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05-22-2013, 09:13 AM - 1 Like   #1
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SDM vs Screw Drive, Why is only one of them silent?

Is there an inherent difference between the SDM motor in a lens and the screw drive motor in a camera body? If there is no difference, than why is one much louder than the other? If there is a difference, than why wouldn't Pentax put an SDM motor inside the camera body to make it more quiet?

05-22-2013, 09:21 AM   #2
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SDM motors are in-lens motors. They are newer and designed to be silent motors.
The screw drive is an in-body motor from the film age and was kept in all Pentax cameras since then. This allows Pentax dslr users to use the old AF lenses without any problems.
05-22-2013, 09:27 AM   #3
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QuoteOriginally posted by JinDesu Quote
SDM motors are in-lens motors. They are newer and designed to be silent motors.
The screw drive is an in-body motor from the film age and was kept in all Pentax cameras since then. This allows Pentax dslr users to use the old AF lenses without any problems.
I am aware of this. I My quest is if one is newer tech and more quiet, than why not put a newer and more silent motor in the body? Pentax put SR in the camera, effectively stabilizing all old lenses. A quiet in-body motor would do the same in effectively silencing all old auto focus lenses.
05-22-2013, 09:30 AM   #4
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The loudness comes at least partially from the positioning in the body. You need more transmissions and have longer distances to bridge to reach the focus system in the camera. Therefore more possibilities for vibrations.

05-22-2013, 09:32 AM   #5
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QuoteOriginally posted by geomez Quote
I am aware of this. I My quest is if one is newer tech and more quiet, than why not put a newer and more silent motor in the body? Pentax put SR in the camera, effectively stabilizing all old lenses. A quiet in-body motor would do the same in effectively silencing all old auto focus lenses.
I don't think the motor itself is loud. I think the combination of the motor, linkage to the lens, and the movement of the focus mechanism in the lens is what makes things so loud.

Older AF lenses are a lot louder than newer DA lenses for example
05-22-2013, 09:32 AM   #6
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Nikon leaves the screw drive out of the D3200 & D5200, that means they can't use non AF-S lenses. This goes all the way back to the D40. By having a screw drive Pentax has full compatibility across its lineup. Also, screw drive lenses are far less likely to fail than lenses with SDM.

The big complaint around here for years has been that Pentax doesn't allow cameras to override the SDM and use screw drive in the event of SDM failure...

Why Camera Lens have "Ultrasonic" or "Wave" On Them | The Learning Lensman

Last edited by boriscleto; 05-22-2013 at 09:37 AM.
05-22-2013, 09:36 AM   #7
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Much of the loudness comes from the gears and linkages in the lens. The in-body motor is probably much stronger than the mini-SDM motors as well (as it should be), and so is just naturally louder...
05-22-2013, 09:51 AM   #8
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There is a set of gears inside srew-driven lenses that connects to a "screwdriver" on the body mount (the motor is in the body). The sound is caused by the gears as they turn. SDM on the other hand, uses a "ring-type motor" which is on the lens and this allows a design with a quieter mechanism. To power up this ring motor, the lens uses two special contacts located on the body, near the mount.
So, it does not make sense to put SDM inside the body.

here is an image of the screw-drive mechanism inside a lens:
http://3.static.img-dpreview.com/files/w/TS1600x1600?url=http%3A%2F%2Fkrg.pa...NlB4ElBM07GtY=

part number 5 is what you see on a lens mount (if you turn the focus youŽll see how the screw turns)
In my opinion, screw drive is pretty cool... Has few and simple parts so it is very unlikely to brake and it is very light and small

05-22-2013, 09:59 AM   #9
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I've never heard of the in-camera motor breaking -- anybody had that happen? Maybe in the case where someone forced the focus ring when they shouldn't have (on a lens that doesn't allow "quick-shift" adjustments)?
05-22-2013, 10:02 AM   #10
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QuoteOriginally posted by geomez Quote
I am aware of this. I My quest is if one is newer tech and more quiet, than why not put a newer and more silent motor in the body? Pentax put SR in the camera, effectively stabilizing all old lenses. A quiet in-body motor would do the same in effectively silencing all old auto focus lenses.
As others have said, the screw drive motor itself isn't that loud. It's the linkage with the lens, and the lens driving mechanism. For example, the DA 21 Ltd is very quiet on my camera. My Tamron 70-200 on the other hand, is very loud.
05-22-2013, 10:35 AM   #11
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QuoteOriginally posted by geomez Quote
Is there an inherent difference between the SDM motor in a lens and the screw drive motor in a camera body? If there is no difference, than why is one much louder than the other? If there is a difference, than why wouldn't Pentax put an SDM motor inside the camera body to make it more quiet?
SDM motors are in the lens, and are , as far as I know, direct drive, with no gears between the moving elements and the motors. In body motors all contain gears as do the lenses, gears make noise.
05-24-2013, 05:59 AM   #12
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It sounds like a linkage/transmission rather than an electric motor. I think it only really sounds loud because it's up near your head!
Anyway, it's probably something which could be replaced with a motor without linkages, but I don't think there's space in the camera right at the lens mount area - and if a bit of noise is the tradeoff for durability, I'll take it.

I will admit the only motor-driven lens I have, the 18-135 is really nice to use, it seems very fast and quiet and makes me with a few of my other lenses were built like this.
05-24-2013, 08:34 AM   #13
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Noise of any of these drives is coming mainly from gears. There is obviously more gear transmission needed if the motor is in the body. Silently running gearboxes need perfect evolute forms of the teeth, and extreme low tolerances in the calculated distance of the teeth to each other.

From my professional experience in advicing QC of gear production, I would estimate that the form error of the evolute of a tooth flanc should in no location exceed 1/1000 of a millimeter, and the calculated distance likewise. Mass production of such gearboxes would be possible, but prices would explode. To keep such tolerances for a longer time, also the teeth would have to be of hardened steel, and grinded after the hardening process. I am afraid the production cost would make the AF mechanism more expensive than the production of the lenses.

Noise of any gearbox comes from scratching of a tooth flanc along the surface of the neighbouring tooth. With a perfect gearbox, teeth would NEVER scratch when transmitting power, but just touch each other along a defined line (rolling along like a cylindric shape on a table).
07-16-2016, 07:45 AM   #14
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I realize this is an old thread, but I have relevant info LOL.

Buying and trading gear, I temporarily had a Nikon D90 with a Tamron 18-250mm lens, and a Pentax K-R with a Tamron 18-250mm lens. Both lenses are A18, identical, both are AF screw.

(Tamron also has an 18-250 lens with a motor inside for D40, etc)

OK, I can tell you that between the K-R and the D90, the same AF lens, the K-R is at least 3X louder when focusing, if not more.

Let's stop fooling ourselves, the Pentax AF is louder than competitors - it may be in an effort to sell more high end lenses, or to cut costs, but the result is a loud AF.



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