Just a little background here.
"ISO" is an intialism that stands for an international standards body.There is no property of photographic film or of a digital imaging chain whose actual designation is "ISO".
The ISO publishes various standards that give ways of stating what we might call the "sensitivity" of a photographic film or of a digital imaging chain.
With regard to film, the basic sensitivity metric, determined as prescribed by the appropriate ISO standard, is called the ISO speed
. The name of course comes from the fact that with film of a higher sensitivity, an appropriate exposure (for given scene luminance and given aperture) is given by a shorter exposure - a "faster" exposure process.
To provide continuity with practice in the film realm, the basic metric for the sensitivity of a digital imaging chain is also called the ISO speed
. (It is of course determined in a wholly different way.)
Putting aside some complications, the basic "equation" we use (for example, in an exposure meter or automatic exposure control system) to arrive at a "recommended" photographic exposure (that is, combination of exposure time and aperture) for a given scene luminance, taking into account the sensitivity of the film or digital imaging chain (as its ISO speed
) is the same for either medium.
But, especially taking into account the greater sophistication of modern exposure metering and control systems, it was observed that, in the case of digital cameras, this equation typically led to a lesser exposure than was "optimal" - typically about 1/2 stop "short". This gives a less-good noise result than we might actually enjoy.
Camera manufactures could have "tweaked" the operation of their exposure control systems to "take advantage" of this in two ways:
a. Change the "equation" used for automatic exposure control.
b. Keep the equation the same but "rate" the sensor system at a lesser ISO speed
than would be determined by the ISO test method.
Had they done (a), the result would have been that the internal automatic exposure system would have produced a different exposure for a given shot than would have been "recommended" by a properly-calibrated free-standing exposure meter. This would have led to complaints about "inaccuracy" of the exposure control system of the camera.
So they did (b) instead.
But of course this led knowledgeable enthusiasts to complain that the "ISO speed
ratings" of the digital camera were incorrect.
So the ISO defined a new metric for the sensitivity of a digital camera chain, the ISO standard output sensitivity
). This is a different measure than the ISO speed. In fact, again putting aside some complications, for a given sensor chain, the ISO SOS
is about "1/2 stop" less than the ISO speed
. (That is, the ISO SOS
is about 0.7 times the ISO speed
And that "solved" the problem!
Today, the sensitivity of many digital cameras is stated in terms of the ISO SOS
, not the ISO speed
(and that often is stated in the "specifications", although it is sometimes hard to find).
Now for a second chapter of this story, i must refer to the matter of exposure index
. For our purposes we can say that exposure index is "what we tell the exposure meter
is the ISO speed of the film of digital camera chain".
Going back to traditional analog exposure meters, we recall that if we want to bump the exposure the meter recommends for some reason (compensate for backlighting, perhaps), we can set the "ISO speed" dial to a lower value than the actual ISO speed
of the film or digital sensor. And of course, what we set on that little dial is the exposure index (and, as I just illustrated, it is not always the ISO speed of the film or digital sensor chain).
Now, returning to recent developments in the description of digital camera sensitivity, when the ISO introduced the ISO SOS, it also introduced another new "metric", the ISO Recommended Exposure Index
This is a metric that works the same way as the ISO speed or ISO SOS (as a parameter of the exposure equation). It is defined as (and I paraphrase) "the exposure index that the camera manufacturer feels will, as an input to the exposure equation, give a 'desirable' exposure result in many cases."
In many case, digital camera manufacturers now indicate that the sensitivity "ratings" of their camera (at its various "ISO" settings) are in terms of ISO REI. That is, there is not any way that these values can be "correct" of "incorrect".
As a practical matter, in most of these cases, the "ISO" ratings are actually intended to be the ISO SOS values (and often turn out to closely conform to that definition), but the manufacturers do not care to commit to that.
A more extensive discussion of this matter is given here:http://dougkerr.net/pumpkin/articles/SOS_REI.pdf