Imaging is Everything

Loren Alldrin
April 1999

As videographers, we love to debate the merits of tripods, tape formulations and fancy lens filters. And while these goodies do have an impact on the quality of your images, they're not the real deciding factor in how good your videos look.

In the grand scheme, it's your camcorder's lens, iris and sensor design that really control your image quality. These components make up your camcorder's imaging system, where light is bent, massaged and converted into an electronic signal. In this article, we'll dissect the imaging system of the camcorder and explore what makes it work.

The goal of this article, however, is not to just fill your mind with interesting camcorder trivia. We'll take a hard look at how the camcorder's imaging system affects your video quality. We'll explore the implications of zoom settings, stabilization schemes and sensor designs. Finally, we'll suggest some ways to get the best-possible image quality from the camcorder you own right now.

Light begins its journey into the camcorder through the lens, which is little more than a black tunnel containing numerous pieces of glass. These aren't your ordinary pieces of glass, however--they're "elements" carefully designed to bend light in a specific fashion. Some elements are concave, some are convex, and others may be convex on one side and concave (or flat) on the other. Elements of a certain shape fan light out, while others narrow the dispersion of the rays.

It's not uncommon for a camcorder to have a half-dozen or more individual lens elements arranged into several groups. Why so many elements? Because there's a lot of manipulation going on inside a lens to deliver a focused image over a broad range of subject distances and zoom settings. The focusing group moves in response to the focusing motor, and is responsible for generating a sharp image at a given distance between camcorder and subject. Zoom elements do just what their name implies: they change the dispersion of light through the lens to narrow or widen the camcorder's field of view and increase or decrease the amount of magnification.

Lens designers also have the challenge of minimizing chromatic aberration (or color distortion), the inevitable by-product of running light through a lens element. This distortion causes the different colors of the visible light spectrum to separate and focus at different points, a result of the same principle that allows water drops to fan sunlight into a rainbow of color. Left unchecked, this rainbow effect would turn a camcorder's image to mud. By following one lens element with a counterpart that has an opposite effect on the light, lens designers can cancel out these color aberrations.

Lens designers can also control rowdy light rays by layering elements with special coatings. These coatings help minimize image distortions, but can be rather easy to damage. Lens coatings help control flare as well, which occurs when a bright light (the sun, for example) hits the lens at a slight angle. Flare can cause halos of light to appear on the image, and may wash out colors and contrast.

For the past several years, lens designers have been experimenting with light-friendly plastics for lens elements. Some camcorders now have lenses partially (or completely) made up of plastic lens elements. Plastic elements are cheaper to manufacture, weigh less and offer optical performance nearly on-par with their glass counterparts. Size and weight disadvantages aside, higher-end camcorders usually include glass elements exclusively.

Another key component in the camcorder's lens assembly is the iris. Just like the iris in your eye, the camcorder's iris regulates the amount of light coming through the lens by changing diameter. Typically, the iris in consumer camcorders operates automatically, although some camcorders offer manual iris control. When light levels are low, the automatic iris swings all the way open (to an inch or more in diameter in some lenses) to let as much light as possible pass. In extremely bright conditions, the auto iris closes down to as small as 1/8 of an inch, or more, to reduce the light to a more usable level.

The setting of the iris does more than just control the amount of light hitting your camcorder's sensor. It also has major impacts on the look of your image. The most visible effect of iris diameter is in depth of field. This term relates to how large an area in front of the lens is in sharp focus. A shallow depth of field means that, with your lens focused at 10 feet, objects 9.5 and 11 feet from the lens might be on the ragged edge of being blurry. A deep depth of field means objects positioned anywhere between three feet from the lens and the distant horizon may be in sharp focus.

The iris setting affects another aspect of the image as well: sharpness. Every lens has an iris range over which it will deliver its crispest, highest-contrast image. This range usually sits near the middle of the iris' travel. When the iris is stopped all the way down, lens performance and resolution may suffer slightly. A stopped-down iris creates a very broad depth of field, which ensures that nearly everything in front of the lens is in optimum focus. This helps counteract the slight loss of image quality in very bright light.

Much worse is when the iris is all the way open. The performance of some lenses can drop noticeably when the iris is all the way open, delivering a softer image with washed-out contrast. The ultra-shallow depth of field that results from a large iris setting only makes things worse. The implications of a wide-open iris are significant--we'll explore these implications further in a moment.

Another lens performance factor that affects how much light hits the sensor could be called "zoom loss." By nature of zoom lens design, tighter zoom settings cause the lens to pass less light than wider settings. And we're not talking just a slight reduction in light--a modern lens, fully zoomed in, may pass less than 10% as much light as compared to fully wide setting. Better-quality lenses hold zoom loss to about one f-stop, or a 50% reduction in light levels between fully wide and fully tight.

This loss of light, when zoomed in, can tax the camcorder's imaging system and compromise image quality. If light levels are high, the camcorder's iris will swing open to compensate for the reduced light transmission of the zoomed-in lens. If you don't have much light to work with, the iris won't be able to open up far enough to compensate for the loss of light. Your camcorder's gain-up circuit will electronically boost the signal, with a resulting increase in video noise.

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