Lens question

[ahab1372]

The EF-S lenses that say they are made for digital cameras are optimized for the smaller sensor on the crop camera. If you were to mount an EF-S lens on a full frame camera you would see the image was a circle with soft edges we call vignetting. This is because the image projected by the lens is smaller than the full frame sensor. Because of this the EF-S lenses can be made smaller and at lower cost.
Hope this helps.

To be exact, the image is not smaller, it is the same size as an image of a Full Frame lens with the same focal length. But the corners are very dark and not very sharp, so the usable part of the image is smaller - which is fine, because the smaller sensor in APS-C cameras doesn't capture those outer parts of the image anyway.
So 50mm is 50mm, no matter if EF or EF-S. So why do camera/lens manufacturer build lenses for crop cameras? Because it allows them to built smaller, lighter, and possibly less expensive cameras and lenses.

I always found it easier to wrap my brain around these things by thinking about the angle of view, or how much of the scene will fit in the picture:

Mount a 50mm on a full frame, shoot from your kitchen window into the backyard, and a certain part of the backyard will be captured in the picture.

Mount a 50mm lens (EF or EF-S doesn't matter) on an APS-C camera, take the same shot, and the camera will capture a smaller part of the backyard - even if you used the very same EF lens as with the full frame. The reason is the smaller sensor, which captures only the center part of the image that is projected on the sensor plane.

Now if you want to capture exactly the same picture as with the full frame camera, you would have to use a lens that has 50mm/1.6 = 31.25mm focal length.

Back to your example: 24mm on full frame is considered wide. On a crop (APS-C) camera, 24mm gives you an angle of view that is the same as 24mm*1.6 = 38.4mm focal length on a full frame. That is not considered really wide. You would need a 24mm/1.6 = 15mm lens for the same wide angle of view. Again, if that is an EF or EF-S lens doesn't matter.

Regarding image quality: Using the 17-40L on a crop camera doesn't have any advantage (image quality wise) over the EF-S 17-55mm or EF-S 15-85mm. The 17-40L is heavier and has its price tag because it needs more glass to project a decent image on a larger image circle (well, and build quality and sealing). The quality of the center part of the image (the part that is captured by the APS-C sensor) is not better than the one projected by the two EF-S lenses. But L is still nice I hear

Edit: Re-reading this thread, it occurs to me that I did not really contribute much that others haven't said already Oh well, got carried away.