[font color=\'#000000\']What is often called "ISO" for digital cameras should be called "Exposure Index" (EI). It is not governed by any ISO standard. (Some trivia: a proper ISO film speed specification always combines both the ASA and DIN numbers, like "ISO 100/21", so the numbers used these days for digital are better described as "speeds in ASA units".)
This exposure index determines the combinations of aperture ratio and shutter speed that will make the subject that the light meter sees come out as a "mid-tone", after appropriate processing in the camera.
If we want to compare to real ISO film speeds, we have to look at things like the shadow noise levels, which should rise above the background noise about four stops below the mid-tones, so two things should be noted.
a) many DSLR's exceed this ISO standard for shadow handling at their lower EI settings, so that the closest thing to a true ISO speed might be a middle setting, like 400 or 800.
at least one DSLR, the Fuji S2, applies different degrees of noise filtering as the ISO setting is increased, so that the shadow noise levels do not increase nearly as much as with some other cameras. Instead, resolution probably decreases, much as it does with films of higher speeds. It seems that some Canon DSLRs do this too, though they have not started it so clearly as Fuji apparently did in an interview with Phil Askey of DPReview. In fact, the rather slow rise in measured noise levels of JPEG output as "ISO" setting increases on most digital cameras suggests that something like this is quite common: noise levels do not increase as fast as rough physics suggests it should.
With a strategy like this, it is conceivable that some or all of the ISO settings of a digital camera could roughly satisfy the ISO standard for shadow handling in films, at the cost of decreased resolution at higher speeds.[/font]
