How to Use Lens Aperture in Digital Photography

Critical in Focusing, Composition, Lens Choice as Well as Exposure

lens with aperture at smallest setting

Photography tips on how setting the aperture on a digital camera is more than controlling the amount of light passing through the lens.

Lens aperture is the opening made by the normally adjustable internal diaphragm in the lens used to set the amount of light reaching sensor or film so that, in partnership with shutter speed, the image is correctly exposed.

The size of the diaphragm opening is measured in f – stops with seemingly meaningless numbers 1.4 – 2 – 2.8 – 4 – 5.6 – 8 etc. However each of the steps in this scale indicate 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-stop settings, especially as their widest setting, indicated by their smallest f-stop number.

Camera Settings

Aperturepriority, or Av, is one of the most popular automatic metering modes, or really, it should be called a semi-automatic mode, as the photographer sets the lens aperture and the camera chooses a suitable shutter speed as the photo is taken.

Depth of Field

A significant side effect of aperture is the effect on depth of field, where the bigger the aperture, indicated by smaller f-stop numbers, the smaller the distance from the front to the back of the focus area.

Landscape photographers use the smallest aperture available, or stop the lens down, to maximize the depth of field. However, this comes at a cost 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.

Older lenses have a ring to adjust aperture

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 while sports buffs should look for images or data from wide apertures.

Faster Autofocus

Apart from the direct image quality the size of the aperture has other implications. Autofocus systems work with the lens at its biggest opening, and the automatically stopping down as the picture is being taken. 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.

Quality Clue

One guide to lens quality is the maximum aperture of the lens, bigger the aperture the ‘faster’ the lens. It costs more to design and build a fast lens, so that the benefit from this extra effort should be evident in other aspects of lens performance. To go from a zoom with maximum of f4 to one which opens to f2.8 can raise the price of the lens several times over.

Using a wide aperture (f3.5) to help isolate a face in a crowd