I was explained the function of an optical shutter. Apparently that thing opens and closes very fast, so light falls on a film or, nowadays, a chip and that light is captured: it's a picture that stays if no more light falls on the film or chip. The faster the shutter opens and closes, the less light falls on the film or chip, but also the faster the objects can move while still keeping a sharp picture. This can be improved by using large lenses. This way more light is allowed in and there is a better chance of having a sharp picture. Nice to know!