How Does a Camera Lens Aperture Work?


The Light Metering System

  • A camera lens aperture works in conjunction with several other key features to create the perfect exposure for an image, either digitally or on traditional film. When the aperture and shutter speed are adjusted together correctly, the light meter inside the camera reads the correct exposure, leading to a good image. When looking through the viewfinder of an older manual film camera, the light meter is usually represented by a line hovering between two points (a plus for overexposure and a minus for underexposure). In newer film cameras and almost all digital cameras, the light meter is represented by a digital readout similar to the plus and minus readout of an older camera. When the line (or digital dot) hovers directly in between the two points, the camera is set to the correct exposure.

Using the Aperture for Exposure

  • The aperture controls the amount of light entering the camera. It is controlled by numbers called f-stops, which usually range from 5.6 to 45, depending on the brand of lens. Smaller f-stops let more light in because the aperture opens to create a bigger "hole," which is represented in the illustration above. Larger f-stops actually close up the hole, making a smaller area for light to pass through. The shutter speed controls how long the aperture is open for, and therefore how long a specific amount of light is let through. For instance, if the f-stop of a lens is set to 5.6 and the shutter speed of the camera is set to 1/1000, a good deal of light is allowed through the lens for one 1,000th of a second. The shutter speed/aperture combination varies based on the lighting situation; an aperture set at 16 and a shutter speed of 1/100 with 100 ASA film speed would be appropriate for a sunny day. Different lighting situations mean different aperture settings, which is why a light meter is is so important.

Shooting Manually, with Aperture Priority, or on Automatic

  • The above section explains shooting manually by changing the shutter speed and aperture by using the light meter as a gauge. Some cameras (both film and digital) come with an aperture priority mode, which lets the user decide the aperture opening while the camera decides the correct shutter speed. Many cameras also come with a fully automatic mode, which allows the camera to choose both the shutter speed and aperture for completely automatic shooting. All three options can be useful in various shooting situations.

Depth of Field

  • Aside from controlling exposure, the aperture of a lens also controls the depth of field in a photograph. If the aperture is set to a very small aperture opening, like 5.6, a good deal of light streams into the camera; this also limits the amount of the image that will be in focus. The point of critical focus would be in focus, but much of the background would be out of focus. For example, if one was photographing a tree with a large field behind it, the tree would be in focus but the majority of the background field would not. If the aperture is set to a larger aperture opening, like 42, most of the image would be in focus. Using the previous example, the tree and the field would both be crisply focused. The use of varied depth of field and exposure techniques can make for very interesting images, all courtesy of the lens aperture.

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