Why do digital cameras need a long exposure time in low light conditions? Sometimes the exposures are 15-30 seconds or longer. Doesn't the electronic sensor achieve a steady state under 1 second or so? I can understand long exposures in film, where the light effects are cumulative. But in an electronic sensor? Do they do digital signal processing behind the scenes to build an image?
Electronic – Digital camera sensors and long exposures
cameraimage-sensorlight-sensorsensorsignal processing
Best Answer
The light conversion element in most image sensors as actually a reverse biased diode carefully crafted to retain charge until reset. A photon enters the bulk of the photodiode and interacts with the Silicon lattice creating a electron/Hole pair. The electric field within the diode from being reversed biased sweeps the minority carrier away while the majority carrier is kept within the photodiode. As more photons are intercepted more carriers are collected. With a lower rate of generation (i.e. with lower rate of photon arrival) one must accumulate for a longer period to have the same signal level.
In other words, photodiodes are integrating sensors.