----- Original Message -----
From: "Sandy Harris"
Subject: How useful is SDM?
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?
I think the factor now is can you get the lens you want with screwdrive or
SDM?
I have two SDM lenses, the 55/1.4 and the 60/250 zoom.
I actually did a direct comparison between SDM and screwdrive AF with the 55
and the FA50/1.4.
The 55 does focus somewhat faster in real life, though if all you do is
compare lock to lock, the 50 is much faster.
One thing to consider with SDM is failure rate.
Over on ForumsNeurotica, there have been a lot of threads dedicated to SDM
failure, especially the 16-50 and to a somewhat lesser extent, the 50-135.
As optically good as these two lenses are (if you can get a 16-50 that isn't
decentered anyway), I'd run like hell from buying either of those lenses
until Pentax fixes the problem.
So far, I haven't read of any SDM failures in the SDM only lenses, so I do
wonder if there is an issue with the dual AF integration.
William Robb
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