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08-18-2013, 06:26 PM   #1
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Screw-drive Auto-focus Noise Source

The latest camera review here (the K-500, by Adam) once more brings the topic of noisy screw-drive AF to the fore, and it seems to be noted frequently in lens discussions. Reviewing various threads here, opinions on the source of noise seem to vary between the body and the lens, though with different emphasis, depending on which body and which lens is discussed. Some objective data would help, and wouldn't be difficult for anyone with a sound-level meter to set up, provided a good variety of bodies and lenses were available to test. Even smart-phones with sound-level apps could be pressed into service, but only for relative measurements, as calibration is more problematic for these devices.

The noise probably comes from both sources, with mechanical noise arising within the lens and the body, and motor noise (probably magneto-strictive, which would account for the higher-pitched squeal) from the body. Mechanical noise would be more likely to arise from the gear trains and bearings in lenses, due to manufacturing tolerances allowing gaps in both, and imperfections in the gear profiles. My small experience with dismantling lenses indicates that gear trains tend to be injection moulded plastic, including shafts, which is perfectly reasonable from a manufacturing viewpoint, as they can be produced cheaply and with adequate strength, but with widening variation on accuracy as the production moulds wear. I haven't dismantled a body drive unit, so I don't know if there is any gearing used at that end of the drive.

All this is academic, of course, as I doubt anyone will be modifying their camera bodies or lenses to reduce AF noise, but it probably counts for greater relevance in extending the boundaries of user knowledge than much of the debate here. No offense intended, naturally. So, if anyone does have some knowledge or willingness to devote a little time and effort in identifying more precisely the source of the noise that irritates some of us, please feel free to contribute.

08-18-2013, 06:35 PM   #2
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The reason why I think it's mostly the lens and not so much the body is because when I use my Tamron 70-200, it is very loud, but when I used my DA 21 Ltd, it is pretty quiet. Given the same motor, I can only attribute the difference due to the lens - unless the Tamron 70-200 also takes more torque to focus and increases the load of the camera motor.
08-18-2013, 08:13 PM   #3
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I also wonder if there is a difference in the screw drive motors between cameras. My K100 is always loud. The first thing I noticed when my K20 arrived was how quiet it was. My K5 extended the tradition by being quiet.

08-18-2013, 08:51 PM   #4
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Thanks for your comments. My experience is similar, having owned (still own, actually) an *istD, K20D and K-5. I don't have a calibrated sound-level meter, but I might have a shot at comparing the iPhone app readings from all bodies with my range of AF lenses, which should give some comparisons, at least. I don't expect too much from the iPhone microphone, in the way of linearity and saturation levels, let alone noise isolation, but seeing we're not talking about very loud noise, it might be possible to detect some differences. I used to have access to calibrated equipment and quiet rooms, but that was in another working life.

08-19-2013, 03:06 PM   #5
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I don't really care much about AF motor noise since i've yet to have any negative experiences from screw drive noise with the shooting I do. I would imagine that the weight (and inertia) of the focusing elements and in-lens drive train have a lot to do with the actual sound. The relative strength of th motor will also have some effect, as will motor isolation in the body and any sound insulation.

I have two Tokina AF lenses -- an 80-200 f2.8 AT-X Pro and an 80-400 AT-X and both are not only loud, but I can also feel the camera/lens torquing in my hands when AF is activated, which doesn't happen with my other big AF glass. I always assumed that they had the heaviest focusing element groups and maybe even carried their "tank-like" build quality to possibly using brass gears in the drive train (never took one apart to see though).

Scott
08-19-2013, 04:35 PM   #6
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That's an interesting observation, thank you. The power a motor delivers depends on the load applied and its own characteristics, so the Pentax in-body AF motor is clearly more powerful than it needs to be to drive their own lenses, and noise tends to increase with load, all of which fits with the observations made in this thread and some others.

Edit: it would be interesting to know what materials are used for the gear trains in various lenses. I would expect that brass or zinc would be noisier than synthetics, all other things being equal. Deflection under load would cause gears to move away from their ideal meshing positions, so the quality of the bearings and support structures would be an influence, too.
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