From: Sandy Harris
I have never used an autofocus system. I find myself comparing lenses
with an SDM motor to ones without. For example, the 17-70 f4 has SDM
but 16-45 does not. 55/1.4 does but 50/1.4 and 70/2.4 do not.
There are plenty of other differences -- notably price, weight and
focal length -- and I have some idea how to balance those in relation
to my usage and preferences. However, I don't have much idea how
important or useful the SDM motor is, so I am asking here.
Is this something that should be ignored in making a choice? An
important factor? Somewhere between?
Fast auto-focus. How important that is depends on what you're doing with
your camera.
Should be useful in tracking shots (like for wildlife) where you're
using continuous auto-focus, or other places where you need really fast
auto-focus (sports).
Those are the two uses that come to mind for me. I'm sure there are
others I haven't thought of.
Doesn't float my boat right now because Pentax doesn't offer a lens in
the focal length I want for shooting wildlife (300f2.8, 400f2.8 or
600f4), nor do second tier lens makers offer a lens of that focal length
in Pentax KAF-mount with SDM [that I'm aware of].
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