Remember, an EF 24-105 mm on a "cropped" camera body (such as the 4Ti) is actually a 38.4mm x 168 mm focal length lens. Therefore, the FOV with the lens set to 24mm on the full frame camera would be different than the FOV with the lens set to 24mm on the "cropped" body camera. As I understand it (and boy, could I be wrong!) as the focal length changes so does the field of view. The EF-S lens is simply a lens that will only fit onto the cropped body camera, not the full frame. Nevertheless, at 24mm, the lens focal length is still actually 38.4mm, not 24mm. Therefore, see above.
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Originally Posted by Steve in HB
At least partially. Now if the focal length actually did change, then the field of view would change accordingly. However, the focal length can't change, since it's a physical characteristic of the lens itself. It has nothing to do with the camera it's mounted on. Instead, the field of view changes because the smaller sensor in the APS-C camera doesn't see as much of the subject as a full frame sensor would, if it was sitting behind the same lens. This is the same as would happen if you look at something with your naked eye, then look at the same thing through the core of a toilet paper roll. In the second case you see far less, but nothing has changed inside your eye. Regarding lenses not really having the focal length they are marked with, the EF-S 17-55 mm f/2.8 IS USM is a known offender. The wide end is more like 18 mm than 17. At these focal lengths, the difference of 1 mm is quite noticeable. As far as focusing goes, the 17-55 mm would rather be better than the EF-S 15-85 mm f/3.5-5.6 IS USM, since the max aperture of f/2.8 allows the center point to use the high-accuracy sensor pairs instead of just the regular ones.
