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05-25-2014, 04:51 AM   #1
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Is SDM autofocus better?

Hi there. I have a K30 and all my AF lenses are screwdrive. And although K30's AF is said to be decent I kinda think that it has some major flaws. For example the continuous AF is unusable for me - shooting walking people is a hit or miss game. And when the light is lower it often hunts, often locks on the background and sometimes is almost there but another secong goes before it locks and the moment is gone. *sigh* My question is - when using SDM lenses is the AF better (quicker, snapier, more reliable) or it is only quieter?

05-25-2014, 05:26 AM   #2
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it is only quieter, that is really the only advantage it has over the more common pentax screw-drive focusing mechanisms. However with Sigma HSM lenses it is quicker, though that is more of a reflection of the type of motors sigma use.
05-25-2014, 05:50 AM   #3
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So for example with the 50-135 2.8 I will still have awful continuous AF?
05-25-2014, 05:53 AM   #4
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It's the same AF system, just a different motor.

If it's any compensation, at 2.8 there's a lot of light coming through to help the K-30's contrast detection.

But why are walking people a problem? Are you panning to keep a focus point on them?

If you find others cross your line of view, consider back button focus.

Last edited by clackers; 05-25-2014 at 06:23 AM.
05-25-2014, 06:43 AM   #5
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Screw drive is reliable and actually quite fast with the k-series. SDM or micro motor is smoother and more accurate with small adjustments. This is an advantage in tracking focus shots. You can see this in imaging resource k-3 review where he used both types and the SDM/DC had an advantage in shots where a fast moving object comes toward the camera.
05-25-2014, 06:46 AM   #6
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In continuous AF I find it helpful to set the AF point to center point if I know my subject will be centered in the frame. That way the camera doesn't try to focus on any of the other AF points (after all, the camera has no knowledge of what you're trying to compose). I find that works well. If you have an off-center subject, manually select the AF point in advance to reflect your intended composition.
05-25-2014, 06:57 AM - 1 Like   #7
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QuoteOriginally posted by muzicantini Quote
So for example with the 50-135 2.8 I will still have awful continuous AF?
Muzicantini - the SDM AF accuracy is fine - it is just slow from end to end. If you are shooting an activity which puts your subject matter at a similar distance from you it should perform fine. If you are shooting an activity which causes the AF to rack from MFD to Infinity regularly you will be disappointed because the AF speed is slower than screw drive.

Make sure your expanded AF zone is active.
The hunting of the lens has more to do with the Aperture value than the AF system of the body/lens
Are you using TC's? A 1.4x TC knocks one stop of light off your scene, and a 2x TC knocks two stops.

The other thing to think of is the actual EXIF of your shots. Slow shutter speeds will blur, and might appear to be out of focus but just lack sharpness.

Post a couple of photos with EXIF and identify what lens you are using.

I've posted a couple of examples from my Kr - which arguably is much less featured - the AF-C while not 100% gives acceptible results. All photos where shot in RAW.

Runners in sporting Life 10K in Toronto - AF-C Hi Speed Drive mode - can be found in the album here:

A day at African Lion Safari - a great water park for the kids to cool off Similar to above

And some of my earlier attempts... it is a skill that gets better with practice/experience

05-27-2014, 03:32 AM   #8
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So you say that Sigma's Hsm is better than the SDM?


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