusesmore thanone inch at 6 feet. Ugh. This lens is about six months old.