aizan said:
doesn't it focus on what you point it at? i thought it worked just like a rangefinder spot.
You're referring to the Contax G1/G2? There are two fixed black brackets in the finder area. The AF system is supposed to focus on what's within those brackets.
But since the brackets occupy a certain area, they very often wind up covering parts of the subject (such as both the near eye and the far eye in a close-up portrait) that are at different distances, or part of the subject plus foreground or background.
In these situations the AF system has no way of knowing which target is the one you intended; it will focus on the one that produces the strongest signal on the AF sensor.
There's a distance scale (meters only) on the LCD display at the bottom of the finder. If you remember to check this, it can help you determine when a focusing error has occurred -- for example, if your intended subject is a person's head at 2 meters, and the scale reads 5 meters, it's a good bet that the AF system has focused on the wall behind him/her instead of on your intended target. Once you're aware of the error, you can try again -- either by using the focus lock to select a more "focusable" area of the subject, or by switching to manual focusing.
However, in fast-breaking situations it's easy to overlook the display, and it doesn't really solve the problem when the target distances aren't very different (such as the near eye/far eye example above.)
When I was operating the Contax G User Pages, I got a lot of emails from people lamenting the fact that they couldn't actually
see a direct indication of what it was on which the AF system had chosen to focus. Although with practice I got experienced enough with the Gs to anticipate and avoid possible focus errors, I still got a few of them myself, and I can understand why it would make many users feel insecure.