[Rol]

I have very little knowledge in AF and DSLR structure.

As I know, the light come from Lens is not all go to optical viewfinder, part of light is reflected to the detector for AF and expo metering. So some people guess that D70 / 50 with smaller viewfinder because it take more light to AF detector for faster and more accurate focus.

Pls correct me if any wrong.

[walkingfuture]

All DSLR (SLR) has a seperated sensor working as an AF sensor and metering sensor. Just at the top side.
So, it's totally different from normal non-DSLR DC, non-DSLR DC will do all the things (AF, metering, taking photo..etc) on the same sensor.

[akirabbq]

The AF sensor are NOT on the top side, it is at the bottom under the mirror. However metering sensors are on the top side, behind one of the sides of the pentaprism/mirror.

Believe it or not, the main mirror is semi transparent, so some light can pass through the main mirror. There is a second mirror behind the main mirror, which reflects light to the AF sensors at the bottom.

DC uses CCD for live preview, AF and metering can be computed by live data fed from the sensor. When a DSLR has its mirror up, there is no way to meter and AF, except using the same method as a DC, but this will increase the cost of the DSLR. That's also why E330 can not do AF when using LCD to preview.