Every camera lens has an opening that allows light in and onto the film/image sensor. This opening is called the ‘aperture’. The amount of light entering the camera by increasing or decreasing the size of the aperture. An f-stop is a fraction that indicates the diameter of the aperture.
f = focal length of the lens
/ = divided by
Therefore the diameter of a 50mm lens with f/1.4 would be 35.7mm.
The hole on vari-focal lenses is created by 6 overlapping blades. Twisting the lens moves theses blades either closer together or further apart, enabling the photographer to alter the diameter (aperture) of the lens. The widest aperture is noted by a small f/stop. Therefore as the hole in the lens decreases the f/stop increases. Subsequently, as you decrease from one aperture to the next, such as f/4 to f/5.6, the amount of light entering the lens is halved. Therefore increasing the size of the lens opening by a full stop doubles the amount of light.
Peterson (2004, p. 32) has categorised the various apertures depending on the creative exposures they can produce. These are:
- Small apertures (f/16, f/22 and f/32) create story telling exposures, images that show a great dof.
- Large apertures (f/2.8, f/4 and f/5.6) produce singular theme or isolation exposures.
- Middle apertures (f/8 and f/11) are used when dof is not a concern.
- Close-up, macro