Pixel Density
Pixel Density is a calculation of the number of pixels on a sensor,
divided by the imaging area of that sensor. It can be used to
understand how closely packed a sensor is and helps when comparing two
cameras with different sensor sizes or numbers of photosites (pixels).
Because the light collecting area and efficiency of each photosite will
vary between technologies and manufacturers, pixel density should not be
used as a predictor for image quality but instead as a parameter to
help understand the sensor.
Diagram comparing some common sensor sizes
The APS-C sensors used in most modern DSLRs
have an area of approximately 3.5 cm², while the 1/1.7" and 1/2.3"
sensors commonly used in compact cameras have areas of 0.43 and 0.29
cm², respectively.
To get some idea of what this means, here is a
diagram representing a pixel density of 28 MP/cm² (the pixel density of
the Canon G9). As you can see, this density equates to 12 MP on the G9's
1/1.7" sensor but would be 91 MP if applied to a sensor as large as the
one in a Canon 450D.
The calculation is based on the number of pixels produced at the camera's native resolution (Effective pixels),
so both for conventional Bayer sensors and Foveon type, one photosite
is considered equal to one pixel in the final image. For Fujifilm's Super CCD SR
technology, each photosite contains one 's' and one 'r' photodiode but
contribute only one pixel to the final image, so are classed as a single
pixel.
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