The mirror being between the sensor and lens means that there is a dead zone where there can be no lens, which severely restricts lens designs. For example, fo

The mirror being between the sensor and lens means that there is a dead zone where there can be no lens, which severely restricts lens designs. For example, for many years fast normal lenses on SLRs were 58mm “Biotar” not 50mm because a a new kind of focusing (retro focus) had to be adapted to work with a mirror in the way.