Camera lenses explained: Aperture
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A lens aperture is the hole that is used to control how much light enters the camera. This hole is created by a series of blades inside the lens that make up an iris that can move to open or close and change the size of the aperture.
Aperture value is calculated using the following formulae:
Aperture value = Focal length (f)/Aperture diameter in mm
Even though the aperture diameter of two different lenses maybe different, if the aperture value (or f/number) is the same, they will let the same amount of light through.
For example, an aperture of f/11 on a 100mm lens indicates an aperture with a diameter of 100/11, which is 9.09mm.
With a 50mm lens the same aperture (f/11) has a diameter of 4.54mm.
Despite this size difference, the two lenses let the same amount of light through when they are set to f/11.
Camera lenses explained: Fast vs slow lenses
Lenses are marked with their maximum aperture setting. Short focal length prime lenses typically have maximum apertures of f/1.4, f/1.8 or f/2 while telephoto lens are more likely to have maximum aperture settings of f/2.8, f/4 or even f/5.6.
High-end, expensive zoom lenses are able to maintain the same maximum aperture setting throughout their focal length range. Less expensive zoom lenses, however, have a variable maximum aperture settings.
The kit lenses sold with many DSLRs and compact system cameras often have a maximum aperture of f/3.5-5.6.
This means that at the widest setting the aperture can be opened up as far as f/3.5, while at the longest telephoto point the maximum aperture is f/5.6.
If you set the lens to f/3.5 at the widest point and zoom in, the aperture changes to f/5.6 by the time you reach the longest point.
Long lenses with a maximum aperture of f/2.8 or greater and shorter lenses with apertures of f/2 or f/1.8 are often referred to as 'fast' lenses.
This is because they allow more light through than their slower counterparts with smaller maximum apertures, and this enables faster shutter speeds to be used.
Fast telephoto lenses are therefore very popular with professional sports photographers because the long focal length allows tight framing of the distant subject while the wide maximum aperture enables movement-freezing shutter speeds to be used.
Camera lenses explained: Stabilisation
Some lenses have an element inside them that can move to compensate for those accidental movements that are made when we hand-hold a camera.
In some cases this stabilisation system has several settings; there maybe one that restricts the correction to vertical movements, for example, which is useful when panning to follow a moving subject.
Some image stabilisation systems can also be set to only activate when the shutter release is fully depressed to take a shot.
This means that the view through the viewfinder isn't stabilised as it would be normally, but the range of correction available at the time of capture is greater, potentially making it more effective.
Camera lenses explained: Focus limiter
Some longer lenses have focus limiting switches which are designed to cut down on the time that a lens takes to focus on a subject when it is within a particular distance range from the camera.
This also cuts down on any distractions for the AF system, making it more accurate. In some cases it is even possible to customise the focus range for a lens. This is especially helpful when there are likely to be objects between you and the subject as the camera will ignore them and only focus within the range specified.
