Aperture and Shutter Speed

A well-exposed image is only attainable if the amount of light that hits the film or CCD, is controlled correctly. The main components in a camera that control the needed quantity of light are the shutter and the aperture.

In the last example, we suggested to close the aperture in order to increase the depth of field. If the aperture is closed without changing the shutter speed it is evident that less light will eventually reach the light sensitive material, therefore, producing an underexposed image. To prevent shooting an underexposed image the shutter speed needs to be extended.

Most modern cameras provide an automatic exposure mode (usually P-Mode because it is programmed). This mode automatically determines the optimum aperture settings and shutter speed with the aid of a built-in exposure meter. Some cameras even display the current exposure settings. This feature can be rather helpful when you want to check the aperture setting for more depth of field control. If you are not satisfied with the automatic settings you can always activate the shift function and instantly change the aperture/shutter speed combination, while maintaining the same exposure value but a different aperture.

In program shift, aperture and shutter speed of a camera are set in a reciprocal way. As one increases the other decreases and vice-versa. Therefore the exposure is always constant. Program shift is a good way to vary the depth of field, without having to adjust the exposure all the time.

Choosing the correct shutter speed and avoiding blurred images

In this image motion blurring is caused by the choice of a slow shutter speed. Very often that can be avoided by choosing a faster shutter speed. But in this case the motion blurring emphasises the speed of the cyclist.

Long exposure times, especially when photographing dark scenes, may produce blurred images. This is due to the fact that we cannot hold a camera completely still while shooting. If movement of the camera or the subject is expected, a fast shutter speed is required to avoid blurred movement of the subject on the film or CCD. The use of long focal lengths with hand-held cameras also calls for fast shutter speeds because even the slightest vibration can cause a loss of image sharpness. Simply remember the following: the longer the focal length, the higher risk of vibrations that lead to loss of image sharpness and the faster the shutter speed that you should set.

It is usual practice to estimate the appropriate, i.e. slowest, shutter speed to avoid blurred images. This rule of thumb helps estimate the slowest shutter speed by taking the reciprocal value of the selected focal length. Therefore, if the selected focal length is 60mm the “safe” shutter speed should would be 1/60sec (one sixtieth of a second). If the selected focal length is 150mm than the slowest shutter speed should be 1/150sec. A slower shutter speed will generally cause a blurred image.

To avoid motion blurring, the shutter speed (in sec. ) should equal the reciprocal of the chosen focal length (in mm). Ie 1/60sec for 60mm.

When taking pictures in dark surroundings, indoors or outdoors, keep an eye on the shutter speed. Although most cameras automatically activate their built-in flash when there is little or no light available you should be aware that the built-in flash has a maximum range of 3m (metres). Another way to reduce the needed light for correct exposure is by selecting a higher ISO setting, thus increasing the shutter speed. If you do change the ISO setting don’t forget to set it back.