From Wikipedia:
In photography, intervalometers are used to trigger exposures. This is often done for a time-lapse series. It may also be used to start taking picture(s) after a set delay. For example, in aerial photography, one may want to delay the start until five minutes after take-off, to allow time to reach altitude.
Most cameras have very basic intervalometer functionality, the “self-timer”. A self-timer can be used to delay taking a picture for short time. The delay is commonly used for the photographer to get into the picture. A self-timer is so basic such that most people do not think of it as an intervalometer.
One of the more common types of intervalometers is the timer that is used to turn lights on and off at set times. These are commonly used by people when they go on vacation. There are also a large number of commercial and industrial applications for even such basic intervalometers.
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