Sensor Cover Glass
The protective glass stack in front of the sensor, incorporating the infrared cut and any low-pass filtering.
Measured in
mm
In depth
The stack's thickness is an optical element in its own right: rays converging steeply must pass through it, and a lens designed for a thin stack can perform poorly behind a thick one. This is why some adapted rangefinder wide-angles show smeared corners on cameras whose stacks are thicker than the film-era air gap they were computed for.
Examples in practice
A classic 21 mm rangefinder lens shows smeared corners on a thick-stack body and clean ones on a thin-stack camera.