A Look at the Inner Workings of the Camcorder Shutter

The shutter is but one of several important camera components used to exercise greater control over the look of your video images. We’ll begin our exploration with a discussion of the benefits the shutter brings us. Before we open a camcorder to take a look at the inner workings of the high-speed shutter, though, let’s review the more general function of all shutters.
A camera’s shutter controls the length of time that light is permitted to enter the lens. The other major exposure control on any camera (still or video) is its iris, which affects the "space" through which light must pass, so our discussion today will be about space and time. Don’t worry, it’s not rocket science. All you really need is to understand a few simple terms and relationships.
Simply stated the shutter and iris work together to control the amount of light entering the lens of a camcorder. The shutter controls the duration of the exposure of light to the CCD (Charge Coupled Device). The iris controls the size of the opening through which the light is allowed to enter the lens. While this is simple enough, each affects the final image: each gives us control over the look and feel of our video images.

Expanding Your Focus

The iris in a camcorder works much like the iris in your eye. If the light is bright, the iris in your eye closes down, making the opening through which light passes smaller, so the light doesn’t blind you. In low light the iris in your eye "dilates", opening wide to allow more light to enter. The same is true of your camcorder’s automatic exposure system. The wonderful thing about the iris is that it lets you control the depth-of-field in your images. In low light, your camcorder’s iris opens up. In bright light, it closes down. But as the opening becomes smaller, the depth-of-field, or range that is in sharp focus, increases. In fact, if the iris is closed down far enough you don’t even need a lens to focus the image. The entire world comes into sharp focus when the opening is extremely small.
The first camera, the camera obscura, didn’t have lens, shutter or even film. Long before anyone thought of such things, the ancients noticed that when light passed through a small opening into a darkened room, an image was projected at the point where the light struck the wall or floor. With a little experimentation it was discovered that the smaller the hole through which the light passed, the sharper the image. This resulted in the development of the camera obscura, which uses a small hole (and in later versions, a lens) in the wall of a darkened room or box, to project images on a drawing surface where they would be traced, producing highly accurate and detailed drawings.
The pinhole camera is a more modern example of the same approach to imaging. All you need is a shoebox, a pin and some film. If you’d like to know more about pinhole photography, you can find loads of information on the Web.

The Problem with the Pinhole

While using a small hole instead of a lens can produce images that are sharply focused from nearly the surface of the lens to infinity, it has one big disadvantage. The tiny hole can’t pass much light. To get more light, you could make the hole larger, but then the sharpness of the image is lost. By using a lens in place of the pinhole, you can collect light over the entire surface of the lens, and still get an image that is in focus at the CCD or film.
The lens allows more light to enter the camera, but the area that will be in sharp focus is reduced. The larger in diameter you make the lens, the worse the problem becomes. With very large diameter lenses, the area in focus, or the depth-of-field as it’s called, can be reduced to a fraction of an inch. Another factor that affects the depth-of-field is the focal length, or magnification, of the lens. Telephoto lenses produce a shallow field, while wider-angle lenses produce a deeper field.

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