We can adjust two camera settings to control the amount of light that reaches the film. Those settings are the shutter speed and lens f-stop, or opening. There are many combinations of shutter speed and lens opening that will allow the same amount of light to reach the film for proper exposure. These are known as equivalent exposures. The shutter speed and lens f-stop can also be used to our advantage to: 1. To reduce the effects of camera motion. Photographers aren’t very steady creatures, and everything we do tends to shake the camera. Longer telephoto or zoom lenses make the problem worse. Not only are they physically harder to hold, but they also magnify any motion. We need to pick a shutter speed that minimizes the shake. With short lenses, up to about 135 mm, 1/125th of a second should be sufficient. Longer lenses need higher speeds. 2. To stop or blur action. A fast shutter speed will stop action; a slow shutter speed can be used to blur action. 3. To control depth of field. We can change the amount of the scene that is in focus in
front of and behind the subject with the size of the lens opening. Sometimes we want
to show a lot in focus, possibly to show depth in the scene, sometimes we want to
have nothing but our subject in focus to emphasize the subject. No matter what we try to do with the settings, we still have to get the right amount of light to the film. If you change from one shutter speed to the next higher speed, this lets half as much light expose the film. You should keep the total amount of light-the exposure-the same by opening the lens to the next larger lens opening. It also works the other way around. If you change to the next slower shutter speed which lets in twice as much light, you should use the next smaller lens opening to let in the same amount of light as before.