Whats *effective* focal length? The lenses I mentioned were both for full frame bodies, yet have the same fov... I'm still a bit lost here. Certainly the 17-40 @17mm would "look" different than the 28-105 @ 28mm?
I got curious (I have the 17-40/4L) and checked the specs for a few lenses on the B&H website. Indeed, a 28/1.8 claims 75 degrees coverage, while the 17-40/4L claims 74 to 29 degrees. The 16-35/2.8L claims 108 to 63 degrees, and the 24-105/4L claims "84-23 degrees (with full frame cameras)."
The listed specs for the 17-40 must
refer to a reduced sensor camera (like the 10D or 20D), which may be what they expected it to be used most for. On a full-frame camera the 17mm end of the 17-40 should give more than 100 degrees.
Yes, the 17mm does look much
wider on an FF camera.