Aperture Area

The amount of light captured by a lens is proportional to the area of the aperture, equal to:

Where f is focal length and N is the f-number.

The focal length value is not required when comparing two lenses of the same focal length; a value of 1 can be used instead, and the other factors can be dropped as well, leaving area proportion to the reciprocal square of the f-number N.

If two cameras of different format sizes and focal lengths have the same angle of view, and the same aperture area, they gather the same amount of light from the scene.

The relative focal-plane illuminance, however, depends only on the f-number N, independent of the focal length, so is less in the camera with the larger format, longer focal length, and higher f-number.

A big (1) and a small (2) aperture