Camera optics

High-quality pictures start with precision optics.
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The sweep of technology fascinates me. In the 45 years I have spent in the business, I've seen incredible change in almost every aspect of the industry. Consumers can buy cameras that produce images superior to that of the best cameras of 20 years ago. HD imaging has risen from laboratory oddity to common production practice since 1978 and into the hands of consumers in the form of exquisite camcorders for less than $1000. Recording technology has moved through analog tape and even analog disk to digital recording on flash memory and optical disks in portable packaging. Electronics advanced from vacuum tubes heating equipment rooms to toasty comfort to microelectronics using a tiny fraction of the energy once required. Signal processing progressed from strictly an analog possibility to image scaling and movement in cell phones.

Optics are stuck in analog

This photo of a 1950s camera shows how lenses were mounted on a turret to enable an operator to choose the appropriate focal length.

There remains one part of the complex system for creating and manipulating content that is permanently stuck in the analog domain — that is if you ignore quantum effects, which in point of fact is becoming difficult to do. Putting an image on a sensor is now and always has been the province of analog optics. Don't get me wrong; optic science has matured and now produces images no one would have thought possible 20 years ago. With sensor sizes from 1/6in to larger than a film frame, lenses must be capable of producing resolution, contrast, uniformity of focus and low chromatic aberration. This is truly science, as well as difficult engineering.

Monochrome cameras used in the 1930s for early commercial experiments in television were fixed focal length. Often the lenses were mounted on a turret to allow the camera operator to choose a focal length appropriate to the scene and taking distance. Size manipulation was done in the same manner as film production, with the camera moving to create an object zoom instead of the common optical zoom done today. In fact, the invention of the zoom lens for television was a huge boon to all. In the 1950s, a zoom lens was available, but it was not until the 1970s that fixed focal length lenses disappeared from common use. By the era of the first plumbicon cameras, fixed focal length was no longer a rational choice.

But optical innovation was barely keeping up with the increasing level of imaging quality cameras were capable of. Early color television cameras suffered from registration errors even with the most careful technical setup, leaving residual optical errors (chromatic aberration and other effects) just observable in the final image. Unfortunately, the most recent improvements in cameras have forced intense scientific and engineering development in optics. As we approach an era when 4k and soon 8k cameras are commonplace, we will bump against the limits of physics, as well as straining the ability of excellent engineering to keep ahead of imaging science. Diffraction limits for some imagers and lens combinations are real effects.

The trade-offs of technology

As the industry inexorably pushes manufacturers for longer zoom ratios and higher performance, it is asking for trade-offs that must be understood. Longer focal length without increased physical length presents an interesting challenge. A 101-1 zoom lens (one commercial model is actually 8.9-900mm) is 660mm long, only two-thirds of the maximum focal length, but more incredibly the wide end of the zoom is accomplished with a lens nearly 75X as long as the optical path we are creating. With a 2X extender, such a lens has a viewing angle of only 18 arc minutes, just over half the diameter of the moon as seen from the earth. This is perhaps as much a high-resolution telescope as a television lens.

Choosing a lens should be done carefully and with consideration of the application and the rest of the camera chain. A lens with a large aperture may be valuable when working in low light conditions, but comes with a trade-off. MTBF is affected by moving either to high or low f-stop settings, reducing lens performance. Illumination uniformity across the sensor is also affected by both the zoom and aperture settings. This can lead to the feeling of looking down a porthole. Low-cost lenses, especially ones not intended for HDTV applications but used on HD cameras nonetheless, will also degrade performance materially. It is tempting to select a camera that might extend the life of a good SD lens on a new HD studio camera. However, the precision with which new lenses are built matches the quality of the new HD cameras available.

It is easy to see why. A camera designed to be high quality in SD needs to support a limiting bandwidth of only 5MHz. An HD camera is capable of nearly 30MHz luminance bandwidth. The SD sensor has one-sixth as many pixels as a modern 1080p camera. Clearly, a lens optimized for SD and built to be cost-effective in that application can make compromises that a good HD lens cannot make, both technical and in manufacturing economics. It is tempting to try to save money when replacing cameras by retaining lenses, but it presents huge trade-offs. Perhaps the most obvious is aspect ratio. An SD lens does not need to provide high performance outside of the 1.33:1 sensor area, which is 25 percent less sensor area than the 1.778:1 HD TV aperture. Masks are put on lenses to prevent flare outside of the intended image, and a 4:3 mask is the wrong idea for a 16:9 image.

Conclusion

As HD cameras become more cost-effective, the lens may again become more expensive than the camera. But keep the analog nature of imaging in mind, and acknowledge that high-quality pictures start with precision optics. That is not possible with low-cost optics, which limit the performance of HDV and other camera formats that use optics packaged with a low-cost camera. The rules of physics are hard to bend.

John Luff is a broadcast technology consultant.

Send questions and comments to:john.luff@penton.com