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06-10-2013, 12:12 PM   #1
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Autofocus comparison : lens motor vs. body motor (screw drive)

Which autofocus methodology works better on a K-30? Screw-driven from the body or motor driven in the lens? Or, is it all about the same here?

Thanks!

-=- Boris

06-10-2013, 12:24 PM   #2
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QuoteOriginally posted by 6BQ5 Quote
Which autofocus methodology works better on a K-30? Screw-driven from the body or motor driven in the lens? Or, is it all about the same here?

Thanks!

-=- Boris
It depends on the lens. Lenses with short throw are generally the fastest, such as the DA limiteds and the 18-135mm WR.

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06-10-2013, 12:37 PM   #3
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QuoteOriginally posted by Adam Quote
It depends on the lens. Lenses with short throw are generally the fastest, such as the DA limiteds and the 18-135mm WR.
Actually, I was thinking more in terms of precision, reliability, and consistency. Does the screw head out quicker? Will the focus change over time due to mechanical wear or does the camera adjust with more focus movements? If I focus on something 1000 times will I always get the same focus?

-- Boris
06-10-2013, 12:45 PM   #4
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QuoteOriginally posted by 6BQ5 Quote
Actually, I was thinking more in terms of precision, reliability, and consistency. Does the screw head out quicker? Will the focus change over time due to mechanical wear or does the camera adjust with more focus movements? If I focus on something 1000 times will I always get the same focus?

-- Boris
In-lens motors are more prone to failure than the old system. Unless you damage your screwdrive lenses externally, chances are they will last for a lifetime.


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06-10-2013, 01:05 PM   #5
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Screw driven motors will have a longer life span. As far as speed goes, as Adam says, it is much more related to the focus throw than to the motor. A lens like the DA 35 limited will often take a long time to get where it needs to, because it is a macro lens and has that whole macro distance to travel, while the DA *16-50 is pretty decent, as is the 18-135.
06-10-2013, 06:29 PM   #6
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Thanks for the feedback!

I would have expected the opposite answers. My guess would have been that the screw drive would have been more prone to wear and slop/gear-lash.

-- Boris
06-12-2013, 12:06 PM   #7
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QuoteOriginally posted by 6BQ5 Quote
Thanks for the feedback!

I would have expected the opposite answers. My guess would have been that the screw drive would have been more prone to wear and slop/gear-lash.

-- Boris
Screw drive lenses do not have much that can go wrong - there is just a pinion and a ring gear and nothing else. It may possibly wear out over time and become a bit loose but I don't see any way it would actually break unless they use some particularly cheapo plastic to make the gears (perhaps in the DA-L lenses?). All the complicated things are in the camera body.

Lenses with in built motors on the other hand have plenty to go wrong as a lot of the electronics, the motor and reduction gearing are all in the lens.

The older Pentax lenses had ultrasonic motors, which one would expect to be more reliable (because they have very few moving parts) but apparently they do break down relatively often. The newer lenses have what Pentax calls DC drives which seem to be more durable, but then again maybe they haven't been around long enough to break down.

I don't know if that 'DC' carries the usual meaning of a DC motor, which would be one with brushes and a commutator (which would seem like a backward step from ultrasonic) or else something else entirely. From the sound it makes it is evidently something spinning fast and having reduction gearing, unlike ultrasonic motors which drive the barrel directly (or at most with a low ratio reduction gear for lenses with particularly long focus throw).

Anyone know what the DC actually means, as used by Pentax? I am curious but I'm not about to dismantle my 18-135WR to have a look inside.
06-12-2013, 12:12 PM   #8
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Not quite - DC is a different and lower tier silent motor for Pentax. SDM is still their preferred silent motor.

SDM - if I remember correctly, uses vibrations to move the lens elements: Ultrasonic motor - Wikipedia, the free encyclopedia
SDM is the same concept as HSM, USM, and SWM. And from what I've read, SDM for Pentax isn't the same for each lens model. For example, the Pentax 18-270 SDM is a re-brand of the Tamron 18-270 VC. As such, it would be unlikely that Pentax would replace the motor just to call it SDM - so it seems likely that SDM is just Pentax's label for any ultrasonic motor used in their lenses.

I am not sure what DC is, but it isn't an ultrasonic motor.

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