The naming scheme for digital camera sensors is derived from old television camera sizes expressed as the diameter of the tube through which light enters to strike the sensor at the end of the tube. The sensor’s diagonal measurement is about one third less than that of the tv camera tube so camera sensor diagonal measurements are smaller than you might expect from their names.
Camera with a Four Thirds Sensor: Micro Four Thirds cameras are mirrorless.
Here are a few film and sensor dimensions, and their relative sizes compared to 35mm film and full frame digital sensors.
| Type | Format | Dimensions (mm) | Surface | Ratio to 35mm | |
| Width | Height | mm² | |||
| film | 4×5 | 121 | 97 | 11737 | 1358% |
| film | 645 | 56 | 42 | 2352 | 272% |
| Sensor | 645 | 43.8 | 32.8 | 1436.6 | 166% |
| Sensor | Full Frame | 36 | 24 | 864 | 100% |
| Sensor | APS-C | 22.3 | 14.9 | 332.27 | 38% |
| Sensor | four thirds sensor | 17.3 | 13 | 224.9 | 26% |
| Sensor | 2/3″ | 8.8 | 6.6 | 58.08 | 7% |
| Sensor | 1/1.6″ | 8 | 6 | 48 | 6% |
| Sensor | 1/1.8″ | 7.2 | 5.3 | 38.16 | 4% |
| Sensor | 1/2.7″ | 5.3 | 4 | 21.2 | 2% |