Why does a camera sometimes need to use a flash?
Answered by HowStuffWorks
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  1. Much light is required in order to expose an image to film. Indoor photography, because of the relatively small amount of ambient light, requires either that film be exposed for a longer stretch of time or that at it be exposed to an increased level of light. Usually, the first option of lengthening exposure time is not a practical solution. This is because any sudden movement can result in an unclear picture being produced -- "say cheese!" needs to happen fast. As such, the electronic flash system is the solution to this problem. The flash releases a quick burst of light as soon as the camera shutter is released, creating the proper amount of light, and mitigating the worry about a photo subject ruining the photo.

    Behind the scenes of a camera flash, it's interesting to note the important role of the capacitor in letting everyone know that the shot is about to be taken. A high-voltage charge is stored in the flash circuit capacitor. The capacitor circuit, in turn, is linked to the camera's gas discharge tube with a device called a resistor. Once the capacitor's voltage hits a certain level, a current is able to stream from the resistor. This process illuminates the xenon gas in the discharge tube. Subsequently, this light will indicate to photographer and subject that a flash is about to go off.

    Of course, today many people use digital cameras, which typically have an automatic flash. It's helpful to understand a bit about the general settings many flashes employ, based upon lighting conditions. Usually there is an "Auto" flash setting for those who'd rather not mess with things and prefer to just aim the camera and click. Often times there is a lower-light, "twilight" setting that can be used in darker settings and allows time for more natural light to enter the picture. Another setting will typically handle the ubiquitous "red-eye" problem, to keep subjects from looking like vampires. These are some of the common settings, although cameras will offer others as well [source: Digital Camera Web Guide].

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