On Dec 4, 2005, at 6:12 AM, Tom Reese wrote:
1. the real world differences between the older EF (electronic
focus) lenses and the newer EF USM (electronic focus ultrasonic
motor) lenses?
Is the newer EF USM demonstrably faster with long telephotos?
If a lens is available in both USM and non-USM versions, keep in mind
that the USM motor is used primarily for marketing purposes. You
won't get full-time manual focus with any lens that lacks a distance
scale.
Focus speed is a matter of lens design. The old 80-200/2.8 lens had a
regular motor, but it focused much more quickly than the 50/1.0 or
85/1.2. Neither of those lenses were internal focus/rear focus, so
the motor had to move the entire lens assembly.
2. Are the newer IS telephotos optically better than the earlier
non IS telephotos?
The optical quality difference between the IS and non-IS telephoto is
minimal. In the real world you will obtain sharper images with the IS
lens, particular with the 600/4.
--
John Chennavasin | This article contains material which may inform and
[EMAIL PROTECTED] | may be quoted, printed, forwarded, or redistributed
www.fobpro.com | as long as the original attribution remains intact.
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