| Wednesday, September 21, 2005 |
11:16 - I wish I could set it to ISO 9000
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I have a question about digital photography that perhaps someone can answer.
Why is it that the ISO ("film speed") equivalency settings on modern digital cameras, particularly those with CMOS sensors (as in my Canon 10D), give you quality and performance so close to what you'd get with film of the same speed?
This is an ISO 1600 shot taken at the San Jose Giants championship game on Monday (they won). Pretty grainy, huh? Which is pretty much as you'd expect: there wasn't much light, and I was using a long lens without much of a huge aperture. Cranking up the ISO to 1600 was the only thing I could do to get a reasonably fast shutter speed.
But why does ISO 1600 on a CMOS sensor look just as grainy as ISO 1600 on film?
Is it by design, or an amazing coincidence?
These are two completely different technologies. ISO speeds on film, as I understand it, depend on fundamental limitations of film chemicals to react with light, and there isn't much that can be done to make an ISO 1600 film roll react as smoothly to light as an ISO 200 roll. But with digital, we're talking about an entirely different set of problems, right? CMOS technology keeps getting better... doesn't it? Who's to say that a CMOS sensor can't be made that reacts to light at a 1600-equivalent speed without going grainy?
Or is it that the photon density really is that low—that in the amount of light that enters the lens from a low-light scene, passes through a tight aperture, and hits the sensor in 1/250 of a second, there are really so few photons that there's a measurable distance between where one photon hit it and where the next one over did? I don't know the physical details well enough to know if that's what's going on, and it seems unlikely to me (I'd always been under the impression that even in the dimmest light, billions of photons were hitting any microscopic square of surface in any given millisecond). But maybe I'm wrong.
And maybe I could have kept from appearing so ignorant by doing a little research or some calculations or something. Hey, it's what I'm supposed to be trained for.
But even so, it seems that digital cameras ought not to be bound by the same restrictions as film cameras, and if CMOS sensors could be made to react that much faster without displaying graininess, just imagine how photography would change...
UPDATE: Several people have noted that what I'm seeing here is noise, not grain; and noise is a necessary side effect of cranking up the gain on the sensor when the signal-to-noise ratio is low, as in low light. Thanks to all who mailed!
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Brian Tiemann
