On Sat, 2013-05-11 at 22:29 +1200, RussellMc wrote:

> STM is a Stepper Motor style of in lens focus motor as opposed to the
> more usual "USM" (ultrasonic motor.)
> USM works on a ring warping piezo electric drive principle.
> STM is newish. in some lenses only and targeted especially at video
> use. It tends to be slower to focus than USM but quieter and smoother.

When it comes to Canon, there are really 3 types of focus motors, with
one type split into 2:

1) DC Motor - if a lens has no modifier on it of USM or STM, it is a DC
focus motor. They are generally slow, loud and have the issue of you
shouldn't touch the focus ring unless you've selected manual focus since
you can strip a gear

2) USM - lens marking is "USM". There are actually TWO types of USM, the
cheaper lenses (i.e. the 75-300 IS USM) has a version of USM that is
slower, louder and doesn't support manual focus override without hitting
the switch. Most other lenses have the "modern" USM that is very quiet
(but not necessarily silent), generally quick, and fully supports manual
focus override during AF.

3) STM - lens marking is "STM". This is the newest from Canon. There are
only 3 lenses with STM in them, the EF-S 18-135mm f/3.5-5.6 IS STM, the
pancake EF 40mm f/2.8 STM, and the newest 18-55mm f/3.5-5.6 IS STM.
Generally they are very quiet to silent (the 18-135 is silent, the 40mm
is very quiet, but can be heard in video). Focus speed is variable, the
18-135 is VERY fast, the 40mm can be a little slower. Manual focus
overide IS supported. Only "odd" bit about these lenses is the focus
ring is NOT physically connected to the focus lens element. The ring is
connected to an encoder, all focus element movement is by the STM, even
in MF mode. The rub is that this lens will not allow manual focus
without power, and it feels a little weird turning a focus ring and not
having the element track immediately. You get used to it though.

TTYL

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