A picture is an instant in time created by the result of hundreds of tiny details merging together. The length of time that instant lasts can be measured by the exposure of the photograph, and this exposure has an immeasurable impact on the final outcome of the image. Like every good photographer, you'll need to understand how to manipulate the exposure to get the results you want and with a little practice you'll soon understand the theory behind it.
What is Exposure?
Put simply, exposure is the length of time a sensor or piece of film is given to absorb light. Long exposures will end in very bright, washed out images, while short exposures will lead to dark and muddy prints. The amount of available light has a major impact on exposure, however in almost any situation it is possible to over or under expose a photograph.
What Does Exposure Do?
Exposure affects a variety of factors in your picture. Most obviously, the length of a photograph's exposure will control how dark or light it turns out. If you're in a dark area, you'll need a longer exposure time to actually see the subject while in a bright lighting, you'll need a short exposure time to avoid the film or sensor from being blinded.
Exposure can also change the focus of subjects whether they're blurry or sharply focused. For example if you were to take a picture of a basketball player making a dunk, a fast exposure would give you a clear frozen image of the player making his shot. Don't worry about a little motion blur, but if the exposure isn't fast enough it's likely that the action shot will turn out bad.
And lastly, exposure also controls the depth of field. Depth of field determines how much of an image is clearly in focus, and how quickly the background loses sharpness. Short exposures don't give a camera very much time to absorb data, so they will result in shallow depth of field. Long exposures will create a greater sense of depth, but as noted above are more subject to motion blur.
Changing Your Exposure
By adjusting the shutter speed, you can control the exposure of any picture. High shutter speeds give you very short exposure times, whereas longer exposure times can be achieved by slow shutter speeds. Whether you choose to go long or short depends on what you're trying to accomplish with the photograph as well as the available light options.
As an example, imagine shooting a photo of a child riding a bicycle on a summer afternoon. If you have a slower shutter speed or a long exposure, you will get motion blur as the child rides by on his bike but you may still see the background in focus. A short exposure, or high shutter speed, will freeze the child in frame and will blur out the majority of the background.
In this particular case, a shorter exposure may be your best option if the light is so intense that a low shutter speed will over-expose your image. You can work with the aperture, or f-stop, on your camera to reduce the amount of light that enters the lens, but in many cases your exposure will be dictated by your light source.
Dealing with exposure is just one of the many ways that you can control the final output of your images beyond framing a subject and hitting the shutter button. Once you understand how shutter speed works, you'll be able to easily change the depth and amount of movement in every picture. And fortunately with SLRs these days you can select your shutter speed and the camera will figure out everything else.
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