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Why is AF so slow in Live View on SLRs ? On compacts, the AF works very smoothly and quickly.

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You could see this question for more information: photo.stackexchange.com/questions/13880/… –  Kiril Kirov Jul 12 '11 at 12:30
    
And see also How does auto-focus work? –  mattdm Jul 12 '11 at 13:05

3 Answers 3

On a DSLR the mirror is up when the camera is in live view, blocking off the AF sensors which are located away from the lens-sensor axis. AF in live view is usually handled with contrast detection, which is much more processor intensive. Compacts are designed to focus when in live view mode, so their mechanism is optimised for that particular setup, which is the same when shooting video or stills.

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In my camera the focus sensors are not located in the pentaprism, but behind the mirror... –  Guffa Jul 12 '11 at 11:55
    
Which camera is that? –  Nick Miners Jul 12 '11 at 11:56
    
of course compacts are always slower than slrs, they just feel faster because of the lower weight which makes holding a dslr with a big lens at arm's length rather cumbersome :) –  jwenting Jul 12 '11 at 11:59
    
@Nick Miners: Canon EOS 5D mk II. I think that it's common for Canon SLRs. –  Guffa Jul 12 '11 at 12:22
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@Guffa - just to make it clear, even though the sensor is behind the mirror, that doesn't mean it is exposed to the image when the mirror is up. Actually it is below the mirror. The mirror is a beam splitter, and a portion of the light is deflected down to the sensor.Thus it does not "see" the image when the mirror is up. –  ysap Jul 12 '11 at 12:36

A typical compact camera uses a smaller sensor, which allows for shorter focal lengths and deeper depth of field for similar field of view and f-number. Therefore, a compact camera has to move its lens much less and also needs less precision for achieving acceptable focus.

Also, expectations for compact camera focusing speed are lower than for dSLR, so slowness is less noticeable.

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In Live View the mirror is folded up, so there is nothing that sends any light to the regular focus sensors.

Instead the focusing is done by analysing the details of the image before and after moving the focus a bit (as analysing a single image doesn't tell you if the focus is right, or if it is behind or in front of the current setting). This means that the camera has to adjust the focus back and forth a lot to find where there is most detail.

Simply, the image sensor is used to do the focusing, and as it's not constructed specifically for that, it gives much less usable information than the regular focus sensors.

Compacts are always in live view mode, so to speak, so they are constructed with that in mind. They may also have other focus sensors than the image sensor.

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I am not convinced that a P&S is more optimized than a DSLR for contrast-detect focus. I find it hard to believe that, say, Canon mastered the AF on their compacts but declined from using the same algorithm in the DSLR (BTW, of the cameras I owned, Canon's P&S AF really sucks) –  ysap Jul 12 '11 at 12:40
    
@ysap: However you look at it, more resources will always be put in the primary focusing method, even if only to make it seem faster... :) –  Guffa Jul 12 '11 at 13:39

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