What is sensor resolution? It is not the number of photosites on the sensor, that is just photosite count. In reality sensor resolution is a measure of density, usually the number of photosites per some area, e.g. MP/cm2. For example a full-frame sensor with 24MP has an area of 36×24mm = 864mm2, or 8.64cm2. Dividing 24MP by this gives us 2.77 MP/cm2. It could also mean the actual area of a photosite, usually expressed in terms of μm2.
Such measures are useful in comparing different sensors from the perspective of density, and characteristics such as the amount of light which is absorbed by the photosites. A series of differing sized sensors with the same pixel count (image resolution) will have differing sized photosites and sensor resolutions. For 16 megapixels, a MFT sensor will have 7.1 MP/cm2, APS-C 4.4 MP/cm2, full-frame 1.85 MP/cm2, and medium format 1.1 MP/cm2. For the same pixel count, the larger the sensor, the larger the photosite.
It can also be used in comparing the same sized sensor. Consider the following three Fujifilm cameras and their associated APS-C sensors (with an area of 366.6mm2):
- The X-T30 has 26MP, 6240×4160 photosites on its sensor. The photosite pitch is 3.74µm (dimensions), and the pixel density is 7.08 MP/cm2.
- The X-T20 has a pixel count of 24.3MP, or 6058×4012 photosites with a photosite pitch is 3.9µm (dimensions), and a pixel density is 6.63 MP/cm2.
- The X-T10 has a pixel count of 16.3MP, or 4962×3286 photosites with a photosite pitch is 4.76µm (dimensions), and a pixel density is 4.45 MP/cm2.
The X-T30 has a higher sensor resolution than both the X-T20 and X-T10. The X-T20 has a higher sensor resolution than the X-T10. The sensor resolution of the X-T30 is 1.61 times as dense as that of the X-T10.
Sometimes different sensors have similar photosite sizes, and similar photosite densities. For example the Leica SL2 (2019), is full-frame 47.3MP sensor with a photosite area of 18.23 µm2 and a density of 5.47 MP/cm2. The antiquated Olympus PEN E-P1 (2009) is MFT 12MP sensor with a photosite area of 18.32 µm2 and a density of 5.47 MP/cm2.