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The HD Quality Puzzle |
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Page 1 of 2 The HD Quality Puzzle
by Mark Schubin
By almost every measure, small-format HD cameras should be worse than their larger counterparts. So why do their pictures seem so good? They are HDTV and do use technologies available 25 years after the first broadcast solid-state camera. But, can a 1/6-inch-format $200 HD camcorder, or even a 1/3-inch-format $5,000 HD camcorder, perform as well as a $25,000-and-up 2/3-inch-format camera?
No. All else being equal, each pixel's individual sensor on the smaller-format imager (chip converting
Originally presented to the Washington Section of SMPTE on May 28, 2009, used with permission.
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images into video signals) is smaller. Less area means less dynamic range (contrast ratio). It also means less sensitivity at any particular f-stop. For sensitivity equivalent to a 2/3-inch-format imager at f/2, a 1/3-inch imager would need to use roughly f/1, but no 1/3-inch-format HD cameras have that large an aperture. Compensating by adding gain yields noisy pictures.
Similarly, 1080-line HD's 1920 active samples per line represent 100 line pairs per millimeter (lp/mm) on a 2/3-inch-format imager. The same resolution on a 1/2-inch-format imager would be 138 lp/mm, 1/3-inch 197, 1/4-inch 245, 1/5-inch 316, and 1/6-inch 379.
A lens for a 1/6-inch format camera, therefore, needs to be almost four times better than a lens for a 2/3-inch camera for similar MTF (modulation-transfer function, indicating contrast delivered at different resolutions). If a 2/3-inch-format lens delivers 80% of contrast at 100 lp/mm, a 1/3-inch-format lens would have to deliver 80% at 197 lp/mm to match the performance.
Unfortunately, small-format lenses are less expensive, not more. Perhaps the mass manufacture of lenses for consumer HD camcorders helps amortize some optical-design expenses, but it doesn't account for that great a disparity.
One gray performance area is depth of field. For comparable shots and lighting conditions, a smaller-format camera will offer more depth of field than a larger-format camera. In shooting news, where it might be desirable for everything to be in focus, that could be desirable. In other videography, where it's desirable to separate the main subject from the background (or even foreground) by means of focus, it's bad.
So the question remains. Why do small-format camcorders seem to look so good? Here are five possible answers.
1. You never see Clark Kent and Superman together.
Small-format camcorders are almost invariably demonstrated by themselves; there is no larger-format camcorder shooting the same scene for comparison on a split-screen monitor. If you want to see what you're missing, arrange for a shootout. Compare image sharpness and texture.
2. Small Begets Small.
According to theories of 20/20 vision, 1080-line HDTV is optimally viewed at a distance of 3.16 times the height of the picture. If you're checking out the image quality of a camcorder from even a distance of just two feet from a monitor, the 3.16x criterion demands a 16:9-aspect-ratio monitor of more than 15 inches.
A small-format camcorder might be demonstrated on just a 7-inch monitor. And a small monitor might have less than full HD resolution, too.
3. Why, Duh.
Fujinon's popular 2/3-inch format wide-angle lens has a minimum focal length of 4.5 mm. Canon introduced an even wider 4.3 mm 2/3-inch-format zoom at NAB 2009. So, if Sony's HVR-V1U HD camcorder has a lens with a wide-end focal length of just 3.9 mm, is that wider still?
No. That camcorder uses a 1/4-inch-format imager. For a shot equivalent to a 2/3-inch-format camera's, it should use a focal length about two-and-a-half times smaller. So, its 3.9 mm is roughly equivalent to 9.6 mm in a 2/3-inch format, a little tighter than the wide end of a normal lens, not a wide-angle lens.
Wide angles are the toughest aspects of lens design. That's why wide-angle lenses cost more than normal lenses, even though they have smaller zoom ranges. By eliminating very wide angles, small-format camcorders preserve what quality their imager size allows.
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