Values of K other than 12.5 are most likely attributed to two factors: spectral sensitivity of the photo cell and lens transmittance. This is the K equation:
From Appendix C3 Spectral Considerations
The spectral response of the film and the detector as well as the spectral quality of the light in the scene, and the light used in calibration and sensitometry, affect the meter calibration. If one factor (r) is chosen to relate the photocell’s spectral response to the scene as compared to that calibration, and a second factor (p) is used to relate the film’s spectral response between scene and the sensitometric evaluations, these factors can be combined with (R) into the constant K
1 of equation C4. (Eq C4 H
g = K
1/S
x). These factors are defined as follows:
Firstly, r = ratio of luminance of uniform surface source used in calibration to luminance of scene when both sources produce the same response of the meter.
Since the calibration color temperature of 4700K was chosen to minimize the spectral effect indicated luminance in daylight compared to the indicated luminance for tungsten, the spectral response ratio determined between 4700K and 2850K is a reasonable measure of the effect of difference in spectral sensitivity between daylight and the 4700K calibration sources.
Secondly, p=ratio of the photographic effectiveness (actinity) of scene luminance to the photographic effectiveness of illuminance used in determining film speed.
It is now possible to define a basic constant K
1 which excludes all variables except film speed Sx and camera exposure Hg due to measured field luminance. K`
1 is defined as the value of K
1 when:
r = 1.0
p = 1.0
R = 1.0
Equation C4 may be modified by substituting the above parameters to obtain:
H
g = (K`1 * r) / (p*S
x*R)
Appendix C5 Assumed Values (excerpt)
Since r and t are subject to change, it is desirable to establish a constant K
o which is not likely to be changed greatly as the above variables change. Therefore, K is defined as follows:
K = (K
o * r) / t
--------------------------
According to
Re-evaluation of Factors Affecting Manual or Automatic Control of Camera Exposure, this equation provides “a better foundation for establishing different values of the constant K used to calibrate meters under a wide variety of applications. In particular,
the loss in transmission due to the many elements in a zoom lens, and the effects of highly red or blue sensitive detectors are taken into account.”
A value of 14 for K means the Ev will be a touch lower and the exposure calculator will have a slightly slower shutter speed or lower f/stop than it would for a value of 12.5 in at least one of the two situations. A K of 14 could easily mean the manufacturer has concluding that zoom lenses are more prevalent and with the additional elements has a lower transmittance than the ~ 0.90 of the standards. The additional exposure will make up for this optical system light loss and place the exposure at H
g. A lens with fewer elements might have higher transmission properties causing the exposure to be slightly higher than required.
(11.4 * 1) / .81 = 14.1
If the value of 14 comes from the photo cell being more sensitive to the color temperature of the calibrating light source, the meter will read the calibrated value for L as higher than it is. A value of K will mathematically reduce it and the exposure calculator should then produce the same exposure recommendation as with K = 12.5. I have to admit this part is just speculation as I’ve not read anything about this specifically nor am I privy to any manufacturer insider information.
(11.4 * 1.1) / .90 = 13.9
One of the reasons why the 18% question continues to persist is that it really isn’t that relevant. Whatever the meter is pointed at, it wants to place it on the same exposure point (log-H) for a given film speed. The f/Stop and shutter speed will probably be slightly different with the different value of K, but the exposure is always supposed to fall at the same point. If the difference in K is from the spectral sensitivity of the photo cell at 4700k, K is compensating for a reading that is either higher or lower than the target value. If the different K comes from the value of t, it may or may not represent the lens you are using, but the exposure difference is small.
