Exposure is the amount of light collected by the sensor in your camera during a single picture. If the shot is exposed too long the photograph will be washed out. If the shot is exposed too short the photograph will appear too dark. Almost all cameras today have light meters which measure the light in the given shot and set an ideal exposure automatically. Most people depend on the light meter which is fine, but if you know how to control your exposures you can get some creative and sometimes better pictures. (The photo on the left is with low shutter speed and narrow aperture (high f/stop).
The two primary controls your camera uses for exposure are shutter speed (the amount of time the sensor is exposed to light) and aperture (the size of the lens opening that lets light into the camera). Shutter speeds are measured in seconds and more commonly fractions of a second. (1/2000 of a second is very fast and 8' seconds is extremely slow). Apertures are measured in something called f/stops (a very wide aperture is f/2.8 and a very small aperture is f/19).
You might wonder why there is not just a constant shutter speed or a constant aperture so that you would only have to worry about one control. The reason is that even though they both control the amount of light getting to the sensor they also control other aspects of the picture. Shutter speed for example can be used to freeze subjects in midair with a fast speed or it can be used to blur water with a slow speed. Aperture controls the depth-of-field which is what is in focus in the picture. Aperture can be used to draw attention to one subject (like the flower on the right) by blurring the background with a wide aperture (low f/stop). Aperture can also be used to focus everything in a picture with a narrow aperture (high f/stop). (The photo on the left is with Wide aperture (low f/stop) and corresponding shutter speed).
On most digital SLR's (Single Lens Reflex) cameras today you can even change the sensitivity of the sensor when collecting light which is called the ISO speed. The common span of ISO speed is 100 to 800. The higher the ISO speed the faster the camera collects light but it also adds more noise to the photograph than the lower speeds. For example if your trying to take pictures in dim light without a tripod you might want to raise the ISO speed in order to get a picture that's not blurry. Most of the time you should keep it at a lower ISO speed if there is enough light, but it makes a big difference when there isn't.
Low shutter speed and slightly narrow aperture (pretty high f/stop)
The best way to learn how to use shutter speed and aperture is to just keep experimenting with them.
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