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02-02-2011, 07:00 AM   #1
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DC motor (18-135) + K-5 1.02 contrast AF = FAST

I think a lot of people may have missed it, but the 1.02 firmware for the K-5 (and 1.01 for the K-r) included improvements to its contrast detection AF speed. And of course the 18-135's DC motor has already been noted to be extremely fast. I don't use live view much, but I decided to give them a rough test together.

I did the test as follows: set the 18-135 to 135mm so that its maximum aperture is f/5.6 (make the AF detection system work a little harder). I turned the ring all the way the minimum focus distance and targeted items that were a moderate distance away (10-20 ft; I would use infinity, but my apartment is not that big ). I confirmed that the focus ring ends very close to infinity anyway (there are no markings, and no physical start or end to the ring, but there is a very subtle click you can feel when you reach infinity or minimum focus). Times are approximate because it's hard to be precise when holding a stopwatch in one hand and operating the camera in the other.

In decent indoor lighting (~EV 8), the combination is blazing. From minimum focus distance to near infinity and confirmed lock in less than 1 second (somewhere around 0.8s)! In noticeably worse light (~EV 4), it still managed to do the task in a very respectable ~1.5s. No hunting at all, just straight to focus and lock. Under EV 4 it starts to lag, though, and under EV 2 it starts hunting and often fails entirely, but in even moderate lighting it's incredibly fast.

Just for a bit of comparison, I did the same tests with the K-x and 55-300 (set to the same focal length). The 55-300 is the closest lens I have to the 18-135 in terms of focus properties: focus throw (a least for moderate distances) and lens weight (so presumably the focus systems would have to move roughly the same weight in glass).

Despite being a faster aperture (f/4) and having close to the same focus throw for the distances involved (around 100 degrees to go from minimum to lock), the combo took a snail-like 6-7 seconds. Heck, for the EV 8 test, the K-5's CDAF + 18-135 managed to beat the K-x + 55-300 in phase detect AF!

If Pentax takes advantage of these two systems in their mirrorless camera and video AF in their DSLRs I would say they definitely have a winning combination on their hands.


Last edited by Cannikin; 02-02-2011 at 07:25 AM.
02-02-2011, 07:09 AM   #2
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Put the 18-135 on the K-x...seems to be mostly the lens...
02-02-2011, 07:21 AM   #3
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QuoteOriginally posted by kenyee Quote
Put the 18-135 on the K-x...seems to be mostly the lens...
Done.

The 18-135 does clearly move much faster on the K-x than the 55-300, but still much slower than the K-5, at least in CDAF. It takes a bit under 2 seconds to get into the right ballpark for focus (~3 times faster than the 55-300, but twice as long as the K-5), but at this point the limitations of the K-x's CDAF become painfully apparent. Once it reaches the right area, it starts hunting and micro focus adjusting for an additional 1-2 seconds, whereas the K-5 went straight to lock.

Also, the K-5 is much faster with the 55-300 than the K-x, but still much slower than with the 18-135.

Both the DC motor and the new CDAF algorithm play equally big parts. Can't wait to see more lenses with the DC motor (and hopefully Pentax reconsiders their decision to keep SDM on their DA* lenses, or at least silently replaces them with DC motors).

Last edited by Cannikin; 02-02-2011 at 07:31 AM.
02-02-2011, 07:31 AM   #4
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I guess there is as much difference between the "SDM" motors in the different DA* lenses as there are between SDM and DC. That is: same motor technology different marketing language.

02-02-2011, 07:39 AM   #5
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QuoteOriginally posted by joakimfors Quote
I guess there is as much difference between the "SDM" motors in the different DA* lenses as there are between SDM and DC. That is: same motor technology different marketing language.
Are you sure you don't mean "same name, different technology"?

It seems to me like SDM has significantly different reliability and speed between the various lenses that have it, and that the DC motor is something different entirely (speed, focus ring operation, very distinctive sound not found on SDM lenses).
02-02-2011, 07:42 AM   #6
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Just guesses, could be that the DC motors aren't piezo electric. The only one I have "heard" is one I tested in a fairly noisy store so couldn't really hear it.

Anyway, nice to hear that the 18-135 + K-5 combo is quick to focus.
02-02-2011, 08:06 AM   #7
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CDAF is an interesting development for dslrs. I tested a 2x AF teleconverter with the 55-300mm (effectively f11) in our dining room with ambient/indirect sunlight - with light that gave a shutter speed of about 1/60th the TC was still able to AF using contrast detect. That is due to the sensor's ability to gain up and find focus using the contrast within the image, regardless of the light coming through the lens. The better CDAF becomes, the better focus with TCs and low light as well.
02-02-2011, 11:06 AM   #8
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QuoteOriginally posted by pxpaulx Quote
CDAF is an interesting development for dslrs. I tested a 2x AF teleconverter with the 55-300mm (effectively f11) in our dining room with ambient/indirect sunlight - with light that gave a shutter speed of about 1/60th the TC was still able to AF using contrast detect. That is due to the sensor's ability to gain up and find focus using the contrast within the image, regardless of the light coming through the lens. The better CDAF becomes, the better focus with TCs and low light as well.
That is a neat component of CDAF here - The limitation on aperture of PDAF is based on lens physics not available light, so PDAF will go down the tubes rapidly past its rated aperture of around f/5.6-6.3.

CDAF, however, is only dependent on available light going into the lens, so if you're in bright light with a small aperture, it'll still likely work.

I should try CDAFing with a Bigma + TC combo - not something I would EVER have considered with phase detect.

02-02-2011, 11:24 AM   #9
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QuoteOriginally posted by Entropy Quote
I should try CDAFing with a Bigma + TC combo - not something I would EVER have considered with phase detect.
I've no doubt it'll work - and outside probably quite well. Definitely report back!
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