How do ultrasonic motors allow continuous manual focus?

Question

I understand the basic pros and cons of ultrasonic lens motors — called USM by Canon, SDM by Pentax, or SWM by Nikon. One of the chief benefits is that they allow "always on" manual focusing — you don't have to disengage the autofocus motor in order to make manual tweaks.

Many Pentax lenses using the traditional non-SDM have drives have a quick-shift clutch mechanism which accomplishes largely the same effect, but this is apparently different. Is the difference here something to do with the motor itself (and something to do with its "ultrasonic" nature), or is it that the gearing used by this type of motor is special?

It's my understanding that some ultrasonic motors are ring-type, but that others, including the one in the DA★ 200mm lens I'm testing, are "micro-motors". Both seem to have this same beneficial property of allowing manual focus without a switch, which leads me to wonder: would it be possible to use an ultrasonic in-body motor and get basically the same benefit — or does it not work that way?

Asked by mattdm

Answer

The "gearing" is special, in that it is essentially non-existent, at least between the motor and the main mechanical drive for the focusing mechanism. In order for a traditional (low-frequency or DC-driven) syncromotor to have enough control fineness (or even control finesse), it needs to be geared down significantly. The mechanical forces involved in gearing down are slight, but working backwards against the system (manually focusing while the motor is trying to drive the system) would mean applying enough torque to damage gears, bushings/bearings, and probably strip the mechanical coupling between the gears and shafts, etc. (Not to mention that if there's a worm drive in the system anywhere, it won't work backwards at all.)

High-frequency systems (and systems with enough poles, even if they operate at lower freqs) can be directly controlled with a lot more fineness because the phase angles involved can be larger for shorter physical intervals. Because they don't need to be geared down, there's no extra mechanical stress on any of the components when you physically oppose the motor.

I don't think it would be viable to have direct drive from the in-body focus motor. The screw drive would need to run at a very different speed for a lens meant to be driven directly at or around 1:1 gearing than it would for an older, geared-down lens. (I don't have a Pentax to play with, but my Nikon screw-drive lenses seem to be at about a 8-12:1 drive rate.) If the motor has enough torque to drive a 1:1-geared lens effectively, then it's probably got enough torque to strip the drive screw on an older lens (or if the driver and receiver are hard enough not to strip, then to destroy some even more expensive parts, like the attachment points on lens barrels or body castings).

Answered by Stan Rogers

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Can I save money on lenses by buying a Nikon camera with an in-body autofocus motor?

Question

I have dilemma what camera is better Nikon D5100 or D90. The first one is newer but the second one has lens servo ("screwdriver") motor. My question is: can I save some money overall with the more expensive camera, by buying lenses without lens-based AF motors?

Asked by garik

Answer

If money is the only consideration and if you plan to buy many lenses, I'm sure the answer is yes. The newer AF-S lenses are more expensive than the AF-D predecessors, or the AF lenses before those.

If price is the only consideration, you might forego AF altogether and get some old MF Nikkors. They're often cheaper still.

I have trouble endorsing the direction this thought process leads, though. AF-S lenses often offer more than just internal motors: better optics are pretty standard. I while back I pulled out an old 35-80mm f4-5.6 AF-D "kit" lens and shot with it a little on my D300 and was amazed at how poor the optical quality of it was as compared to the 18-55mm AF-S "kit" lens. It wasn't bad, but the 18-55 was clearly better. I'm sure that there are many great AF-D lenses, but I suggest that you'll really need to do your research to be sure you're getting something worthwhile.

Of course, the AF-S lenses offer two other great benefits, too: generally faster AF, and much quieter AF.

Answered by Dan Wolfgang

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What is an auto-focus motor?

Question

I recently bought a Canon 550D, and I read it has an auto-focus motor, which not all cameras possess. I'd like to know what it is, and what I can do to keep it running smoothly; AFAIK the manual doesn't come with instructions on how to improve the life of the focus motor.

Asked by the.midget

Answer

An auto-focus motor is just what it sounds like: a small electric or piezoelectric ("ultrasonic") motor which moves the lens elements to facilitate autofocus.

In some camera systems, this motor is in the camera body, and the lens moved by a physical coupling. However, you actually read some misinformation. Canon EOS cameras do not have a motor in the body, and instead, all lenses have a built-in focus motor.

With Pentax, all auto-focus camera bodies (to date) have a motor, and some lenses have their own. (In that case, the lens motor is used.) That used to be the case with Nikon too, but in the last few years (starting with the D40), the entry-level Nikons have dropped this. That's a big deal, because it means Nikon AF lenses without a motor don't autofocus. As Nikon refreshes the lens lineup, this will be less of an issue (except if you have or want to obtain older lenses and have them autofocus) — lenses like the AF-S DX NIKKOR 35mm f/1.8G didn't exist when the Nikon D40 came out, and that's part of why the Internet was all in furor about this.

So, you often see "no focus motor" as a disadvantage for entry-level Nikon cameras. This leads to the incorrect but somewhat widespread assumption that the natural opposite (for cameras for which this is not an issue) is "has focus motor". That's technically incorrect, but the practical point is that for your camera, the lack of a focus motor is a non-issue.

And also, since there isn't one, there's nothing to worry about for maintenance. Focus motor failure isn't one of the big, highly-reported problems of cameras which do have them, and I don't think there's any particular recommended maintenance.

Answered by mattdm

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