July 20 2016
What does aperture mean? The aperture in a camera lens is more than an exposure control. It also influences the Depth of Field (DOF) and the image quality.
The aperture is a camera lens opening using a mechanical diaphragm with movable metal blades. The blades move to set the size of the opening in the lens. This controls the amount of light reaching the sensor or film so in partnership with the shutter speed and ISO, the image is correctly exposed.
The camera f value indicates the size of the lens opening and is measured in f stops. These are seemingly meaningless numbers 1.4, 2, 2.8, 4, 5.6, 8 etc. Each of the steps in this scale indicates a doubling in the amount of light passing through the lens, as in changing the aperture from f5.6 to f4. Often lenses will also have in between f settings, especially at their widest opening, indicated by their smallest f number.
Depth of Field
The aperture influences the Depth Of Field, where the bigger the aperture, indicated by smaller f numbers, the shorter the distance of the focus area.
Landscape photographers use small apertures (larger f number ) to maximise the Depth Of Field. There is a limit to reducing the size of the aperture as light is diffracted at the edges of the diaphragm blades, reducing sharpness. At large aperture settings the percentage of diffracted light passing through the lens is small, but this increases as the aperture gets smaller, reducing sharpness.
Conversely, using large lens openings, or small f numbers, results in a very shallow Depth Of Field. Photographers can use this characteristic to keep the subject in focus and isolated with an out of focus foreground and background. This is useful for portraits.
One guide to lens quality is the maximum aperture of the lens. Lenses with a large maximum aperture are said to be a ‘fast’ lens. It costs more to design and build a fast lens, so the benefit from this extra effort should be evident in other aspects of lens performance. The price of a zoom lens with maximum of f4 is usually less than a lens with an f2.8 maximum aperture.
The choice of aperture has an important influence on the quality of photographs taken with a lens. Most lenses perform below their best at their maximum aperture. At smaller apertures the image quality improves and then declines once more as the aperture becomes smaller. The trick is to find the range of apertures where each lens is at their best. Usually, this is around f7 or f8. Better lenses usually have a wider range of apertures in this “sweet spot. Take a series of the same photographs with different apertures and critically compare the image quality. Particularly look for sharpness away from the centre of the photograph, and coloured fringing along the edge of objects.
This effect varies from lens to lens so when choosing a new lens it is important to consider the aperture used in any tests or sample photographs. Try to find those from the aperture range for your intended use, small apertures for landscape photography, while sports buffs should look for images or data from wide apertures.
The size of the aperture has implications for the Autofocus system. They work with the lens at its biggest opening, and then automatically changing to the selected aperture to take the photograph. This lets the most light through the lens giving the best autofocus performance. Therefore, a lens with a smaller f-number will be better where fast autofocus is required, such as sports and action photography.
Aperture priority, or Av, is one of the most popular automatic metering modes, or really, it should be called a semi-automatic mode. The photographer sets the lens aperture and the camera’s exposure system chooses a suitable shutter speed as the photo is taken.