John Chennavasin wrote:
>
> On Wed, 21 Feb 2001, Rome wrote:
>
> > I'm new to Canon:
> > Are the USM lenses just quieter or are they actually better than non-USM
> > lenses. By better, I mean optically better, or faster focussing. I don't
> > give a hoot about noise. It can be as noisy as a power ripsaw for all I
> > care, just as long as I get the shot.
>
> There are several types of USM out there.
>
> Ring USM: this type of motor consists of two rings. Since there are no
> moving parts, full-time manual focus is possible. This allows you to leave
> the lens in AF mode. When you want to focus manually, just turn the focus
> ring. There is no need to fiddle with tiny toggle switches on the lens.
> Focusing is silent and quick. Very useful with CF4 to move the autofocus
> activation to the * button. Most currently available AF prime lenses
> will have a ring USM motor.
>
> Micro USM: Generally used for the entry-level lenses. This looks more
> like a conventional motor. No full-time manual (except for the 50/1.4).
>
> If a Canon lens is available in both USM and non-USM versions (such as
> 28-80/3.5-5.6, 28-90/4-5.6, 28-200/3.5-5.6, 75-300/4.5-5.6), the USM
> version contains a micro USM motor which is quieter and focuses slightly
> quicker, but doesn't allow full-time manual focusing. Optically they are
> the same lens.
>
> Focusing speed is also dependent on the lens design. I have a EOS 24/2.8,
> which doesn't have a USM motor, but it focuses relatively quickly since
> it is a rear-focus design (only some lens elements move). The lack of
> full-time manual is a slight annoyance.
>
> 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.
>
As an addendum, most, if not all, micro USM lenses have rotating front
elements, ring USM lenses have stationary front elements. This makes it
easier to use filters that require orientation, like polarizers and
gradients.
Skip
--
Shadowcatcher Imagery
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