TV CAMERA LENSES: A MATURING TECHNOLOGY

Aspheric lenses

Spherical lenses are rounded as if made from a slice of a ball.
Because rounded shapes are easy to grind, sherical elements have been
used in mass produced lenses since Galileo. The problem with
spherical lenses is that the edge of the lens focuses light a little differently than the center of the lens; when the middle of the picture is in focus, the outside edges may be a little fuzzy. When the outside edges are sharp, the middle may be a little fuzzy. You have wrestled with this phenomenon while trying to focus a slide or overhead projector onto a large screen: it's hard to get the whole picture sharp at once. The problem was correctable, but required additional lens elements (separate glass discs) to be teamed up
together, each element bringing its own new problems to the equation.
Aspherical lenses are shaped differently, correcting for this focus berration. Aspherical lenses have been in use for years but were prohibitively expensive because they could not be mass produced; they had to ground individually.

One way around this problem was to mold the lenses from plastic. They focused well but plastic lacks some of the other desirable optical properties of glass. Plastic lenses are used in home camcorders where low cost and light weight are more important than high quality. Companies like Tamron even produce hybrid CCTV lenses which use glass coated with an optical resin that is molded into the aspherical shape.

Four years ago Fujinon introduced a process that would squash hot glass into an aspheric mold. The process was so accurate that the glass didn't even require grinding or polishing (a difficult step for aspherical lenses) as it emerged from the mold. The advantages of the aspheric lenses were manifold. Not only would the lens focus sharply from the center to the edge of the image, but one aspheric element could replace three spherical lens elements. With fewer elements came reduced weight and size. With fewer glass-to-air surfaces, the lens exhibited fewer internal reflections and flares. Transmission (the ability to pass light through a lens without scattering any of it) improved while chromatic aberrations (rainbow-like color fringing, especially at the edge of the image) were reduced. Not only could the zoom lens be smaller, but it could have a broader zoom ratio with this new technology.First Fujinon applied the aspheric lenses to their regular lens line and made their lenses heavier, more expensive, and reduced the zoom ratio. Wheels, at first, were not as good as skids. Fujinon then redesigned their lenses from scratch, taking advantage of the aspheric properties, and now bring a mature technology to us. Four years later, the wheels rotate and the new lenses are living up to their potential.Nikon has just started building aspherical elements for their lenses. Nikon offers its S15x8.5III 15X zoom lens (can zoom to 15 times its minimum focal length) with the ability to focus as close as 22 inches (most lenses presently focus only up to 30 inches).


Dead zone

Another area of maturation is the MOD. Four years ago, Angenieux introduced a 15 X 6.5 2/3-inch f/1.6 lens with a minimum focus distance of 18 1/2 inches. With time we can expect to see the MOD (minimum object distance) whittled away until the lens can focus continually from infinity to macro (where the lens focuses on objects nearly touching the glass). This feature has be available in home camcorders for quite some time and it is very handy allowing one to start with a wide shot of the wedding reception and dolly in to a close up of the bride's 3 karat zircon, remaining in focus all the way. This feature has slowly seeped into the prosumer market, but has barely scratched the professional market. Although professional camera lenses have macro capabilities, there is still a "dead zone" between the MOD and the macro distance.A few years ago JVC introduced Auto Macro-Focus on some of its new consumer models. The feature allowed full automatic focusing from the closest macro position (about 1/4" from the lens) to infinity. This is a very useful feature but be aware that most models require you to go to full wide angle in order to use the Auto Macro feature. Any time you move an object to 1/4" from the lens, you cast a nasty shadow on the object. It would be much better if you were able to zoom in on an object shooting it from several feet away in the macro mode. Just a few lenses have this macro-while-zoomed-in ability.

Internal focus

For years, lenses focused by rotating the outer elements. Disadvantages: graduated and polarizing filters rotated along with the lens. Also, external moving parts collected dust and dirt. External gear teeth could get stripped. The entire focusing and zooming mechanism, mounted onto the external lens, was exposed to the environment. It would be nice to have the outside element remain stationary and have all the motion occur inside the camera. Not only would the complex motion of the lens elements be easier to control inside the camera, allowing wider zoom ratios, but the mechanism would make a sleeker package, more appealing to consumers. As an added benefit, users found that they could set a camera on a desk, propping the lens on a yogurt container, and not have the thing fall off as the auto-focus lens rotated.
 
Internal focus made it easier for lenses to focus continuously from infinity up to macro. Minolta's VHS-C C3300 was the first internal focus camcorder that allowed videographers to venture through the dead zone and this innovation has now spread to virtually all of the small consumer camcorders. These systems even focus well when you affix supplementary lens attachments such as wide angle or telephoto converters.To manually focus an internal focus lens (sometimes useful in low light when the camera has trouble deciding where to autofocus), some camcorders have switches that move the inner lens electronically; there is no lens barrel to turn. Nikon has recently introduced a professional 20X 8mm internal focus lens using aspheric elements.

Extra low dispersion glass

Nikon calls this ED glass and Canon calls it UD glass. It is a flourite-basedformulation of glass that transmits light efficiently scattering the least amount along the way. The new glass reduces halation, the glow you get when light bounces around inside the lens causing a slight fogginess. You see halation when you are driving into the sun with an older car whose windshield has seen its share of sand and pebbles. Low dispersion glass buys you an extra half f-stop or more of sensitivity depending upon the number of elements in the lens. This gain translates into lower f numbers for a lens and low lux ratings (high sensitivity) for the camera

Zoom torque

For those who still like to zoom and focus manually, there are plenty of lenses with this feature. Introduced in 1993 and spreading among manufacturers is the zoom torque control. This feature increases the drag on the zoom lever allowing for a smoother manual zoom

Image stabilization

How do you keep a picture from bouncing when you're shooting from horseback? The Schwem people had the solution years ago with their GyroZoom image stabilizer. The device is very effective, but heavy and expensive. The consumer market has seen EIS (Electronic Image Stabilization) and DEIS (Digital Electronic Image Stabilization) mature and flourish leaving a slight vacuum in the prosumer and larger vacuum in the professional markets. EIS really works and is a boon to prosumers and wedding videographers who don't have the time to mount their camcorders on a tripod.There are two basic forms of image stabilization used today, optical and electronic. Each has advantages and disadvantages. Canon and Sony employ the optical approach (Sony, however, uses the electronic approach in its CCD-TR500 camcorder). Sony calls its optical stabilization "Steady-Shot". Two piezoelectric velocity sensors respond to camera movement along the camera's vertical and horizontal axes. As the camcorder wiggles or changes position, these sensors activate motors that effect a vari-angle prism inside the camera. This prism consists of two pieces of flat glass with a special silicon-based oil between them. When the camera jiggles, the motors squeeze the bellows, making the glass surfaces no longer parallel, bending the light and moving the picture. Thus if you jiggled the camera to the left (which jars the picture to the right), the optical image stabilizer will shift the picture from your CCD chip to the left. The system can compensate for a 1.4 degrees horizontal or vertical movement, enough to hold the picture still if you are already trying to hold it still. Move the camera too far, of course, and the picture bumps over. You turn off the EIS to perform a smooth pan or tilt and turn it on when you are trying to hold a steady shot.The Mitsubishi "StableCam" system, like the Canon system, uses horizontal and vertical piezoelectric sensors to note motion of the camera. But instead of moving the picture with a prism, Mitsubishi moves the picture totally electronically. Mitsubishi shifts the picture by clocking the data out of the CCD chip earlier or later than normal. By waiting a few microseconds and outputting the top line from the CCD chip, for instance, that line will appear lower on your TV screen (the TV was sweeping lines across its screen even though it wasn't getting data). The late data is thus shifted lower on the screen. The downward movement of the picture would compensate for an accidental downward movement of the camera. Naturally, if you read the data from the CCD chip late, you would create a black band across the top of the screen where there was no data; your image would remain stable but this black frame would be dancing around the periphery of your picture. To solve this problem, the manufacturers electronically zoom in on the chip, using just the pixels in the middle 85% of the chip. Thus the top left pixel of the CCD chip is not the top left pixel of the TV screen image. If the camera were steady, perhaps a pixel that's 30 down and 30 across from the top left corner would begin the top left part of your TV image. Now with this border available, when the camera is bumped downward, the formerly idle pixels at the top of the chip come into play. At the bottom of the chip 30 idle pixels become 60 idle pixels. Similarly, horizontal bumps to the camera cause the first or last pixels in the chip to be read out. Perhaps you've already guessed the disadvantage in this process. If you are only reading 85% of the pixels with DEIS turned on, you are losing 15% of your image sharpness. Thus you get a fuzzier but steadier picture.Hitachi and Sony attempt to solve this problem by installing an oversized CCD with 470,000 pixel elements in their top-of-the-line models. Thus when the middle 85% of the CCD chip is read out, it contains full resolution. Those extra pixels are just waiting for the camera to be bumped.Panasonic and JVC also use digital electronic stabilization, but without motion sensors. The entire image is digitized and subdivided into a number of reference points. Using "fuzzy logic" computations, an internal computer compares all the reference points. If some points change value while others do not, the system presumes the subject is moving and not the camcorder so no correction is made. If, however, all of the reference points have shifted in a similar direction, the computer presumes the camcorder is moving and adjusts the picture accordingly. As with the previous designs, the camera steadies the picture by reading deeper into one or another edge of the chip, thus shifting the picture in the direction of the bump.Each system has its own advantages and disadvantages. The optical systems, since they use the entire chip, tend to have a sharper picture; they are not wasting any pixels. Also, the optical system, sensing physical motion, is unaffected by the picture content; it works well in low light, zoomed in, zoomed out, whatever. The JVC and Panasonic systems need a fairly bright clear picture to "lock on" to the data for steady pictures. Because the systems are electronic, however, they are smaller, lighter, and cheaper to manufacture and require less power from the battery to operate. As mentioned earlier, the electronic systems are not using all the pixels in the chip (unless DEIS is turned off, in which case they can use all of the pixels creating a picture with maximum sharpness), thus yielding a fuzzy picture except for the models that have more pixels in their CCD chips to start with. Incidentally, this blurring is a minor matter when you consider that many of these camcorders are unable to record on tape the full resolution of the CCD. Switching EIS off will show a sharper picture directly on your TV monitor, but from tape, the difference will hardly be noticeable.

HDTV lenses

Lenses that are to be used for high definition television need to focus sharper than their NTSC forbears. The lenses must also shoot a wider aspect ratio (16:9) than NTSC (4:3). Put these together and they spell more expense and workmanship associated with HDTV camera lenses.

Lens mounting

Mounting various lenses on cameras is a bit of a black art. Consumer and prosumer cameras have built-in lenses that are not removable. You can attach filters, close-up attachments, and telephoto and wide angle adapters to the outside of the lens, but with very few exceptions, you cannot remove the lens and upgrade it. One notable exception is the VL (Video Lens) mount adopted in 1990 by Canon, Hitachi, Matsushita, and Sony. The VL mount permits quick release of one lens and reattachment of another while retaining autofocus, iris, and power zoom capability. The Canon L1 and L2 Hi8 camcorders employ this lens which can be used on similarly equipped 35mm cameras as well.In the professional world, cameras and lenses are slightly difficult to mate because there are so many variables. Standards exist but details like back focus (the distance between the CCD chip and the closest lens element), mounting standard, chip size, focal length, and manufacturer models need to be taken into account. The major lens manufacturers keep track of the camera models and publish lists that allow you to mate one of their lenses to your camera. Many of the camera manufacturers also publish lists of what lenses work with their cameras. As a generality, you have to take into consideration whether you are using 2/3" or 1/2" CCD chips; the lens needs to be designed for a particular size chip. ENG cameras generally use bayonet mounts where the lens is inserted into the front of the camera and twisted a quarter turn to lock into place.Sometimes a coupling ring is turned instead of the lens to unite the two. Canon, Nikon, and Sony often use a standard called B4 which describes a popular kind of mount (1/2" and 2/3" mounts both have a B4 style). Ikigami uses a slightly different B3 mount. Adapters exist which go from one kind of mount to another. CCTV, monochrome, and older cameras, especially those taking fixed focal length lenses, still use the common C-mount where the lens is screwed onto the front of the camera until snug.

Lens cleaning tips

If you see a dark speck on your picture that moves with the image, perhaps you have a piece of dirt on your lens. If the outermost element of your lens rotates and the dirt rotates with it, you pretty much know that that's where the dirt resides. If it happens to be a flake of dust, try to avoid blowing it off with your breath; unless you've been in the desert awhile, tiny strands of spit will likely fly onto the lens taking the place of the dust which is long gone. There are little bellows and squeeze blowers made for removing dust from lenses. Some have soft brushes on them to help with the process. Cans of compressed air are also good for removing dust. With compressed air you can also clean the inside element in your lens, or your camera's prism block or CCD sensor. Fingerprints are harder to remove. Simply rubbing lenses with dry cloth or lens tissue may scratch their delicate lens coatings. Also, never apply lens cleaning fluid directly to a lens; the solution will dribble into the crevices of the lens mount possibly dampening the interior of the lens. Dampness in the lens can cause the glass to fog. To remove water spots or fingerprints, dampen a rolled up piece of lens tissue, about the diameter of a cigarette, with lens cleaning solution. Gently swab the surface of the lens from the center out. Roll up a dry piece of lens tissue to finish the job buffing the lens lightly. Camera lenses may seem like a ho hum topic, much like the nonrotating wheel in prehistoric times, but the recent perfection of several technologies has made lenses an exciting part of the camera again.