Understanding Light and Exposure
In photography, exposure is defined as the total amount of light allowed to fall on film or image sensors when taking a photograph. Exposure is dependent on two factors: the aperture and time. The aperture controls how much light is entering the camera through the lens and the time factor determines how long that light is hitting the film or image sensor.
The aperture is a hole between the lens and the photographic medium. The larger the hole, the more light passes through it. The size of these holes is measured in f-stops. Typically f-numbers range from f/1.4 to f/16, with the larger number being a smaller hole.
Also referred to as shutter speed, this refers to how long the light is exposed to the film or image sensor. For example, when shooting in conditions with low light, like night, your camera compensates with a slower shutter speed. The slower shutter speed exposes the existing light to the film longer so a brighter image is obtained. An image with the shutter open for one second will look considerably darker than an image that was exposed for ten seconds.
Overexposure and Underexposure
If an image appears to be too bright and blown out, then it is overexposed. This means that there was more light than the camera could read. In digital photography, overexposure is very bad when it's not used for artistic reasons. It is nearly impossible to bring an overexposed image back to normal because information has been lost. That's the opposite problem with film. If a film image is overexposed, it means that the information is there and it can be partially restored.
Underexposure means that there was a lack of light and the images appears to be dark. In digital photography, we can restore images that are partially underexposed. You cannot do that in film, however, because if it was underexposed, it means the image was never recorded.
Blur and Special Effects
Because of the time factor, images can be manipulated to create special effects. Photos where fast action is taking place can appear blurred, like a waters current or a horse race. This can also be used to manipulate actions that are not fast. A fifteen second exposure can make ordinary things seem extraordinary.
If you're shooting in low light and have a long exposure, then you can take a small, considerably brighter light and move it through the frame. When the image is developed, you will see the entire path that light followed.
So to sum it up, exposure is the amount of light that enters the camera through the lens to the photographic medium, factored with how long the shutter cycle is.
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