Lens technology

All televisions are at least partially analog. The process of acquiring and displaying images is light. One might argue that due to the dual wave and quantum particle nature of light, it is not “analog,” but in the context of replicating the experience of human vision, light causes responses on the basis of analog intensity (flux density) variations over an area, as well as color, as measured by wavelength or frequency. Similarly, the display must emit light that can be interpreted by the eye as a copy, entomologically an “analog,” of the original image.

An RCA TK41 with a turret of lenses is shown above. Photos courtesy Chuck Pharis.

To make the television system work, light is processed using at least one strictly analog device: the camera lens. Lens technology may have been the first of several enabling technologies that led to the development of television. In this article, images electronically generated, as well as the display, will not be discussed. In the German language, “television” is pronounced “fernsehen,” which quite literally means “distant seeing.” The role of a lens in a telescope, binoculars or other optical aid is to allow us to see further and the same could be said about the role of television.

Early commercial television cameras were modeled after film cameras, with fixed focal length lenses and a mechanically adjustable iris. Camera output gain was controlled by adjusting the gain of the chain, rather than by changing exposure. This worked fine in controlled conditions, but remote locations often provided a challenge that the camera could not handle. Television cameras have been asked over the last 75 years to acquire images with flux densities varying from starlight to nuclear explosions, which simple gain controls cannot track. Hence, a key development in the modern lens was the attachment of a motor and servo to the iris to allow exposure control from the camera CCU. With modern CCD cameras, exposure can be controlled by an electronic “shutter,” which controls the time the light is allowed to build up on the sensor before being read out. Still the range of light a CCD can read is only a few 100:1, and the iris remote is still needed. It also serves to allow creative control over depth of field for the lens, which is particularly important for long, focal-length zoom lenses.

Early monochrome cameras had fixed lenses or a “turret” of several fixed lenses that an operator could select without moving the camera relative to the scene. The first zoom lens for television was introduced in 1946, but it was many years before the performance was good enough to be widely used.

By the 1970s, zoom lenses were common, with 10:1 and even 18:1 ratios readily available for studio and field applications. When 30:1 lenses were introduced, it became clear that the race was on for even longer zoom ratios. At that time, the quality of a lens seemed to be in direct proportion to the zoom ratio. Long zooms came with the unavoidable fact that full aperture was not usable because the lens at wide angle might be f2.8, but at full zoom it might be three f-stops slower. The falloff of light meant that in many applications, the full zoom range was not usable.

The Zoomar lens was commonly used in the 1950s.

Today zoom lenses are taken for granted. Modern sports remotes command the longest possible zoom ratios, as high as 100:1. The vertical image size with such a lens is about 1/3 of a degree, less than the diameter of the moon (using a 1150mm focal length from an 87 × 13.5mm lens). Such long focal lengths come with unavoidable side effects: They cannot be handheld, and they cannot be held steady without stabilization technology. The good news is that such technology is now readily available; the bad news would certainly be the price of these lenses, with list prices well over $100,000. HDTV lenses in this class can cost more than the list price of the camera head they mount on.

Over the last 30 years, lenses have improved in quality dramatically. Zoom lenses were not readily available early in television history because of the difficulty of getting all colors to focus at precisely the same point and with the precise same image size. Lenses inherently do not pass all colors of light through with the same refraction. As a result, the lens must be carefully corrected to be achromatic. In tube cameras, corrections could be made for the lens to adjust the image on all three pickups to be perfectly in registration despite minor flaws in the optical path. CCD cameras do not allow corrections due to the fixed pixel locations, so the lenses must be considerably more precise. HDTV cameras raise the bar even further.

Other improvements have included advanced servo electronics. Modern lenses have a complex microprocessor-controlled servo with digital readouts of focal length, f-stop and other data communicated back to the camera for relay to the operator and the CCU. Though a good camera operator can do a wonderful job manually, it is a joy to run a camera with memory presets for focus and zoom. It can turn a mediocre camera operator into a star.

Today the future of our industry makes even passive optics a complicated technical discussion. For instance, the movement to 16:9 aspect ratio is now well under way, and many cameras are asked to do double duty for 4:3 and 16:9 uses. This requires a ratio converter in the lens to make the image size optimum for both. In addition, some cameras today can operate with variable aspect ratio, to mimic the 2.35:1 aspect ratio commonly used in film. It is not clear what kind of lens is required when a single camera/lens combination might be used for 1.33:1 to 2.35:1 images. Television lens manufacturers are now building “prime” cine style lenses for HDTV acquisition applications, a sign of the movement to electronic acquisition using film techniques.

Modern lenses also have corrected other flaws to an amazing level of precision. Focus breathing (the change of focal length with focus change) is controlled to a large degree in all modern lenses. This is done in part with internal focus elements, rather than the front element moving as was common in older zoom lenses.

Certainly as time goes on, the march of zoom length will continue unabated, though it is sometimes hard to understand why. Mechanical and optical precision will continue to improve and (hopefully) costs will come down to levels that will make the best optics more affordable.

John Luff is senior vice president of business development for AZCAR.

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