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How to Calculate Aperture Diameter

A camera lens' aperture refers to the circular opening in the lens that lets in light. When cameras must reduce the amount of light coming in and hitting the film strip or digital sensor, the aperture is "stopped down" and closes slightly, similar to the way the pupils in your eyes work. Aperture is expressed in cameras by the F/stop, which is a value that compares the focal length of the camera to the aperture diameter. To calculate the aperture diameter, you need only know the camera's F/stop setting and focal length.

Instructions

- 1
  Determine the focal length of your camera lens. The focal length of any given image will be present in the photograph's metadata (visible on your computer), in your camera's advanced view mode (press, for example, the "Info" button when viewing a photograph in playback mode) or in your camera's LCD when the photograph is being taken. The focal length will be expressed as a number in mm (for instance, 200mm). The method of finding focal length varies based on your camera. Consult your camera's user manual if you cannot find the focal length of a particular shot.
- 2
  Determine the F/stop value of a particular exposure. Like focal length, the camera's F/stop or aperture will be present in the metadata, in the camera's advanced view mode or in your camera's LCD when the photograph is being taken. The aperture will be expressed as the letter F followed by a slash and then the aperture number; for instance F/2.8.
- 3
  Divide the focal length by the image's F/stop number to calculate the aperture's diameter. For instance, an image taken with a 200mm focal length and an F/4 aperture would have an aperture diameter of 200/4 or 50mm.

Photography