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. To quote the website:
Most single-lens reflex cameras use an autofocus method called the "phase detection system." Using a separator lens in autofocus module, this system produces two images from the image information of the subject captured through the lens. It then measures the distance between those two images using a line sensor, and detects defocus amount. If the focal plane is in front of the camera lens relative to the image surface (front focus), the distance between the two images will be narrower than the optimum value. If the focal point is behind the subject (rear focus), the distance between the two images will be wider than the optimum value (Fig. 1). In other words, if the distance between the two images is narrower than the corresponding distance in an in-focus condition, the autofocus system determines that the focal point is in front of the subject. If the distance is wider than the reference value, the system determines that the focal point is behind the subject. Then, the autofocus system computes instantaneously how much the lens position should be moved and in which direction, and moves the lens accordingly. This focusing operation is extremely fast.
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. [link ]
* * *
Too be honest my faith in Canon (Not Cannon) was completely lost when I bought my 40D... I seem to have non stop problems with the camera, here is a list of things that have gone wrong.
-When I let the camera choose the focus point, my fence is in focus and not the bird.
-When I set it to Auto Mode or Sports Mode, my pictures still come out blurry even if I try to push the button faster...
-When I tested the focus on a paper with lines on it, the lines weren't in focus, I even tried it manually by setting the white balance to shade, and shutter speed to 1/10s but it is still blurry, I tried fixing the focus with setting the camera sharpening to 7 but it still has blurry pictures. Clearly it is back focusing and a bad camera... [link ]
* * *
After many failures at Canon Service to calibrate my 350d focus I reached the conclusion that it is impossible to have it calibrated.
I continuously moved from strong back-focus to strong front-focus (as it is today).
Never inside the deep of field, with all the 3 lenses I have.
For this reason, I'm thinking to the possibility to buy another camera, rather than sending again my 350d for calibration (after 7 times).
Actually I don't need another camera. I just would like to solve my focus issue. [link]
So you see, DSLRs may have faster autofocus, but the autofocus system on compact digital cameras is more reliable.