Why is the aperture always written with a slash between the âfâ and the number – e.g. f/8 ?
An aperture is a hole in the lens. For each aperture number, this hole has a corresponding diameter, which is different for each lens focal length.
The longer the lens gets, the less light can pass through. In order to achieve consistent exposures between lenses of different focal lengths for the same aperture number, the size of the hole needs to get bigger on longer lenses.
Thus the aperture is expressed as a ratio of focal length to aperture number. This ratio expresses the diameter of the hole – i.e. f/8 on a 100mm lens means the hole diameter will be 100/8 = 12.5mm.