A is for Aperture

This is the first in a series of posts about photography. Let’s hope I can make it to Z!


Cameras capture images by allowing light to fall on the sensor (it used to be the film, but those days are over for the vast majority of photographers.) the amount of light falling on the sensor is the exposure. The exposure has two dimensions: the size of the light (aperture) and the time that it lasts (shutter speed.). Think of filing a bucket with water. The amount of water in the bucket depends on the size of the hose used to fill it, and how long the hose is turned on.
A given exposure can be achieved with a number of different combinations of aperture and shutter speed, for example 1/125 sec at f2.0 is equivalent to 1/250 sec at f1.4. A bigger “hole” for a shorter time.
Aperture is measured in f-numbers. The f-number indicates the ratio of the focal length of the lens and the diameter of the opening in the lens formed by the blades of the diaphragm. So a 50mm f2.0 lens has a maximum opening of 25mm diameter.
The photo above shows a Canon 85f1.2LII lens. The maximum aperture of this lens is 71mm (2 3/4inches!)
The maximum aperture of a lens indicates its speed. A lens with a large aperture (small f number) is a fast lens. You can use these in low light (indoors) to get photos that might otherwise need a slow shutter speed and cause blur.
The aperture of a lens is adjustable–you can “stop down” a lens to make the size of the hole smaller. Why? The aperture controls the depth of focus, or depth of field. This indicates the parts of the scene that are in focus. An image exposed with a large aperture will have a very shallow depth of field — only those things very close to the point of focus will be in focus. All other parts of the scene will be out of focus. Using a small aperture will make nearly everything appear in focus. It’s common to use a large aperture to make a portrait of a person (head shot) and have the background out of focus (see bokeh.) Landscapes are usually made with a small aperture, so that both the flower in the foreground and the mountains in the background are in focus.

As one of the two main controls of exposure, aperture is well worth understanding. Sometimes you can’t do much but open up the lens to maximum aperture (low light photos), but when there’s a choice, use the aperture as a creative control.

1 thought on “A is for Aperture

  1. Pingback: F is for Focus, f-stop | Philip Ganderton

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