Automatic Exposure

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Unlike analogue cameras, modern SLR cameras have an automatic exposure. This automatic exposure control works by interlinking the aperture and shutter, which both significantly influence light intensity as well as exposure time. Both of these elements produce the exposure of a photo. In other words, the exposure is the product of light intensity and exposure time (Exposure = Light Intensity x Exposure Time).

While analogue photography allows for the manual adjustment of aperture and shutter, digital photography grants you the option of using an automatic exposure. This automatic setting is especially designed for users who just want to point and shoot without having to put too much thought into the exposure measurements of their camera. There are three basic types of settings:

Automatic aperture

On the mode selector: S/Tv (short for Shutter priority / Time value)
The shutter speed is pre-selected and the aperture will automatically be adjusted

When using an automatic aperture control, which is also called shutter priority, the photographer has to pre-select the exposure time. The camera’s integrated exposure meter then adjusts the opening of the aperture accordingly. The shutter priority is often used to influence the dynamics and sharpness of a photo. On most mode selectors, this mode is displayed as S or Tv.

Aperture priority

On the mode selector: A/Av (short for Aperture priority / Aperture value)’
The aperture is pre-selected and the shutter speed will automatically be adjusted

This mode allows the photographer to select a specific aperture value; the camera is then responsible for using a suitable shutter speed, which ensures proper exposure.

The aperture has an effect on both exposure time and depth of field. In portrait photography, people are especially emphasized when the background appears blurry. To create this effect, the f-number should be relatively low (e.g. 1.4 or 2.0) to ensure that the aperture area is as open as possible. The f-number is a relative value that results from dividing the focal length by the diameter of the entrance pupil.

When photographing landscapes, you should choose a higher f-number (e.g. 16 or 22) to make the larger area appear as sharp as possible. The opening of the aperture is then relatively small.

Program mode

On the mode selector: P
The aperture and shutter speed are both automatically adjusted by the camera

This popular mode is mostly used for taking pictures when there is not too much time available for adjusting the camera settings. Depending on the amount of light detected, the camera automatically adjusts the exposure time and the aperture accordingly. Depending on the model of your camera, the way its program mode operates can be quite different from that of other models. The provided handbook that came with your camera can give you more information specific to your camera model.

Amateur and professional photographers versed in the technical aspects of their cameras like to stick to using the manual mode (displayed as M on the mode selector) since it allows them to avoid problems that can come about when using automatic exposure. When using automatic exposure, it can be difficult to take reproducible photos since it’s possible that the camera will not automatically use the same settings twice in a row or the white balance could be different. This can be observed when looking at wedding photos of the bride and groom. Depending on the illumination perceived by the camera, the groom’s black suit can appear to be dark grey (if the black value is too light) while the bride’s dress can also appear grey if the white value is too dark.