HD Upgrade: It's Not Just the Electronics

December 3, 2008
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?



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.




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.



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.


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.


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!


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