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The aperture

The amount of light hitting the sensor is controlled by the aperture in the lens. The aperture is a mechanical copy of the iris in the human eye. The human iris opens up in dim light and closes down in bright light to control the amount of light reaching the retina. The aperture of the camera lens can also be opened and closed in different brightness levels to control the amount of light reaching the image sensor.

The right amount of light is required for correct exposure. Too much light and the image will be overexposed, not enough light and it will be underexposed. As the aperture is opened or closed it is given a numerical value called an f-stop. When the value of the f-stop decreases by one stop exactly twice as much light reaches the image sensor as the previous number.

When the value of the f-stop increases by one stop half as much light reaches the image sensor as the previous number. The only confusing part is that the biggest aperture is the f-stop with the smallest value and the smallest aperture is the f-stop with the largest number. The larger the f-stop the smaller the aperture.

Note > When using the aperture you may find the aperture is opened or closed in 1/3 stop increments instead of one stop increments, e.g. the difference between f/5.6 and f/8 is one stop. The camera’s LCD read-out may, however, indicate values of 6.4 and 7.1 as the aperture is stopped down from f/5.6 to f/8.

The shutter

The length of time the shutter is open also controls the amount of light that reaches the image sensor, each shutter speed doubling or halving the amount of light. Exposure, therefore, is a combination of aperture and shutter speed.

To slow the shutter speed down is to leave the shutter open for a greater length of time. Shutter speeds slower than 1/60 second can cause movement blur or camera shake unless you hold the camera steady with a tripod or by some other means.

To use a shutter speed faster than 1/250 second usually requires a wide aperture or a high ISO setting in order to compensate for the small amount of light that can pass through a shutter that is open for such a short amount of time. It is suggested that you use shutter speeds faster than 1/60 second until you are sure you can hold the camera steady using slower speeds.

Choosing an exposure mode

The disadvantage of a fully automatic or program mode is it can often take away the creative input the photographer can make to their images. A camera set to fully automatic is programmed to make decisions not necessarily correct for every situation. If your camera is selecting both the aperture and shutter speed you will need to spend some time finding out how the camera can be switched to semi-automatic or manual operation.

Semi-automatic exposure control, whether aperture priority (Av) or shutter priority (Tv), allows creative input from the photographer (depth of field and movement blur) but still ensures the meter-indicated exposure or ‘MIE’ is obtained automatically.

Aperture priority (aperture variable or Av)

This is a semi-automatic function where the photographer chooses the aperture and the camera selects the shutter speed to achieve ‘MIE’. This is the most common semi-automatic function used by professional photographers as the depth of field is usually a primary consideration. The photographer using aperture priority needs to be aware of slow shutter speeds being selected by the automatic function of the camera when selecting small apertures in low-light conditions. To avoid camera shake and unintended blur the aperture has to be opened and the depth of field sacrificed.