More on Shutters

{mosgoogle}Cameras traditionally have two shutters, or curtains, that cover the film ready to be exposed -- one shutter sits over the film and one rests off to the side. Once the shutter button has been pressed, the first curtain is moved away so that the film is exposed to the light coming through the lens. Once the film has been exposed for the length of the set shutter speed, the second curtain moves into the position that the first curtain initially sat. Why can't there be just one shutter? For two reasons.

First, if the shutter speed is set to 1/1000 of a second, the shutter would have to move away from the film over a short period of time. Once the shutter has completely moved away from the film, the edge of the film that was last uncovered by the shutter has been exposed less than the edge of film that was first uncovered. This means that if a single shutter were to come back the way it left across the piece of film, it would again underexpose the already underexposed edge of the film, leaving the more exposed edge exposed longer still. Secondly, 1/1000 of a second is not even enough time for the first shutter to completely uncover the film; hence, a second shutter comes behind the first shutter and begins covering the film the first shutter has started to reveal. This has the effect of computer scanner moving across a piece of paper. This is why you cannot flash-fire a single burst of light for a photo with a shutter speed greater than 1/250th, you'll have one part of the photo more exposed than the other.

Larry Huckabee has been a photography enthusiast for over 25 years.  Visit his blog here.