Doug Kerr
Well-known member
Free-standing exposure meters
International standard ISO-2720 gives specifications for "free-standing" photographic exposure meters. Part of it defines the "exposure equation" which controls (in the case of "reflected light" metering), for a certain measured average scene luminance and a certain user-set "exposure index", what photographic exposure (combination of f-number and shutter speed) the meter will "recommend." This is a "procedure-based", rather than "results based", process.
The equation includes a "calibration constant", the choice of which affects the actual exposure to be recommended for a given combination of scene luminance and exposure index.
The standard "allows" a range of this calibration constant from 10.6 to 13.4. We often hear that a value of 12.5 is commonly adopted by exposure meter manufacturers. (My Miranda Cadius is supposed to - I don't think it does these days!)
Note that the actual photometric exposure produced from a certain scene luminance depends not only on f-number and shutter speed but as well on such factors as less-than-100% lens transmission and "bellows factor" (the decline in photometric exposure when the camera is focused at a fairly short distance).
Thus, when we use a free-standing exposure meter, we may get different photometric exposures (and thus different exposure results) with different lenses, and for objects at different distances. The meter itself is in no position to compensate for that.
For what lens transmission and what bellows factor should the exposure recommendation be "correct"? Well, there is no "correct" exposure. That is in part why the ISO standard allows the meter manufacturer a range of "calibration factors": so that he can "play that" situation in the way he thinks will be best for his product's users.
Inbuilt automatic exposure control systems
International standard ISO-2721 gives specifications for inbuilt automatic exposure control systems. It does not define the "equation" for the systems, but rather expresses a "result-oriented" prescription: the system should provide on the focal plane a certain average photometric exposure (whose level is dependent only on the exposure index, that is, on what we or the camera tells the exposure control system is the sensitivity of the film or digital sensor). The prescription of this in the standard is predicated on the exposure index being in terms of the ISO speed of the film or digital sensor.
This standard does not care how the system manages to do this. If the system uses through-the-lens metering, it will probably automatically compensate for lens transmission and bellows factor. If the system does not look through the lens, it probably doesn't. In the latter case, the result may not be consistent (just as when we use a free-standing exposure meter.
Comparing the results from the two
Now, how can we relate the expose "equation" of a free-standing exposure meter to the exposure equation of an inbuilt automatic exposure control system.
Well, it turns out that (if we ignore lens transmission and bellows factor), the result of automatic exposure (according to ISO 2721) would be the same as the result of metered exposure with a K of about 12.7.
Now, how do lens transmission and bellows factor enter into this?
Well, if we meter with a free-standing exposure meter, even though it precisely complies with the ISO standard (with whatever value of K the manufacturer has adopted), the photometric exposure for any given situation will vary with the lens transmission and the bellows factor.
If we control exposure with an inbuilt automatic exposure control system and it precisely complies with the ISO standard for that, the the photometric exposure for any given situation will not vary with the lens transmission and the bellows factor.
How can that be so if the camera does not use through the lens metering? Well, the catch is that if it doesn't, it really can't precisely comply with the ISO standard! (So I'm off the hook there.)
Exposure index - ISO speed or ISO SOS
Note that in the above I spoke of exposure index without saying whether that was ISO speed or ISO SOS.
In fact, the comparison of the two exposure determination methods, and of the value of "K" that the standard inbuilt automatic exposure control system exhibits, will be the same for either so long as the same premise for the index is used in both methods.
But the actual (absolute) result will vary between the two.
ISO SOS
If we operate an inbuilt automatic exposure control system using the prescribed relationship but use ISO SOS as the exposure index, for a uniform-luminance scene, the average photometric exposure on the focal plan should be about 18.1% of the saturation photometric exposure.
If we use a free-standing exposure meter with K of 12.7, and our lens has transmission of 100% and we have a bellows factor of 1, and again use the ISO SOS as the exposure index into the meter, then we would expect that same result.
ISO speed
If we operate an inbuilt automatic exposure control system using the prescribed relationship but use ISO speed as the exposure index, for a uniform-luminance scene, the average photometric exposure on the focal plan should be about 12.7% of the saturation photometric exposure (almost exactly 1/2 stop lower).
If we use a free-standing exposure meter with K of 12.7, and our lens has transmission of 100% and we have a bellows factor of 1, and again use the ISO speed as the exposure index into the meter, then we would expect that same result.
Canon EOS metering
Finally, to link up with another observation I recently made here, it seems like the overall Canon automatic exposure control scheme in EOS cameras typically results in a photometric exposure on the focal plane of about 17.3 percent of the saturation photometric exposure (about 1/15 stop less than the value I derived for the use of the ISO SOS as the exposure index into the standard scheme of things).
Best regards,
Doug
International standard ISO-2720 gives specifications for "free-standing" photographic exposure meters. Part of it defines the "exposure equation" which controls (in the case of "reflected light" metering), for a certain measured average scene luminance and a certain user-set "exposure index", what photographic exposure (combination of f-number and shutter speed) the meter will "recommend." This is a "procedure-based", rather than "results based", process.
The equation includes a "calibration constant", the choice of which affects the actual exposure to be recommended for a given combination of scene luminance and exposure index.
The standard "allows" a range of this calibration constant from 10.6 to 13.4. We often hear that a value of 12.5 is commonly adopted by exposure meter manufacturers. (My Miranda Cadius is supposed to - I don't think it does these days!)
Note that the actual photometric exposure produced from a certain scene luminance depends not only on f-number and shutter speed but as well on such factors as less-than-100% lens transmission and "bellows factor" (the decline in photometric exposure when the camera is focused at a fairly short distance).
Thus, when we use a free-standing exposure meter, we may get different photometric exposures (and thus different exposure results) with different lenses, and for objects at different distances. The meter itself is in no position to compensate for that.
For what lens transmission and what bellows factor should the exposure recommendation be "correct"? Well, there is no "correct" exposure. That is in part why the ISO standard allows the meter manufacturer a range of "calibration factors": so that he can "play that" situation in the way he thinks will be best for his product's users.
Inbuilt automatic exposure control systems
International standard ISO-2721 gives specifications for inbuilt automatic exposure control systems. It does not define the "equation" for the systems, but rather expresses a "result-oriented" prescription: the system should provide on the focal plane a certain average photometric exposure (whose level is dependent only on the exposure index, that is, on what we or the camera tells the exposure control system is the sensitivity of the film or digital sensor). The prescription of this in the standard is predicated on the exposure index being in terms of the ISO speed of the film or digital sensor.
This standard does not care how the system manages to do this. If the system uses through-the-lens metering, it will probably automatically compensate for lens transmission and bellows factor. If the system does not look through the lens, it probably doesn't. In the latter case, the result may not be consistent (just as when we use a free-standing exposure meter.
Comparing the results from the two
Now, how can we relate the expose "equation" of a free-standing exposure meter to the exposure equation of an inbuilt automatic exposure control system.
Well, it turns out that (if we ignore lens transmission and bellows factor), the result of automatic exposure (according to ISO 2721) would be the same as the result of metered exposure with a K of about 12.7.
Now, how do lens transmission and bellows factor enter into this?
Well, if we meter with a free-standing exposure meter, even though it precisely complies with the ISO standard (with whatever value of K the manufacturer has adopted), the photometric exposure for any given situation will vary with the lens transmission and the bellows factor.
If we control exposure with an inbuilt automatic exposure control system and it precisely complies with the ISO standard for that, the the photometric exposure for any given situation will not vary with the lens transmission and the bellows factor.
How can that be so if the camera does not use through the lens metering? Well, the catch is that if it doesn't, it really can't precisely comply with the ISO standard! (So I'm off the hook there.)
Exposure index - ISO speed or ISO SOS
Note that in the above I spoke of exposure index without saying whether that was ISO speed or ISO SOS.
In fact, the comparison of the two exposure determination methods, and of the value of "K" that the standard inbuilt automatic exposure control system exhibits, will be the same for either so long as the same premise for the index is used in both methods.
But the actual (absolute) result will vary between the two.
ISO SOS
If we operate an inbuilt automatic exposure control system using the prescribed relationship but use ISO SOS as the exposure index, for a uniform-luminance scene, the average photometric exposure on the focal plan should be about 18.1% of the saturation photometric exposure.
If we use a free-standing exposure meter with K of 12.7, and our lens has transmission of 100% and we have a bellows factor of 1, and again use the ISO SOS as the exposure index into the meter, then we would expect that same result.
ISO speed
If we operate an inbuilt automatic exposure control system using the prescribed relationship but use ISO speed as the exposure index, for a uniform-luminance scene, the average photometric exposure on the focal plan should be about 12.7% of the saturation photometric exposure (almost exactly 1/2 stop lower).
If we use a free-standing exposure meter with K of 12.7, and our lens has transmission of 100% and we have a bellows factor of 1, and again use the ISO speed as the exposure index into the meter, then we would expect that same result.
Canon EOS metering
Finally, to link up with another observation I recently made here, it seems like the overall Canon automatic exposure control scheme in EOS cameras typically results in a photometric exposure on the focal plane of about 17.3 percent of the saturation photometric exposure (about 1/15 stop less than the value I derived for the use of the ISO SOS as the exposure index into the standard scheme of things).
Best regards,
Doug