A Look Inside Your Camcorder's Imaging System

Your camcorder's inner workings may seem impossibly complicated, but there's really nothing inside that should scare you. It's really just a collection of theoretical smoke and a couple of actual mirrors (and maybe a prism and a dichronic filter if you've got a really cool camcorder).
Let's take a look at the lens, the f-stop, CCD and iris. Along the way, you'll learn how they work and how that knowledge can help you take control over the quality of the images you record to tape.

The Lens and the Iris

Much like your eye, a camcorder has a lens and an iris. The lens is a piece of glass that focuses light onto a small series of mirrors and has the ability to change its focal length (zoom in and out). The mirrors bounce light onto the CCD, which converts the light into an electronic signal that is recorded to tape. In the case of DV, that information is recorded to tape (or disc) as digital code.
Between the lens and mirrors is the iris. It acts much like the iris in your eye, which dilates (opens wider) when light is dim and contracts into a smaller opening when light is plentiful. The iris in your eye adjusts automatically. Many camcorders allow you to adjust the aperture manually.
A side effect of opening or closing the iris is that the depth of field will change. Depth of field describes the area that is in focus in front and behind your subject (see Figure 1). The iris changes the depth of field depending on the amount of light it lets through. Closing down the iris (higher f-stop numbers, smaller opening) lets less light into the camera and simultaneously lengthens the depth of field. As the iris is opened up, allowing more light in, the depth of field shortens. You've observed shallow depth of field if you've ever seen a shot where the subject is in crisp focus while the background just a few feet away is blurry. A shot of a person standing in a field with a mountain far behind them with both the person (subject) and the mountain in sharp focus illustrates a long depth of field. Where a long depth of field can be miles, a short depth of field may be fractions of an inch. To change or manipulate the depth of field with your lens and iris, you need to understand what each will do to expand or shrink the depth of field.
The lens shortens the depth of field as you shorten the focal length (zoom in). Conversely as you zoom out, the depth of field will lengthen. The shutter also works in conjunction with the iris. If you close down the iris you can slow the shutter speed to allow light to hit the CCD longer. If you open up the iris, you must speed up the shutter to restrict the amount of light on the CCD.

The CCD

The CCD (Charge Coupled Device) converts light that enters the lens of the camcorder into an electrical signal. It is a light-sensitive receptor (kind of like the retina in your eye) that reads the intensity of the light on each of its receptor cells, or pixels. In most consumer camcorders, a single CCD handles all the imaging, capturing the whole visible spectrum. It uses a multi-colored filter to add color into the video signal.
Some camcorders use three CCDs. With a prism (see Figure 2) and a dichronic filter, the light is broken down into its component parts of red, green and blue. Each CCD captures a separate color and, when added together, composes the full image. (see the It Takes Three CCDs to be a Pro sidebar).

Shutter Speeds without the Shutte…