A digital camera sensor can be exposed over time, much like film. During the exposure, photons hit each photosite and generate electrical charge; the longer the exposure, the more charge builds up. When the exposure ends, the camera reads out that accumulated signal and forms the image.

So for a normal long exposure, the camera is not usually taking many separate images and combining them. Instead, each pixel integrates light continuously over the exposure period.

The exposure interval may be controlled by a mechanical shutter, an electronic shutter, or sensor timing that resets and reads the sensor at the desired times.

One practical difference from film is that digital sensors tend to show more noise during long exposures, because they accumulate not only light-generated charge but also unwanted electronic noise. Also, unlike film, digital does not suffer from reciprocity failure in the same way.