As said before, the correct exposure is achieved by choosing the right combination of shutter speed and aperture.
This is done by establishing the light level with either a hand held light meter or one built into the camera. These devices measure the amount of available light and give a reading which is a combination of shutter speed and aperture. On a hand held meter a dial shows what aperture to use if you want to use a different shutter speed than that indicated.
Let's assume the given combination of shutter speed and aperture to achieve correct exposure is:
1/125th shutter speed and Aperture F8
The same amount of light will fall on the film if the shutter speed were halved to 1/60th and the aperture were stopped down one stop to F11 to compensate for the extra amount of light falling on the film. The following list will all give exactly the same amount of light on the film:
Mixing any of these combinations will result in incorrect exposure.
The faster the shutter speed, the more you will be able to "FREEZE" the action and get sharp results of fast moving subjects. With slower shutter speeds you will have to hold the camera steadier to get sharp results.
As a rough guide to the slowest speed you can use when hand holding a camera, the speed will be the reciprocal of the focal length of the lens. For example with a 35mm lens the slowest sensible speed would be 1/35th sec i.e. 1/30th and for a 135mm telephoto it would be 1/135th sec i.e. 1/125th.
The effect of changing aperture
Changing aperture produces a much more important effect and is often fundamental to achieving sharper results overall.
For simplicity's sake we'll assume that, at Aperture F2.8, only the subject focused on will be sharp. Stopping the aperture "DOWN" will bring more into sharp focus either side of the
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