Inside a lens is a ring of overlapping blades collectively known as an iris diaphragm, or iris. The expansion or contraction of the iris blades adjusts the size of the opening at its centre, which is called an aperture. Changing the size of the aperture controls the intensity of light passing through the lens.
The most fitting analogue for understanding the structure and function of a lens aperture is the eye. The iris of an eye regulates the retina’s exposure to light by either dilating or contracting the pupil. Inside a camera lens, a large aperture is similar to an enlarged pupil; it raises exposure by increasing the flow of light. A small aperture is like a contracted pupil; it reduces exposure by decreasing the flow of light.
F‑Numbers and f‑stops
Aperture size settings are indicated by f‑numbers, which are also known as f‑ratios or f‑stops (the latter used most commonly in photography lingo). They are expressed with the hooked f followed by a number, such as ƒ/2, ƒ/5.6, and so on. Each f‑stop corresponds to a specific aperture size in each lens. The range of possible f‑stops varies depending on the lens. The standard series of f‑stops is:
ƒ/1 ƒ/1.4 ƒ/2 ƒ/2.8 ƒ/4 ƒ/5.6 ƒ/8 ƒ/11 ƒ/16 ƒ/22 ƒ/32
They are considered standard f‑stops for two reasons: the difference in exposure when shifting between adjacent numbers is equal to one stop (1 EV); traditionally, lenses with physical aperture selection rings would indicate the available aperture range using these stepped intervals. Most cameras permit the selection of intermediate f‑stops in one-third stop increments.
F‑numbers have an inverse relationship to the aperture size: a lower f‑number indicates a bigger aperture that collects a considerable volume of light; a higher f‑number means a smaller aperture that gathers less light. For instance, adjusting your aperture setting from ƒ/4 to ƒ/2.8 doubles your exposure (adds 1 EV); shifting from ƒ/4 to ƒ/5.6 halves your exposure (subtracts 1 EV); going from ƒ/5.6 to ƒ/2 increases exposure by three stops (adds 3 EV), which is eight times more light (2×2×2=8). Many beginners find this relationship unintuitive, even confusing; the following mnemonic device could help you memorize the inverse connection (adapted from Digital Photography for Dummies):
raising the ƒ stops more light
The f‑number is a ratio of the lens focal length divided by the diameter of the entrance pupil, also known as the effective aperture. (An entrance pupil is the image of the aperture as seen through the front of the lens.) The equation is N=ƒ/D, where N is the f‑number, ƒ the lens focal length, and D the entrance pupil diameter. For example, a lens with a 50 mm focal length and an entrance pupil diameter of 25 mm would have an aperture of ƒ/2 (50÷25=2). A lens with a 100 mm focal length and an entrance pupil diameter of 50 mm would also have an aperture of ƒ/2 despite having an entrance pupil diameter that’s twice as large.
Since no lens specifies the diameter of its entrance pupil, but every lens provides you with focal length and f‑stop values, we can express the equation as D=ƒ/N. For example, take a 50 mm lens set to ƒ/4, and you obtain an entrance pupil of 12.5 mm. Set the same lens to ƒ/1.4, and the entrance pupil becomes 35.7 mm. Now, the reason for the unintuitive inverse relationship of f‑stop numbers and aperture size is apparent: the entrance pupil can only be larger when setting the aperture to a smaller f‑stop.
This also demonstrates that an increase in focal length decreases the intensity of light reaching the image sensor. However, because apertures are calibrated in proportion to focal length, all lenses set to the same f‑stop will theoretically transmit the same amount of light regardless of focal length. The equation takes everything into account and provides you with an aperture f‑stop that represents equal light intensity across all lenses.