Photosites on image sensors come in different sizes. The size of a photosite on a sensor is based on the size of the sensor, and number of photosites on the sensor. Some sensor sizes have differing sizes of photosites, because more have been crammed onto the sensor. However different sensor sizes can also have the same sized photosites. For example the Olympus E-M5(II) (16.1MP) has a photosite size of 13.99 µm², and a Fujifilm X-T3 sporting 26.1MP has the same photosite size.
The size of a photosite, is often termed pixel pitch, and is measured in micrometres (or in old terms microns). A micrometre, represented by the symbol µm, is a unit of measure equivalent to one millionth of a metre. It is equivalent to 0.001mm. To put this into context, the nominal diameter of a human hair is 75µm. The area of a photosite is represented by µm². For example, the Olympus E-M5(II) has a pitch of 3.74µm, or 0.00374mm, which is 20 times smaller than a human hair.
In order to increase the number of photosites a sensor has, their size has to decrease. Consider an example using a Micro-Four-Thirds (MFT) sensor. An Olympus OM-D E-M5 Mark II fits 16.1 million photosites onto the sensor, whereas an Olympus OM-D E-M1 Mark II fits 20.4 million. This means the pixels on the E-M1(II) will be smaller. This works out to a pixel area of roughly 13.99 µm² versus 11.02µm². This may seem trivial, but even a small difference in size may impact how a photosite functions.
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