Apertures

 

Within every lens there is an aperture, which like the iris in the eye, can be opened and closed to vary the amount of light that is allowed to pass though to the recording medium.

Below are two pictures of the same lens, one with the aperture fully open, and the other with the aperture nearly closed:

f2.8	f32

As you can clearly see the lens on the left has the aperture wide open which would allow the maximum amount of light to enter the lens, useful where there is little light, where as the lens on the right has the aperture nearly closed and therefore reduces the amount of light entering the lens.

The amount of which the aperture is open can be used to determine the depth of field, or the amount of the foreground and background of the subject that is in focus.

Below are two diagrams showing a representation of the depths of field for the two apertures above:

f2.8

f32

As you can see the first aperture has a shallow depth of field, which would be suitable for portraits, where you would only want the subject in focus with the background thrown of of focus.

The second aperture with its deeper depth of field would be more suitable for landscapes, where you want everything in focus.

 

Controlling the Aperture

When taking of apertures we talk of stops, or f stops. The lens aperture is usually specified as an f number, the ratio of focal length to effective aperture diameter. A lens typically has a set of marked "f stops" that the f-number can be set to. A lower f-number denotes a greater aperture opening which allows more light to reach the film or image sensor. The photography term "one f-stop" refers to a factor of √2 (approx. 1.41) change in f-number, which in turn corresponds to a factor of 2 change in light intensity.

Below is a diagram showing some of the different f stops in relation to the opening and closing of the aperture:

Each lens has its own range of aperture f stops, with lenses the lower f stop values (f 1.2 and f 1.4) demanding far higher prices than starting lenses with slightly higher f stops (f2.8, f3.2 and f4.5), because they allow far higher shutter speeds whilst keeping the sensitivity lower to ensure clearer images. Zoom lenses usually have two lower f stops listed as these cover the different focal lengths available (minimum and maximum), although more expensive zoom lenses may carry a single high aperture, f4 for example, across the whole zoom range.

A typical lens will have an f-number range from f/16 (small aperture) to f/2 (large aperture) (these values vary). The maximum aperture (minimum f-number) tends to be of most interest (and is always included when describing a lens). This value is also known as the lens speed, because it is proportional to the square of accepted light, and thus inversely proportional to the square of required exposure time (i.e. using a lens with f/2, one can take pictures at one quarter of the exposure time necessary using a f/4 lens). Lenses for 35mm cameras can have f-numbers as low as f/1.2.  These are known as "fast" lenses because they allow in more light and therefore reduce the exposure time

Prime lenses (non-zoom lenses) have a fixed focal length (FFL) and large aperture and are favoured by professionals, especially by photojournalists who often work in dim light, have no opportunity to introduce supplementary lighting, and need to capture fast breaking events.

Zoom lenses typically have a maximum aperture (minimum f-number) of f/2.8 to f/6.3 through their range. A very fast zoom lens will be constant f/2.8 or f/2, which means the relative aperture will stay the same throughout the zoom range. A more typical consumer zoom will have a variable relative aperture, since it is harder and more expensive to keep the effective aperture proportional to focal length at long focal lengths; f/3.5 to f/5.6 is an example of a common variable aperture range in a consumer zoom lens.