The exposure time, respectively period of exposure is generally understood to mean the time span for which the film of a traditional camera or a sensor of a modern digital camera is actually exposed to the light so as to record a picture. The exposure time is given in seconds. In combination with the aperture it determines the incidental light quantity.

As a result, only an exactly determined light quantity leads to the correct exposure. If the exposure time was too short, you obtain underexposed pictures, respectively if the exposure time is too long, the pictures will be overexposed. The necessary exposure time can be calculated by means of an exposure meter, either with an autonomous device or with a camera-integrated component.

Technically, with a pre-set light sensitivity, the exposure time is primarily controlled through the shutter opening and set on the camera. In case of a short exposure time of less than 1/5000 s, one talks about short-time photography. For times over 5 s one refers to long-time exposure. Longer exposure times are necessary especially for night-time recordings and recordings of microscopic objects. Also astro-photography, with its very small light quantities, can only function on condition that a long exposure time is set. However, in the case of moving objects, too long exposure times can lead to blurred, respectively too fuzzy pictures. On the other hand, the exposure time selection is therefore an important design means. Fuzziness can be used intentionally in photographic works, by blurring a certain motif or giving moving objects a special dynamic through a sharp camera carriage against a fuzzy, not moving background (sports photography).

The exposure time also plays a very important role for the optimal optical reproduction in the process of laser scanning. Nonetheless, also factors such as object surface are important. The optimal exposure time can vary depending on the reflection degree of the surface.