Quote Originally Posted by Franglais View Post
The D600 sensor has the same dimensions as the D3X sensor (which was made by Sony). The A99 sensor dimensions are slightly smaller. But it might just be that they don't use all of the available surface.

Now my understanding of focussing on a traditional DSLR is that 100% of the light arrives on the mirror and is used for viewing or focussing (OK tell me that's not the case and just part gets diverted to a focussing unit somewhere else). When the mirror flips up 100% of the light is used for exposure.
In a traditional DSLR there are small semi-transparent areas (slits or crosses, I believe - hence the line or cross focusing points) on the main mirror and a small mirror behind it that transmits some of the light to the PDAF sensor on the bottom of the camera. Here is a diagram.

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Terry