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Exposure: Getting it Right Everytime, Part IV

© Wendy Folse

Three ways of controlling exposure

The camera companies have come up with all sorts of on-board computer functions to take some of the confusion out of the calculations. Now days, the camera knows what lens is connected and will give you a reading for that particular lens. However, for the ease of explaining how to achieve proper exposure it helps to forget that the camera knows everything, because it doesn't. It doesn't know what you want to do, only what it was programmed to do.

So far, we have discussed three ways to control the total exposure necessary to produce an image, using shutter speed, aperture setting, or film speed. Where do we get the correct ratio? From the meter, either a hand held meter or the camera's built in meter. Once you have a correct exposure reading for the scene, then you can alter the ratio to suit your needs by adjusting either the shutter speed, aperture setting, or film speed. As long as the ratio stays the same, the exposure will stay the same. That is the beauty of the f-stop system.

It is like a bartering system. If you trade one f-stop for this effect, you give an f-stop somewhere else. The object is to keep the ratio in perfect balance for the correct exposure. If you change it here, you must also change it there. It all starts with an accurate or at least an educated guess at the correct exposure. From there, it is a trading game. Do you need more opening in the aperture for a creative effect? If so, then you will have to compensate for the extra f-stop by changing the shutter speed to match. If you need a faster shutter speed to stop the movement of the subject, then you will have to give it up in the aperture setting.

Say for instance you go out one morning and want to capture a beautiful garden scene, but your meter is giving you a setting of say f2.8 at 1/60 and your lens is not capable of handling f2.8. What do you do? Say your smallest aperture setting is f5.6. That means that your lens opens two f-stops less than the setting for f2.8. Therefore in order to preserve the meter suggested ratio, you will need to increase the amount of time that the shutter remains open in order to let in more light. That means setting the shutter speed at two stops slower or to 1/15sec. That is one way, now lets consider the other two.