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MillimeterLeitner's Cinematography Corner, No. 6
Nov 12, 2009 12:00 PM, By D.W. Leitner
When is sharp sharp?
Infinity mark on focus ring of Fujinon 7.6-130mm HD ENG zoom (ZA17x7.6BERM-M58H) travels past witness mark. Why?
Photo by D. W. Leitner
It's rarely convenient to wait until dark to train a lens on a star field to verify proper infinity focus, so collimators—reverse telescopes—were invented. A collimator is a tube with a miniature, rear-illuminated focus chart or starburst-like Siemens star at one end and a simple lens at the other, set to perfect infinity focus relative to the plane of the focus pattern. Instead of being brought to focus, the image of the focus pattern is projected in reverse out the front of the lens, its rays emerging in parallel.
If a camera lens, in turn, is set to infinity focus and pointed directly at the output of the collimator, it will focus the collimator's images perfectly—that is, if properly mounted to the camera.
Infinity mark on focus ring of Zeiss 7mm DigiPrime stops precisely at witness mark, as it does on all motion-picture lenses. Zeiss, world's oldest lens manufacturer, continues to set benchmarks.
Photo by D. W. Leitner
With 16mm, it's necessary to use an autocollimator to check focus. An autocollimator projects an infinity image into the lens, then by use of a partially silvered mirror and viewfinder, provides a direct look at the miniature image actually projected on film running through the camera. (Since each successive frame of film is unique, poor registration or field flatness can skew the results of examination by autocollimation, which is why I invented a stroboscopic autocollimator in the early '80s. But that's a tale for another day.)
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Video camcorders, by comparison, can output the image that falls on the focal plane of their sensor(s) directly, so complicated autocollimation is avoided. But even so, collimation of videocameras and camcorders never caught on. In the early '80s, before CCDs, professional color videocameras and camcorders utilized three pick-up tubes for RGB, each of which was shock-mounted in rubber, grew hot in use, and routinely had to be mechanically aligned relative to the shared focal plane and each other. Electronically aligned, too, to match linearity, geometry, shading, etc. What a pain!
Unlike 16mm, NTSC video was not a high-resolution format—and this fact forgave many optical sins, not only poor corner resolution but degradations inflicted by 2x extenders (which you'll never find on a film zoom). All this changed with the advent of digital high definition. Suddenly video zooms had to match a higher standard of optical performance, one closer to 16mm.
This led to breakthroughs like the introduction by Zeiss of its DigiPrime series seven years ago, B4-mount video lenses that can arguably outperform any film lens. Which, in turn, upped the ante with regard to both precision mounting and backfocus adjustment of HD lenses—why Zeiss felt it necessary to introduce its own superb video collimation device, Sharp Max.
With Sharp Max, perfect backfocus at infinity is achieved readily, with confidence. The back-illuminated Siemens star seen in the camera's viewfinder leaves no doubt when best results are achieved. Since Sharp Max was designed as a complement to DigiPrimes, it attaches directly to them by means of a clamping ring. But I've tried holding it in front of a variety of other lenses, such as a Fujinon 7.6-130mm HD ENG zoom, which works remarkably well.
In a perfect world, video collimators akin to Sharp Max would be familiar and common location tools.
Don't mean to pull an Andy Rooney here, but have you ever noticed how video zooms focus past infinity? What's up with that?
And why are focus marks measured from the focal plane for all photographic systems, for more than 160 years—except video? Video M.O.D. (minimum object distance), for instance, is always indicated from the front of the lens.
Grist for future columns...
Continue the discussion on “Crosstalk” the Millimeter Forum.