In photography, film consists of cellulose nitrate or a cellulose acetate composition made in thin sheets or strips and coated with a sensitive emulsion for taking photographs. When the emulsion is exposed to light, it forms a latent image. Chemical processes can then be applied to the film to create a visible image, in a process called film developing.
A digital camera uses a sensor array of millions of tiny pixels in order to produce the final image. When you press your camera's shutter button and the exposure begins, each of these pixels has a "photosite" which is uncovered to collect and store photons in a cavity. Once the exposure finishes, the camera closes each of these photosites, and then tries to assess how many photons fell into each. The relative quantity of photons in each cavity are then sorted into various intensity levels, whose precision is determined by bit depth (0 - 255 for an 8-bit image).
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