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 Shutter

Unlike a still camera, a video camera doesn’t actually have a shutter to adjust the amount of time the CCD is exposed to light. The CCD itself acts like the shutter in a still camera. In video, "shutter speed" refers to the interval in which the CCD releases its image and gathers a new one.

If your subject is in motion, a fast shutter speed helps keep the image from blurring, but cuts down on the duration the CCD is exposed to light, making the image appear darker. A slow shutter exposes the CCD to light for a longer period, making the image appear brighter and helping to achieve a satisfactory picture in a situation where you have less available light. Shooting a quickly-moving subject in a dimly-lit setting, however, is difficult. Generally, you should either utilize the subject’s blurring motion as an artistic effect, or you’ll need to obtain more light for your subject in order to raise your shutter speed. High shutter speeds are used mostly in outdoor situations under bright sunlight conditions.



Intuitive Preset Features

Many of today’s point-and-shoot camcorders don’t have manual iris and shutter speed controls. To simplify things, manufacturers replaced manual settings in these camcorders with a series of preset exposure settings. The camcorder sets the iris and shutter speed to a level that should work for a variety of shooting situations.

For example, many point-and-shoot camcorders offer a "surf and ski" setting. This setting raises the shutter speed to catch the fast action of a skier, while closing the iris so that the bright white snow doesn’t overexpose the shot. The "low light" setting opens the iris and slows down the shutter, allowing for a properly exposed shot when there is very little light to work with. These presets are nice for casual shooters, but choosy shooters who want to have precise control over lighting should purchase a camcorder that provides manual shutter and iris control.

The high-speed shutter will clearly capture a race car, a deep depth of field will represent the grandeur of a natural vista or a shallow depth of field can be used to focus on and isolate a person from her surroundings.

Knowing how the lens, iris and shutter work together to present light to your camcorder’s CCD, or CCDs, will help you improve your control over light, and gaining control over lighting conditions will improve your chances of getting exactly the shots you need. And, for some of us tech-no-philes, it’s just plain interesting to know.

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