Doug Kerr
Well-known member
The introduction of the sensor sensitivity metric ISO SOS (to be used as an alternative to ISO speed) was intended to "bump" the exposure calculated by the "standard" exposure equation in a camera automatic exposure system.
The motivation was that the sophisticated exposure control systems of modern digital cameras did not require the 1/2-stop "headroom" that was built into the traditional metering equations, since traditional exposure meters used a principle that was dependent on assumptions that might not hold true for any given scene, while the sophisticated systems are not (so much) dependent on such assumptions, but instead did more "thorough" measuring.
Thus, the normal use of the traditional exposure equations (with the ISO speed as the exposure index) in effect "leaves 1/2 stop of sensor range on the table". The 1/2-stop exposure bump from using ISO SOS as the exposure index "eats the headroom" (thus giving better dynamic range, noise performance, and such).
Suppose we use an properly-calibrated free-standing exposure meter in connection with a camera whose sensitivity is denominated in terms of ISO SOS (e.g., typical current Canon cameras), and set the "ISO number" from the camera into the meter as our exposure index.
The result is that the meter will recommend an exposure 1/2 stop greater than contemplated by the meter manufacturer.
Now of course a traditional reflected light exposure meter cannot reliably produce the "best exposure" over the range of scenes we may encounter.
But if our meter would have produced a "very suitable" exposure for a particular scene when the exposure index is set to the ISO speed (as the meter design contemplates), then when we set the exposure index to the ISO SOS we may in fact encounter overexposure of highlights.
If, for use with a camera operating on an ISO SOS basis, we wish to encounter just what the meter manufacturer had in mind for each scene (for better or worse), then we should set the exposure index into our meter at about 1/2 stop greater than the "ISO number" to which we have set the camera (about 1.4 times the numerical value).
Best regards,
Doug
For a given "sensitivity", ISO SOS is about 1/2 stop less than ISO speed. The result, when ISO SOS is used as the exposure index into the classical exposure equations is a 1/2-stop greater photographic exposure than if the exposure index were set to the ISO speed.
The motivation was that the sophisticated exposure control systems of modern digital cameras did not require the 1/2-stop "headroom" that was built into the traditional metering equations, since traditional exposure meters used a principle that was dependent on assumptions that might not hold true for any given scene, while the sophisticated systems are not (so much) dependent on such assumptions, but instead did more "thorough" measuring.
Thus, the normal use of the traditional exposure equations (with the ISO speed as the exposure index) in effect "leaves 1/2 stop of sensor range on the table". The 1/2-stop exposure bump from using ISO SOS as the exposure index "eats the headroom" (thus giving better dynamic range, noise performance, and such).
Suppose we use an properly-calibrated free-standing exposure meter in connection with a camera whose sensitivity is denominated in terms of ISO SOS (e.g., typical current Canon cameras), and set the "ISO number" from the camera into the meter as our exposure index.
The result is that the meter will recommend an exposure 1/2 stop greater than contemplated by the meter manufacturer.
Now of course a traditional reflected light exposure meter cannot reliably produce the "best exposure" over the range of scenes we may encounter.
But if our meter would have produced a "very suitable" exposure for a particular scene when the exposure index is set to the ISO speed (as the meter design contemplates), then when we set the exposure index to the ISO SOS we may in fact encounter overexposure of highlights.
If, for use with a camera operating on an ISO SOS basis, we wish to encounter just what the meter manufacturer had in mind for each scene (for better or worse), then we should set the exposure index into our meter at about 1/2 stop greater than the "ISO number" to which we have set the camera (about 1.4 times the numerical value).
Best regards,
Doug