September 12th, 2011
The cameras speed setting on modern DSLRs is set using control dials on the camera something similar to those shown in fig 1. In order for the user to select the speed the camera must be set either to manual control or shutter priority. In manual the user has full control over both speed and aperture settings, where as in shutter priority the camera will automatically select the aperture based on it’s calculation of correct exposure.
The shutter speed has no influence over the depth of field of an image, it does however have direct relationship with aperture in controlling the exposure of an image. It can also be used creatively by the photographer to control movement in an image. The faster the shutter speed the easier it is to freeze motion where as slower shutter speeds can be used to show movement or blurring.
Shutter speeds are set in camera by either halving or doubling the shutter time which is called a stop, this controls the amount of light getting though to the sensor. Doubling the shutter speed doubles the amount of light getting through, were as halving the shutter speed will half the amount of light. If the aperture remains unchanged this will have the effect of over or underexposing the image. Modern cameras now allow you to set half stops or even third stop settings allowing even more control of the camera.
When using shutter priority the camera automatically sets the aperture, this is done depending on the settings of the exposure meter. Modern cameras tend to have a few different settings available including spot metering, centre weighted metering, average metering and matrix metering. Almost all digital cameras take a reflective reading of the scene to calculate the correct exposure. Although modern cameras are pretty accurate they do not always get it right and scenes with very bright or dark subjects can throw the meter, this is when manual mode will give the best results.
A typical shutter speed sequence found on a camera would be as follows 1/1000, 1/500, 1/250, 1/125, 1/60, 1/30, 1/15, 1/8, 1/4, 1/2, 1, 2, 4, 8, 15, and 30secs. Very fast shutter speeds such as 1/1000 of a second can be used to freeze motion of fast moving subjects such as cars, bikes, galloping horses etc. Where as slower shutter speeds of a couple of seconds can be used to show some motion such as moving water, or blowing branches in trees etc. Very slow shutter speeds can also be achieved by using the bulb setting on the camera and this can be used to capture star trails or lightning etc.
Another important decision when selecting shutter speed is to eliminate unintentional camera shake. This can happen when a slow shutter speed is selected and the camera is not supported on a tri-pod the result being a blurry image. To prevent this, the camera either needs to be mounted on a tripod or in some other way supported, or the user must select a high enough shutter speed in camera. The shutter speed required to successfully handhold a camera without camera shake depends on the focal length of lens- the longer the focal length the more magnification and the more chance of blurry images. As a general rule of thumb you can use the 1/focal length rule to calculate the minimum shutter speed to handhold for any given focal length. For example if you are shooting with a 50mm lens you can work on a 1/50 of sec shutter speed to eliminate blur or for a 200mm lens 1/200 sec should be fast enough. This is not a hard fast rule and should be used as a guide only; the actual speed will depend on the user and the conditions.