These days many digicams have pretty fast CDAF. The KM A2 was pretty fast too. The Panasonic GH-1 is also pretty fast. Why is the CDAF in DSLRs generally so slow?

Planned obsolescence?

Because the lens is not designed for CDAF. The E-mount lenses have an entirely different type of focus motor, which is capable of the cyclical refined oscillation towards a target contrast after a peak is detected - sine wave with reducing amplitude until a stable flat wave is locked (in graphical terms). Regular AF motors can't do this at all, and have to do an elaborate cycle of detection and target actions.

The best camera to experience this visually is the Pentax K-5. It can do very accurate CDAF with regular lenses, but it takes about 2 seconds and the visual firework display accompanied by focusing sounds is impressive.

First, it traverses the entire focus range of the lens to estabish the peak contrast (one cycle). Then it moves to that approximate distance, and focuses back and forth one time over a shorter range to refine the peak contrast. Finally it does a little wiggle which fixes the locked AF.

David

Okay, that is a really good explanation. I have seen this sort of question come up several times on forums, but no one has ever answered it. Just lots of guessing and speculation and none of it sounded credible at all to me. I am glad to know. The next time I see someone ask about I will point them to this.

So....both K5 and D7000 have this high speed CDAF...
How does this compare against A55's AF?

There's no comparison. The D7000 with the latest AF-S lenses may be able to achieve something (the Pentax definitely struggles). But the A55 can follow action during video with any Alpha mount AF lens, because it is not CD-AF.

The A580 has a kind of CD-AF which detects whether an SSM/SAM lens is fitted. If one is, it will do a fairly normal CD-AF with some attempt at tracking, but this doesn't seem to work with my Sigma lenses (maybe needing a rechip or reprogramming) even if they work with the NEX and LA-EA1. If you fit a screw drive lens, it immediately detects this and uses a phase detect cycle which only works before the shot (no C-AF during video for example). This cycle is much, MUCH faster than Canon's Live View AF mode which switches to phase detect. It seems almost instant.

David

Sorry to reply in this old thread, but I happen to know the worst reason why DSLR CDAF is so slow. It's the shallow DOF. Too little contrast to analyse while away from focus.

Then the native Nex lenses are stopped down during focusing as the available light permits, thus increasing the DOF for CDAF efficiency. Hence they focus pretty fast. The same Nex if used with adapted SAM/SSM does not stop the lenses down while focusing, making the CDAF abysmally slow. Same goes for the DSLR CDAF in general.

So the main reason for slow CDAF is with the ancient aperture control routines in the DSLRs.

But doesn't this decrease accuracy? If your object is in focus at lets say F8, it might not be at F4, when you take the pic.

The worst effect of focus shift happens with regular PDAF that always focuses at lens' largest aperture

So the answer to your question is - yes, it does, but less than other systems do.

P.S. But sure the lens should not be stopped too much, the system needs to stop it down only till there's enough contrast for CDAF to analyse effectively and only if the light permits.