>>3715943Basically: Flange focal distance, which is the distance between the camera mount and the sensor. A short focal flange distance means that the back of the lens (blue oval in this shitty ass diagram i made) is very close to the sensor. Sony cameras have a very short flange focal distance, which means that light hits the sensor at a pretty extreme angle on the edges. Cameras with longer focal flange distances have light hitting the sensor at a gentler angle and thus with better optical quality at the edges of the frame. That extended distance comes at the expense of compatibility with other lens systems and at the expense of size.
