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10-12-2009, 01:43 PM   #1
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SDM and HSM focus speed - dependent on body?

Guys,

This is more aimed at the lens than the body, so I've posted in the lens section.

I was wondering how much, if any, difference the body will make to a Pentax SDM or Sigma HSM lens? Surely the speed at which it changes focal point will be dependent on the speed of the motor in the lens, and not the body? Sure, the body might be able to lock focus or have a more accurate autofocus mechanism, but I'm looking at this from a 'point to point' basis.

The reason I'm asking, is the HSM lenses seem to be pretty rapid indeed - for example:

YouTube - Sigma 50 f/1.4 HSM Hunting on a Pentax K20D

Now this seems quicker than the DA* 55mm to me:

YouTube - DA*55mm F1.4 SDM AF Speed K20D

Then I found this, the 70-200mm f/2.8 which also seemed to be quicker than similar lengthed Pentax equivalents

YouTube - SIGMA 70-200HSM II PENTAX.AVI

Getting back to my original point, here is a demo of the same lens on a Nikon D300:

YouTube - AF speed Sigma 70-200mm HSM Macro II

That seemds very nippy indeed. Given that the Nikon D300 has very quick autofocus, how much of it is the body and how much of it is the lens? Will the Pentax equivalent be as quick as the D300? From the two vids above, it certainly seems possible.

10-12-2009, 02:19 PM   #2
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Join Date: Oct 2009
Location: Lille
Posts: 12
Hello,

From what I understand, HSM does not reply on the body. It almost merely depends on the size of the motor internally built onto the lens. On may be a bit of the circuit current sent to/from the body to the lens might have very little impact though.

As the old time Nikon shooter, my 30 1.4 HSM sigma works wonder on both my D300 and my girlfriend D60. However, a cheapo AFs lens (kit) works slowly on both cameras, because the camera screw-drive cannot override the internal motor speed of the lens. The non-motorised lens seems to work faster than those AFs though (I think it is the same case apply to Pentax lens, as we try to minimize the size of the lens that's why the motor might be smaller)

On K-7, as I've tested, 55 SDM is somewhat slower than a screw-drive DA40 ltd.
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