# F stop = Illuminance (quantity of light) ?

Started May 27, 2012 | Discussions thread
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 F stop = Illuminance (quantity of light) ? May 27, 2012

Hi All,

As we all know F stop is the ratio of lens focal length to the diameter of the lens (entrance pupil). F stop = f/D.

This bit is from wikipedia -
"

A 100 mm focal length lens with an aperture setting of f/4 will have a pupil diameter of 25 mm. A 200 mm focal length lens with a setting of f/4 will have a pupil diameter of 50 mm. The 200 mm lens's f/4 opening is larger than that of the 100 mm lens but both will produce the same illuminance in the focal plane when imaging an object of a given luminance.

Does this mean, for an example -

If you compare four-third and 1/1.7 sort of a sensor with same megapixels (say 10Mpixels) and same sensor technology etc,. and use the same F stop (say F/1.8). Regardless of the focal length in use (zoom) both senors will be exposed to same quantity of light (luminance ? ).