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Autofocus: DSLR vs. compact digital camera Anonymous
There’s a web page at the Nikon website which explains the difference between phase detection autofocus and contrast detection autofocus.

Contrast detection autofocus is pretty simple. The camera moves the lens around until the image hitting the sensor is in focus. In focus is determined by evaluating the contrast of the area within the focusing square. This is the type of focusing system used in all non-SLR digital cameras.

DSLRs use the much more complicated phase detection system.

http://www.nikon.co.jp/main/eng/portfolio/about/technology/nikon_technology/caf/index.htm

I can’t say that I understand all that, but what I get out of it is that phase detection autofocus requires separate hardware for determining focus, and that the separate hardware and the lens all have to be calibrated perfectly, otherwise the system will misfocus.

This explains why DSLRs are often plagued with focusing problems, while you never hear about such problems on compact digital cameras. With complexity comes the potential for more things to go wrong.

Here are some examples of messages at a popular internet photography forum complaining about focusing problems on Canon DSLRs:

"After two 40Ds and consistently soft focus, I sent my camera into Canon. I recently photographed an award ceremony on Capital Hill and every shot was soft. I never had this problem with my 20D. Occasionally, I do get sharp photographs out of the 40D, but rarely. I am using all L glass and am a true believer in Canon. I am anxious to see what Canon does to the camera. I see other posts with sample pics and they look so sharp. It can't be operator error all the time."
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>> Anonymous
cannon owners sure to retaliate.....
>> Vincent !!8LCSE0Zp1mL
no point to this post, if you are trying to say DSLR's suck because things can go wrong easier, then well you may be true, however when DSLR's do work (which 99% of the time new ones will) This point is moot.