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02-09-2012, 01:28 PM   #1
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Once more round again - DC vs SDM

Seeing these threads:

https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/115100-dc-vs-sdm-autofocus.html
https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/117278-pentax-...-dc-motor.html
etc..

Not much info is on this forum about DC motors. I actually haven't ever played with a 18-135 so I don't even know how it compares to SDM. However, word of mouth on this forum seems to make the DC motor in the 18-135 the end-all to SDM problems, and I'd love to hear more information.

First up - SDM failures. Some people report success with repairing the SDM issue by tightening the screw holding the SDM motor in place. Most people get their motor replaced. What fails when the SDM fails, does anyone know? Also, it seems mostly it's the zooms (specifically the 16-50) that suffer most.

Secondly, why is DC different then?

Thanks if anyone can provide me any information!

02-09-2012, 01:41 PM   #2
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DC is a different type of motor, and the focusing system is in the back of the lens, allowing them to use a ring-style motor and fewer mechanical parts. Pentax has offered surprinsingly few official remarks about this system, though. This might make for a great interview question!

I have yet to hear any reports of the AF failing on an 18-135mm, but the bigger question is why is this technology not being used on more of the newer lenses?

My personal experience with DC is that it is actually faster than regular screwdrive AF, unlike SDM.

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02-09-2012, 01:45 PM   #3
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How is it compared to SDM in other ways? Accuracy steps, noise, vibrations, etc?

With the focusing system at the back of the lens, is the lens element associated with the focusing also at the back? Does that effect anything in lens construction?

And yes, I'd love to know more about DC, and also SDM. Searching on this forum for DC brings up few results, while searching for SDM brings up a few hundred threads talking about failure.
02-09-2012, 01:59 PM   #4
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Even between de DA* lenses you see very different preformance depending on the lens so you can not say anything with only one model DC lens when it comes to accuracy, speed and even reliablity, the DA* prime lenses for example and the DA*60-250 looks relative quite problem free.
Besides that the DA* lenses have bigger elements and the lens are tighter shield, so those SDm motors need to move more weight with more firction then the DA18-135 lens so is it a fair comparison?

About the technology.
DC is a burshless direct current electro motor, so it's like a regular elctro motor that you probably know.
SDM is different, it's a ultra/super sonic motor, it has some kind of feets and they vibrate in a certain way and they more or less walk actually.
I believe that when you tighten the screw on the SDM you actually bring those feets closer to their contact but this is speculating on my side though.

As for the elements with the DA*16-50 and 50-135 the front elements focus and the back ones are used for zooming.

As to what fails with SDM, i've no idea but since it only effect the zooms i'm actually inclined to say it might be design flaw in the AF mechanisme/gears rather then the SDM themselves. But SDM use friction to move the focus so they will wear out over time but this doesn't explain the failures.


Last edited by Anvh; 02-09-2012 at 02:14 PM.
02-09-2012, 02:08 PM   #5
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In the pre-Ricoh era a Pentax representative told in an interview that SDM will be used in higher end lenses, DC in consumer ones and screw drive in Limited lenses. Go figure.
02-09-2012, 02:18 PM   #6
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QuoteOriginally posted by Anvh Quote
Even between de DA* lenses you see very different preformance depending on the lens so you can not say anything with only one model DC lens when it comes to accuracy, speed and even reliablity, the DA* prime lenses for example and the DA*60-250 looks relative quite problem free.
Besides that the DA* lenses have bigger elements and the lens are tighter shield, so those SDm motors need to move more weight with more firction then the DA18-135 lens so is it a fair comparison?

About the technology.
DC is a burshless direct current electro motor, so it's like a regular elctro motor that you probably know.
SDM is different, it's a ultra/super sonic motor, it has some kind of feets and they vibrate in a certain way and they more or less walk actually.
I believe that when you tighten the screw on the SDM you actually bring those feets closer to their contact but this is speculating on my side though.

As for the elements with the DA*16-50 and 50-135 the front elements focus and the back ones are used for zooming.
Thank you for this - so essentially, the DC is similar to the little motors inside RC cars? How quiet is it compared to SDM? I havent' actually heard SDM (have heard HSM, Nikon SWM, and assuming SDM is similar).

I can see that SDM should be quieter than DC/screw drive based on your description of it's mechanics. I can also see why it isn't necessarily faster. And your point of lens elements being rotated in the DA* series is an interesting point. Does the 18-135 rotate internally? Which lens element rotates? Internal elements should be lighter than the front elements AFAIK (where the 16-50 and the 50-135 focus).

It'd be nice to see what Pentax would say, except that I imagine Pentax doesn't want to put the SDM in any sort of bad light considering it's what they use in the prized DA* series. I myself want the DA*55mm and I don't even see any mention of SDM failure on that lens.
02-09-2012, 02:30 PM   #7
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Yes they would be like that.

I've played with the 18-135 and it's quiet but haven't compared with SDM though.
And yes SDM, HSM, USM all the same type of motor but i some of them use ring type motors in these form and thsoe are quite a bit faster from what i know.
The SDM can make a very quiet quite a high pitch noise sometimes but you need to listen very carefully and have young ears.

Actually i made a littly mistake, with the DA*16-50 the front element is used for zooming however de element right after that focus though so the elements are still in the front of the lens.
But alle focus elements are internal, never found a lens that used the front element to focus,


Like Raffwal says pentax have said DC for DA lenses and SDM for DA*.
SDM motors have a lot of torque so maybe that's the reason behind that.

As for the failures, already added that comment above but i remember i've read about that Pentax had explained that some failures in the DA*16-50 were due to misaligment of a shaft so not the actual motor.
02-09-2012, 02:37 PM   #8
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Sorry, I meant an element that is closer to the front generally is bigger (albeit maybe not as thick) as a more rear element. However, if a 18-135 user can chime in on which lens element is used in focusing...

If the SDM sounds the way you described it, then it's very close to the SWM of Nikons. If you stick one close to your ear, there's a very light, high pitch whine when focusing. It's adorable.

If torque is to be considered, it'd be nice to have DC for the smaller primes we use - like the limited primes. It would make a lot more sense compared to the screw drive if we're talking about exceptional lenses..

Can the 18-135 be screw driven also? Or is it like the 55 1.4 in that it is DC/SDM only?

02-09-2012, 02:50 PM   #9
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QuoteOriginally posted by JinDesu Quote
Sorry, I meant an element that is closer to the front generally is bigger (albeit maybe not as thick) as a more rear element. However, if a 18-135 user can chime in on which lens element is used in focusing...

If the SDM sounds the way you described it, then it's very close to the SWM of Nikons. If you stick one close to your ear, there's a very light, high pitch whine when focusing. It's adorable.

If torque is to be considered, it'd be nice to have DC for the smaller primes we use - like the limited primes. It would make a lot more sense compared to the screw drive if we're talking about exceptional lenses..

Can the 18-135 be screw driven also? Or is it like the 55 1.4 in that it is DC/SDM only?
The 18-135mm is KAF3 (no screwdrive).

The element used for focusing is directly under the focusing ring. The lack of the distance scale also speeds things up, I think, as nothing needs to rotate on the lens itself.

My experience has been that the 18-135mm is a little queter than SDM- it emits a lower frequency.

I think its success is due to the rear element design, and the very short throw. Regardless of how it's done, that lens is definitely up modern standards, though, and that's what counts.

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02-09-2012, 02:58 PM   #10
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Yes the SWM of the Nikon looks to sound the same =]

I doub they can build-in a motor in those limited lenses, certainly in the DA40 there is very little room.
They should put in a DC or SDM in the macro lenses, those things really make a lot of noise when focusing...

Pentax has announced that the idea of SDM and screwdrive focusing in one lens was to ambistious and complicated to design so they won't bring out new lens with a KAF2 mount.
So new lenses will either be KAF (screw) or KAF3 (SDM/DC) from now on.
Here is the list so far
KAF2 = 16-50, 50-135, 60-250 (not 100% sure about this one), 200, 300
KAF3 = 17-70, 18-135, 55
02-09-2012, 03:06 PM   #11
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I would still be nice to have more DC lenses introduced - having one DC lens in the whole lineup (and it's not a ridiculously priced lens either) is a little lacking, considering how many of us love DC.

Agree on the macro lenses - would make the lack of a focus limiter a little more bearable for non-macro use. My Tamron 90 sounded like a bandsaw when I missed focus.

@Adam - agree on the 18-135. I really want one when I get a k-5 because of the WR. It's just that as a superzoom, it makes me wish the 16-45 came in DC.
02-09-2012, 03:19 PM   #12
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I was able to play with a fellow board member's 18-135 briefly, but I've never held an SDM. I can't imagine anything faster or quieter than the DC motor, it works so well.
02-09-2012, 03:32 PM   #13
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QuoteOriginally posted by JinDesu Quote
I would still be nice to have more DC lenses introduced - having one DC lens in the whole lineup (and it's not a ridiculously priced lens either) is a little lacking, considering how many of us love DC.
I agree with the first part but not with the later.
We have afterall no idea how DC will preform on other lenses, like Adam say the DA18-135 could be so fast thanks to the lens design.
Lets hope they will go up this line with their upcoming lenses but DA* will probably stay slower to focus because they often have a longer focus throw for beter manual focusing.

Doesn't Nikon use special gears for the focus ring to overcome this problem?
02-09-2012, 03:38 PM   #14
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Something like different gear ratios for the motor specifically? I actually have no idea - I could look into that later.

Disregarding the lack of knowledge of how DC will perform on other lenses - I'd at least like to find out. It is unfortunate though, as I don't expect Pentax to just release a bunch of lenses in a short time soon.
03-02-2012, 02:49 PM   #15
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I just got my 18-135mm in the mail from B&H today and I must say... I'm kinda disappointed. The whole time I was waiting for it to be "super quiet" as described by many in the forum.. Compared to my DA* 55mm f1.4, it is noisy! In fact, it is SLIGHTLY less noisier than my 35mm f2.4... did I get a dud?

It sounds similar to this video:

Now granted, I probably won't use it in situations where I'm focusing and someone turns around like "what the heck was that oink oink sound" (church, wedding, indoor candle light vigil), but to me this lens is unacceptably loud.... Maybe my expectations were to high? My wife thought I was crazy for a second because she thought it was quiet but started to agree when I mounted my DA* 55mm....

Does anyone else agree with me?
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