Lens, Iris and Sensor Design Combine for Best Quality Imaging

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.

The Looking Glass

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.

Wild Iris

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.


Sensor Science

Having passed through the lens and iris, light then hits the image sensor. At the heart of the sensor assembly is a Charge Coupled Device (CCD) made up of hundreds of thousands of light-sensitive pixels arranged in a rectangular grid. These pixels store up an electrical charge in proportion to the amount and duration of light hitting them. Every 60th of a second, the camcorder reads these charges, combines them to create an image and discharges the CCD.

Without any engineering trickery, a CCD would respond only to the amount of light hitting it, not the color of the light. In other words, a CCD is by nature a color-blind device. Camcorder designers use two different approaches to extract color information from monochrome sensors, approaches that split the camcorder field into two camps.

Single-CCD camcorders are just what their name implies: models that use a single CCD sensor to handle all the image-making duties. Such a camcorder derives color information from the sensor by covering it with a mosaic of colored lenses. Like a microscopic stained-glass window, this “color mask” alternates red, green and blue (or cyan, yellow, magenta) panes over individual pixels. With some clever electronic processing, the camcorder can derive both a brightness (luminance) and color (chrominance) signal from the single CCD chip.

Three-chip camcorders use a trio of CCDs, each specializing in a certain color. By using a complex prism block or arrangement of mirrors and filters, a 3-CCD camcorder splits the light coming through the lens into three color components. Light from each of the color components (red, green and blue, for example) goes to its own sensor. The camcorder combines the output of these three chips to create a full-color video signal.

Single-CCD imaging systems offer the advantage of being smaller, lighter, less complex and cheaper to manufacturer. Three-CCD systems, though larger and costlier, generally deliver more accurate color at a higher resolution. Three-CCD designs may also deliver a hard-to-define improvement in image depth and realism. Three-CCD camcorders often have better lenses than their single-chip counterparts, a fact which also contributes to an improvement in video quality.

Recent years have seen a trend toward smaller and smaller image sensors in camcorders, from 1/2-inch to 1/3-inch to today’s tiny 1/4-inch designs. A smaller CCD doesn’t just mean a smaller sensor assembly; it means a proportionally smaller lens as well. Every aspect of a lens’s design points back to how large an image it needs to create. If a lens needs to bathe a 1/4-inch sensor in light instead of a 1/2-inch sensor, designers can shrink the lens assembly considerably. This translates to smaller, cheaper and more compact camcorders.

Since a CCD’s sensitivity is proportional to the surface area of each pixel, a smaller sensor will be less-sensitive to light if all other variables are held equal. In reality, the variables aren’t held equal. CCD manufacturers have found ways to gather more light onto a smaller sensor. This gives today’s smaller CCD designs low-light sensitivity on par with that of larger sensors.

Sensor resolution also plays a factor in image quality, up to a point. Once a sensor’s resolution exceeds that of the recording system and tape format, there’s little to be gained by increasing the sensor’s pixel count. A 270,000-pixel CCD delivers ample resolution for a standard format like 8mm or VHS. Will a 470,000-pixel sensor result in sharper images in these formats? Probably not. Where extra pixels can be put to good use in with digital zoom and image stabilization.

Steady as She Goes

Camcorder manufacturers have been trying to put a damper on the shakycam for many years now, introducing various image stabilization schemes that sense camera movements and compensate for them by shifting the image. Some systems use tiny motion sensors to detect motion, while others analyze the CCD image itself.

The simplest image stabilization is called electronic image stabilization (EIS). It shifts the CCD image to compensate for what it perceives to be unwanted movements. First, it crops the outer edges of the CCD image and expands the inner portion to fill the screen. It then slides this active image area around on the face of the CCD to compensate for camera movement. If a camera wiggle causes your subject to jump left 14 pixels on the CCD, the active image area also moves 14 pixels to compensate.

Because EIS throws away image pixels, some systems cause a noticeable drop in image resolution. Other EIS schemes use a higher-resolution CCD than the format really requires, allowing for lossless EIS.

The other method of quelling camera shake is optical image stabilization (OIS). OIS shifts elements in the lens itself to compensate for camera movement. This basically “steers” the light in the opposite direction of the camera shake. Because OIS performs its magic in the lens itself, there’s no resolution loss whatsoever.

In the past, OIS systems have used a variable prism to steer the light. This prism is basically two pieces of glass connected with a flexible bellows and filled with optical fluid. Motors flex the bellows on two axes to change the path of the light. Manufacturers couldn’t make the variable prism small enough for use in ultra-compact camcorders, so Canon recently came up with a different optical stabilization system.

Called an “optical lens shift” mechanism, this OIS scheme moves a group of elements on two axes to eliminate shake. This system is simpler than the variable prism, and can be made much smaller. Future compact camcorders will use optical lens shift technology.

The Perfect Image

There’s a lot going on inside your camcorder to turn light into a usable video image. Some of this engineering wizardry is beyond your control, and some of it is yours to command. Armed with a better knowledge of what you can and can’t control, it’s time for you to start coaxing the best-possible images out of your camcorder.

Videomaker
The Videomaker Editors are dedicated to bringing you the information you need to produce and share better video.

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