Sensor Format
The physical dimensions of a camera's sensor, which set its angle of view, depth of field characteristics and light-gathering area.
Formula / rule
Area ∝ width × height
Measured in
mm
In depth
Format is the most consequential specification in a camera system, because everything else scales from it: lens size, depth of field at a given framing, and total light captured. A larger format gathers more light for the same exposure time and f-number, which is why the noise advantage of full frame over Micro Four Thirds is roughly two stops.
Examples in practice
Full frame measures 36×24 mm, APS-C about 23.5×15.6 mm, and Micro Four Thirds 17.3×13 mm — a four-fold difference in area from largest to smallest.