Doug Kerr
Well-known member
In its 2006 edition, the ISO standard for determining the "ISO speed" of digital still cameras took on two new alternative measures of camera sensitivity, the standard output sensitivity (SOS) and the recommended exposure index (REI). As I understand it, these were included at the behest of CIPA, the Japanese camera technical organization, which had developed these measures and earlier promulgated them in one their own standards.
Before I go further, let me clarify two terms:
ISO speed - a property of a particular type of film or a particular digital camera (in any given "ISO" setting mode). It tells us (in a certain sense) the sensitivity of the medium to exposure. It is determined in a specified way under specified conditions.
Exposure index - what we tell an exposure meter the ISO speed of the medium is. Most commonly, we set the exposure index to the applicable ISO speed, but we might not.
The new measures are:
ISO standard output sensitivity (SOS) - Going right to the bottom line (skipping the definitions and algebra), this is always a number that is 0.71 times the (saturation-based) ISO speed for the camera. It can be presented this way:
"ISO 400 (SOS)" or "ISO 400 (standard output sensitivity)".
ISO recommended exposure index (REI) - This is what the manufacturer of a particular camera recommends we set as the exposure index on our meter. It is the value the manufacturer feels will give good results in a lot of situations. It is not (necessarily) determined under any standardized procedure or based on any standardized defintion. It can be presented this way:
"ISO 400 (REI)" or "ISO 400 (recommended exposure index)"
These terms appear in the specifications for the Canon EOS 1D Mk III (perhaps for the first time in the Canon line - I'm not sure). They are basically presented this way:
"ISO 200, 400, 800 . . . (standard output sensitivity, recommended exposure index)"
My first reaction was, "well, which is it - SOS or REI?" But I think this is what they mean: the ratings that are presented are ISO SOS values (presumably determined under the ISO standard), which by implication are what we should (under ordinary circumstances) set as the exposure index in a meter. They are also what Canon recommends we set as the exposure index in a meter (REI). A little redundant, but . . .
As a matter of fact, a while ago I determined what exposure index values were used in the Canon EOS automatic exposure systems, based on a "kitchen table" test to verify the proper operation of EOS automatic exposure systems passed on by Chuck Westfall. I determined that the exposure index values used by Canon we perhaps 65% of the value that would be determined under the ISO procedures for (saturation-based) ISO speed. I didn't realize then that Canon was ahead of their time - that these "ISO speeds" (as they were called then) were actually (at least very nearly) SOS values (not yet at the time defined by the ISO). (Canon is of course a participant in the CIPA group responsible for this work.)
Before I go further, let me clarify two terms:
ISO speed - a property of a particular type of film or a particular digital camera (in any given "ISO" setting mode). It tells us (in a certain sense) the sensitivity of the medium to exposure. It is determined in a specified way under specified conditions.
Exposure index - what we tell an exposure meter the ISO speed of the medium is. Most commonly, we set the exposure index to the applicable ISO speed, but we might not.
The new measures are:
ISO standard output sensitivity (SOS) - Going right to the bottom line (skipping the definitions and algebra), this is always a number that is 0.71 times the (saturation-based) ISO speed for the camera. It can be presented this way:
"ISO 400 (SOS)" or "ISO 400 (standard output sensitivity)".
ISO recommended exposure index (REI) - This is what the manufacturer of a particular camera recommends we set as the exposure index on our meter. It is the value the manufacturer feels will give good results in a lot of situations. It is not (necessarily) determined under any standardized procedure or based on any standardized defintion. It can be presented this way:
"ISO 400 (REI)" or "ISO 400 (recommended exposure index)"
These terms appear in the specifications for the Canon EOS 1D Mk III (perhaps for the first time in the Canon line - I'm not sure). They are basically presented this way:
"ISO 200, 400, 800 . . . (standard output sensitivity, recommended exposure index)"
My first reaction was, "well, which is it - SOS or REI?" But I think this is what they mean: the ratings that are presented are ISO SOS values (presumably determined under the ISO standard), which by implication are what we should (under ordinary circumstances) set as the exposure index in a meter. They are also what Canon recommends we set as the exposure index in a meter (REI). A little redundant, but . . .
As a matter of fact, a while ago I determined what exposure index values were used in the Canon EOS automatic exposure systems, based on a "kitchen table" test to verify the proper operation of EOS automatic exposure systems passed on by Chuck Westfall. I determined that the exposure index values used by Canon we perhaps 65% of the value that would be determined under the ISO procedures for (saturation-based) ISO speed. I didn't realize then that Canon was ahead of their time - that these "ISO speeds" (as they were called then) were actually (at least very nearly) SOS values (not yet at the time defined by the ISO). (Canon is of course a participant in the CIPA group responsible for this work.)