Mark Schubin /
12.03.2008 12:00AM
HD Upgrade: It's Not Just the Electronics
NEW YORK
Preparing for a long desert drive, should you be more concerned with water, gas stations, tire condition, and sources of assistance or with the displacement and torque of the engine? Shooting HDTV is similar. You'll probably want to be more familiar with lighting, lenses, connections, and even iris settings than with the bit depth of the analog-to-digital converter of your camera.

Consider just that iris setting. You already know the iris controls the amount of light entering the camera, and you might also know smaller holes (higher ƒ-stop numbers) offer greater depth of field (range of distances at which subjects appear to be in focus). But did you know that in HDTV the iris can also control the sharpness of your pictures?

LOOK SHARP

The culprit is diffraction, a physical phenomenon that causes light to spread as it passes an edge. It also affected standard definition cameras, but not very noticeably.

That's because the equation for the proportion of contrast-ratio allowed by diffraction includes a spatial-resolution term; call it line-pairs per millimeter (lp/mm). The equation is CR = 1–(1.22*λ*ƒ*lp/mm), where CR is the amount of contrast ratio (1 being 100 percent), λ is the wavelength of light (red at 630 nm is pretty much worst case) and ƒ is the ƒ-number.

With an old, inch-and-a-quarter-format, tube-based standard-definition camera feeding a typical band-limited NTSC color TV, the lp/mm figure was about 9.6. Plug that into the diffraction equation, and you'd have more than 84 percent contrast at maximum resolution even at ƒ/22. At ƒ/8, you'd be up at 94 percent, pretty close to perfect. Diffraction, in those days, could safely be ignored.

If you'll excuse the pun, contrast that with a 2/3-inch format, 1080-line HDTV camera, which has an lp/mm of 100. At ƒ/8, you're down to 38 percent contrast at maximum resolution. Less than two stops later, at ƒ/13, the contrast at maximum resolution is zero. It doesn't matter what comes before or follows the iris; you're no longer shooting full-resolution HDTV. It gets worse as you go to smaller-format cameras. A full-resolution 1/2-inch-format camera has an lp/mm of about 138 and falls to zero contrast at 1920 pixels per line at about ƒ/9.5. For a 1/3-inch camera, it's 184 and f/7.

You might think, therefore, that you'd get most sharpness at the lowest ƒ-number, and, if diffraction were the only criterion, you'd be right. But some lens aberrations are worse at the largest iris setting. For common 2/3-inch-format camera-lens combos, maximum sharpness will probably be at about ƒ/4 (lower numbers for smaller formats), but you should experiment to see what works best for you.

LENSES & LIGHTING

Spatial resolution is also of concern in lenses. In a way, there's no such thing as an HDTV lens. All lenses can pass HDTV resolution but at differing contrast ratios. A high-end, so-called HDTV lens will offer a much higher contrast ratio at maximum resolution than will a low-end, non-HD lens, and perceived sharpness is proportional to the square of the area under a curve plotting contrast ratio against fineness of detail.

This is another area where imager format comes in. If a 2/3-inch-format lens offers a particular contrast ratio at 1920 pixels per line, that's at 100 lp/mm. To offer the same performance, a 1/3-inch format lens would have to offer the same contrast ratio at 184 lp/mm. That would suggest that the smaller-format lens should be more expensive, the opposite of what manufacturers charge. Draw your own conclusions about resulting image sharpness.

Then there's lighting. Despite one lighting manufacturer's claim that it offered "high-definition" light, there's actually no difference between photons intended for standard definition and photons intended for HD, but there are some lighting considerations to an HD upgrade.

First, all else being equal, HD cameras are slightly less sensitive and have lower dynamic range than their standard-definition counterparts. Of course, all else is never equal, and HD cameras, being newer, can be more sensitive. But then there's detail.

In standard definition, artificial detail is usually added to make pictures seem sharper. HD is often shot with less of that detail added, because the pictures (even when viewed in standard definition) are already sharper. But, depending on the lighting, turning down the detail can reduce the "separation" of foreground from background.

Cameras, even when they have lights attached, don't do their own artistic lighting. If you're unfamiliar with the effects of key lights, back lights, and the like, start learning. A good place to begin is Lowel's online school (www.lowel.com/edu), which offers interactive examples of different lighting techniques.

Another good idea is to learn what different camera-processing settings do. Panasonic has offered a superb demo reel with its Varicam. The Goodman Guides to different cameras are another good source of personal study, and the Santa Fe HD Workshops have offered training in groups.

Lighting, camera-processing settings, and lens iris settings (and optical filtering) all interact. It's not possible to deal with one without dealing with the others.

Although some tripods have been labeled as "high-definition," there's really no such thing (though it could be argued that a tiny bit of mount instability is more significant in HD than in standard definition). But there is another significant difference between some HD and SD cameras, and that's in the cable that connects them to their control units.

After years of triax, many HD users are now faced with SMPTE 311 fiber-optic camera cable with a SMPTE 304 connector. Although there are many advantages to fiber-optic cable, that connector can take some getting used to. For one thing, it needs cleaning in ways that triax connectors don't.

The best source of information on the subject might very well be a SMPTE Engineering Guideline, "Inspection and Cleaning Processes for the 304M Hybrid Connector," which should be published by about the time this article is. It's well worth reading. It'll not only make your fibered HD cameras happier, but it can also save you the cost of more expensive cleaning systems.

Now go forth and shoot HD!

Mark Schubin is an engineering consultant whose clients range from Metropolitan Opera to Sesame Workshop.



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