Photographers often ask why the autofocus sensors on full frame cameras are not spread out more for better coverage. This post explains the reason why that is not possible. It essentially comes down to the fact that it would require the optical viewfinder to be removed.

This diagram by Joseph S. Wisniewski (reproduced here with explicit permission) illustrates the reason:



The main mirror directs most of the light up to the optical viewfinder. There is a small portion in the center that is partially transparent. It lets some light through to a small, secondary mirror directly behind the primary mirror. The secondary mirror reflects light to the autofocus system. As you can see from the diagram, it is physically impossible to make the secondary mirror any bigger.

On the other hand, if the optical viewfinder was replaced by an electronic viewfinder, then the entire mirror could be partially transparent and reflect light into the PD autofocus system, and the AF sensors could be anywhere on the frame. I hope that such a design will occur soon.