exposure
Relationship between aperture and f-number
The reasoning behind the values given to the f-stops can be explained by the following example:
~ The diameter of a selected aperture on a standard 50mm lens for a full frame digital SLR
such as the Canon 1Ds or Canon 5D measures 12.5mm.
~ This measurement divides into the focal length of the lens (50mm) exactly four times.
~ The aperture is therefore given the number f4.
~ The diameter of another selected aperture on the same lens measures 6.25mm.
~ This measurement can similarly be divided into the focal length of the lens to give
an f-number of f8, thereby explaining why the higher f-numbers refer to the smaller
apertures.
The diameter of the same f-number may vary for different focal-length lenses but the intensity of
light reaching the image plane remains constant. F4 on a long focal-length lens is physically much
larger than the same aperture on a shorter focal length lens. The intensity of the light striking the
image sensor is the same, however, due to the increased distance the light has to travel through
the longer focal-length lens. See ‘Inverse Square Law’ on page 97.
ACTIVITY 2
Take exposure readings of a subject in bright sunlight and in deep shade.
Record all of the different combinations of exposure (shutter speed and aperture) for each
lighting condition.Mark Galer