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02-25-2010, 01:43 PM   #16
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If all else was equal, the calculations would be perfect. But of course as things are in the real world, they are *not* equal. Obviously. So the caclulations are not *perfect*. However, even *in the real world*, there is still less variability in noise performance for a given sensor size than many imagine (especially when comparing RAW data). Bottom line is that this simple type of calculation can still be worthwhile to give you a basic idea as to approximately (say, within a stop - probably within half a stop most of the time) what kind of ballpark you're in. If someone wants to disagree that knowing within a stop or or so is useful to you, I can't argue. I'd call it useful to me, if not to you. But if you disagree that's lkely to be accurate to within a stop, I think you'll be surprised if you actually do the math and compare it to real world results.

To be sure, it's more useful in comparing between interchangle lens *systems* than between specific models. One can say FF, for example, is about a stop better than APS-C overall and use this as a basis for comaprison, because long term, that's how it's going to average out, even if at any given point in time the most interesting FF model might be behind of its curve while the most interesting APS-C might be ahead, thus temporarily narrowing the gap. The next year those shoes will be reversed. Overall, though the years, it will average out. But when comparing *specific models*, then there's no overall / "through the years" involved.

02-25-2010, 05:40 PM   #17
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Let's get basic

ISO (or ASA, back in the day) is a measure of sensitivity to light. Plates, film, anything with a photosensitive emulsion, analog phototubes, digital sensors, ALL can be assigned ISO ratings. It just means that a certain intensity of light is needed to form an image of a certain density on the plate/film/sensor/whatever.

Digital: At a specified pixel density, sensors rated at the same ISO level should have similar noise levels, irregardless of the sensor dimensions. Increased density on same-size sensors produces more noise. That's physics.

Comparisons: The K20D density is about 4.1 mpx/cm2 (megapickles per square centimeter). My 5 megapickle PNS's of various brands, with their teeny sensors circa 2004, have densities around 13.7 mpx/cm2, so we can expect them to be about 3x noisier. My old 1 megapickle PNS has a density of about 6.2, with its even teensier sensor circa 2001, so it should be only about 1.5x noisier than the K20D. Squeezing 10 or 12 megapickles onto teensy sensors gives densities up to 45 mpx/cm2. Ay carumba! (But are they 10x noisier? And does CMOS vs CCD make a difference?)

Now, that's all at base ISO. IIRC, various makers rate their base ISO levels at anywhere from 80 to 200. The latter may be cheating, because: to attain higher ISOs, the signals from the sensors are amplified. And here is where we lose any basis for straight-across comparisons: Not all amps are equal, nor components, nor QC standards, nor even sensors from the same production line. Within and between models and makers, are variations, sometimes rather noticeable.

So we can test individual cameras, and say with certainty that THIS camera model 'W' at ISO 'X' has image noise equivalent to THAT camera model 'Y' at ISO 'Z', when tested at the same lab bench at about the same time. Fine. But beyond that, we can only generalize and equivocate. That's life, by Heisenberg!
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