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Do all autofocus cameras need motors to drive the lens? I would have thought today's modern electronics rendered motors obsolete.

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3  
I'm unclear if the question is "are AF motors needed to AF a lens" or if the question is something like "why don't we have mag-lev based AF"? –  Dan Wolfgang Aug 2 '12 at 22:19
    
    
Is the question really "Do the camera need a motor for AF-lenses to work?" ? –  Håkon K. Olafsen Aug 3 '12 at 7:01
    
Simon - Did you mean (1) "For cameras where the lens elements are moved to achieve AF ..." OR (2) can AF be achieved with no lens element movement? Most answered (2). I answered mainly (1) –  Russell McMahon Aug 3 '12 at 8:47
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Yet another possible sense of the question is if the motor has to be in the camera, which i think is the case with some Nikon's. Most DSLR and mirrorless do not have a motor in the body, but the lens will do. –  Jahaziel Aug 3 '12 at 16:22

5 Answers 5

Traditionally lens elements must be moved in order to change the focus, as you must alter the path light takes through the lens, thus a motor is required to move the glass. However this will not always be the case.

Lytro recently produced the worlds first commercial light field camera. The idea is that this camera captures not only the intensity of the incoming light but a measure of the incident angle of different rays. This allows the effect of different focus settings to be calculated mathematically enabling you to "refocus" an image long after it was captured, using a computer. No motors or moving parts are necessary.

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+1 The state of the art, though, means that "light field" cameras require a huge number of sensels (data points) compared to the size of the final image and outboard processing (in fact, a Kodachrome-style single-provider processing API at the moment) to do the recalculation. That's likely to change over the near future (see the Nokia PureView and increasingly powerful low-voltage mobile processors), but we're still at the "not ready for prime time" stage today (and there aren't a lot of early adopters to drive the necessary R&D, so who knows what the future holds). –  user2719 Aug 2 '12 at 22:56

Other than Light Field cameras, movement for autofocus is needed. Some cameras have motors but many do not. The smaller Nikon, like D3100 do not have a motor for example. Instead, there is a motor in each lens. The bigger Nikon, like D4, have motor but also work with lenses having a motor.

So, it is not absolutely needed but is normally needed either in the camera or in the lens. The Light Field camera I have never seen but read that it is very restricted and actually has a focus motor because there is a limit to the possible focus change from Light Field.

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A theoretical light field camera wouldn't have that limit, but the real-world Lytro models today don't gather enough data. –  mattdm Aug 3 '12 at 4:29

There are several good answers and relevant comments already.
I'll address a possibly ambiguous point.

You said - "Do all autofocus cameras need motors to drive the lens?"
Others have explained that a "Light field" or Plenoptic camera (Wikipedia) does not need to move the lens at all to alter focus, so does not need focus motors as nothing moves.

However, you also said " ... I would have thought today's modern electronics rendered motors obsolete."
You may have meant that perhaps some thing other than a motor may be used to move the lens elements in some modern cameras.
Loosely speaking, anything that provides mechanical movement can be regarded as being a motor. By that definition, if anything moves mechanically then a motor must be involved.

However, there are various means by which movement can be achieved with a "motor" that is different or very different from traditional ones. Not all of these are (yet) used in camera lenses.

A common motor which is included inside lenses is the SSM or USM = Supersonic Motor or Ultrasonic motor.This Wikipedia article gives a good description. An ultrasonic motor is often a subset of piezoelectric motors which use electrical constriction or expansion of a crystal ti impart movement. Wikipedia noites -

  • An ultrasonic motor is a type of electric motor powered by the ultrasonic vibration of a component, the stator, placed against another component, the rotor or slider depending on the scheme of operation (rotation or linear translation).

    Ultrasonic motors differ from piezoelectric actuators in several ways, though both typically use some form of piezoelectric material, most often lead zirconate titanate and occasionally lithium niobate or other single-crystal materials.

    The most obvious difference is the use of resonance to amplify the vibration of the stator in contact with the rotor in ultrasonic motors. Ultrasonic motors also offer arbitrarily large rotation or sliding distances, while piezoelectric actuators are limited by the static strain that may be induced in the piezoelectric element.

It is not essential to use piezoelectric action to produce an SSM / USM but this is usual.

Animated demonstration of SSM / USM

Superb short flash animations of piezo actuators in action .
This is the Minolta CCD antishake piezo actuator system at work rather than focus drive but demoinstrates the basic principals superbly.


Related patents here and here

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+1; good point. And now I'm imagining a steampunk camera with litle pneumatic actuators for moving the lens elements.... –  mattdm Aug 3 '12 at 14:19

You always need some means to move the lens, but it may not be a motor in a traditional sense. For example, you can reshape a liquid lens by applying a voltage across it, with no moving parts except the liquid.

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It is also possible that some micro cameras like those on cellphones or integrated webcams use electromagnetic focusing. This is the method used to focus some CD/DVD drive lenses, where part of the elements are mounted in such way that different voltage applied to a number of coils cause movement of the focusing element.

I Have seen this mechanism in action and in some cases they have a wide motion range compared to the size of the elements. The size of a typical CD optic pickup is rather bigger than the lenses found in cellphones.

From an electric design point of view, these mechanisms are more similar to a solenoid than to a motor.

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