system calibrated as prescribed in ISO 2721, whose exposure index
is set to the (saturation-based) ISO speed of the camera. As a result of the automatic exposure system, the average exposure on the sensor will be based on your ISO setting regardless of the average or range of the luminance of the scene. This means that a scene that might want ISO 200 in one area might want ISO 800 in another. Wouldn't it be cool if you could tell your camera to do that, and not blow out your sky while trying to get the darker subject's exposure right?
Well, there might be a new Sheriff in town.
The CIPA (the technical association of the Japanese camera industry) suggests that the introduction of Standard Output Sensitivity (SOS) be created as an objective standard measurement of sensitivity rather than ISO speed. What would this involve?
The use of a consistent exposure metering calibration by setting the meter’s exposure index to SOS rather than ISO speed, the camera could record a higher ISO for low-luminance portions of the scene. At the same time, the brighter, high-luminance portions of the scene would record a lower ISO.
Images taken with the exposure index set to SOS, “direct from the camera”, without any post processing, displayed onscreen in a straightforward way, would appear “brighter” than images of the same scenes taken with the exposure index set to ISO speed.
This could be seen as a marketing advantage among manufacturers. (“Wow! What kind of a camera did you take these with? I have to get one!”)
The use of SOS would then effectively eliminate the 18% reflectance rule of the average light. (We would still use a white or 18% gray card to measure color, but not light metering.
And, by the way, your camera's ISO settings have always provided a cushion of 1/2 stop under-exposing to exposures. This 1/2 stop can be thought of as providing a cushion against the possibility of a high-reflectance region in the scene receiving an exposure area greater than the camera's ability to read it causing “clipping of highlight detail”. So, if you want the actual exposure reading for a given sign, increase your Ev to 1/2 stop and your pictures will have that "pop" that many of them might be missing due to the dreaded "cushion". But beware of blown highlights.
What does all this chatter about a new ISO replacement mean to us, as photographers?
It largely depends on what camera manufacturers do. We could assume that this new technology suggests that camera manufacturers will be moving to an SOS system thereby making all our current cameras obsolete. If a scene were evaluated for its variations in dark and light and the SOS applied the sensitivity as needed instead of the whole scene at ISO 800, it could be that everyone will eventually gravitate to the new system, either out of choice or necessity.
This could be a cool change, expensive probably for us, but none the less, cool.