Canon USM lenses... experiences with them?

[Sean Dempsey]

I underestimated the USM lenses until I got the 17-40L. The USM on this focuses at a blinding speed, and is dead silent. I was shocked at how fast it focused. So I have some questions.

First, do all USM lenses focus this fast? Or does the 17-40 focal length have something to do with it's ability to focus so fast. OR, is that a function of the L series lens? Also, do the non-L lenses with USM focus just as fast?

I am pretty much wanting to eventually end up with all L and USM lesnes... but I want to see if the USM speed is the same in all of them, or just certain ones. If you have experience with any of the above things, please post.

Thanks!

[Lionheart]

A large part of it is the short focal length,(probably-17-40 is a decidedly short range) but most of the USM "L" lenses that are f4 or faster will focus much faster than their consumer USM counterparts. A lot of it has to do with the fact that more light is available for the sensors to lock focus. I'm not sure if engineering has anything to do with it, but it might well be that they're engineered to be faster as well. I know that on my old 1D and 1V, the central 5-7 sensors are cross-basis sensors only with lenses that are f4 or faster (or was f5.6?), and for lenses that are slower aperture than this, only the central sensor is cross basis, to reduce focus fishing according to the manuals that came with the cameras. Makes sense to me, since at f4 and faster, more light is available for a cross basis sensor to function properly, and f5.6 and smaller, the screen starts to get pretty dark even in bright daylight.
I don't see really any focus speed difference on my consumer level 20mm f2.8, 50mm f1.4, 85mm f1.8 lenses versus my f2.8 "L" zooms, but I do recall that the 100-400 f4-5.6 L was noticeably slower to lock focus than any of my other lenses. The 28-135 f3.5 USM was plenty quick as I recall. I've read reviews on the 85 f1.2 as being slow to focus, and that is an L USM lens. Might be the focal length in some cases, might be engineering in some.

[Lemming51]

There is a difference between "ring-USM" which is used in the L lenses and the mid-level consumer zooms (EF 20-35 f/3.5-4.5, EF 24-85 f/3.5-4.5, EF 28-105 f/3.5-4.5, EF 28-135 f/3.5-5.6, EF 70-300 f/4.5-5.6 DO, and EF 100-300 f/4.5-5.6) and the "micro-USM" which is used in the entry-level consumer zooms (EF 28-90, EF 28-105 f/4-5.6, EF 75-300, EF 75-300 IS).

Ring-USM moves the focus elements directly, without reduction gearing, and is used on internal focus elements which are smaller/lighter making AF faster. In "micro-USM" lenses the motors drive reduction gearing (a la conventional af motors) and the focus elements are the larger/heavier front group.