Exposure meter accuracy tolerance

[Doug Kerr]

ISO standard 2720 gives requirements for the behavior of photoelectric photographic exposure meters, both reflected light and incident light types. It does this by way of equations for each type of meter that include a "calibration constant", symbolized as K for reflected light meters and C for incident light meters.

But no values are prescribed (or even suggested) for these constants, only for each one an "acceptable" range (fairly wide). This recognizes the reality that this standard is meant to "codify" (but not normalize) the diverse existing practices of exposure meter manufactures as to the "calibration" of their meters. Thus a meter manufacturer could adopt that value of K (or C) for their meter they have found gives results that suit most of the users most of the time.

The standard then says: "The criterion of overall accuracy of the meter shall be the deviation, from the stated value, of the constant K [or C, as applicable]."

The standard then gives a quite detailed procedure for determining the actual value of K [or C] exhibited by a meter under test.

Yet nowhere does the standard express (or even suggest) a degree of the deviation of the value of K [or C] from the stated value that is "permitted" under the standard.

So for some meter model, the manufacturer may state that it uses a value of 12.5 for K. In a laboratory, we determine by scrupulous and tedious testing the actual value of K practiced by a specimen meter and find that to be 11.2. Does that meter conform to the standard as to its accuracy? That is not defined.

Best regards,

Doug

 

[Ted Cousins]

ISO standard 2720 gives requirements for the behavior of photoelectric photographic exposure meters, both reflected light and incident light types. It does this by way of equations for each type of meter that include a "calibration constant", symbolized as K for reflected light meters and C for incident light meters.

But no values are prescribed (or even suggested) for these constants, only for each one an "acceptable" range (fairly wide). This recognizes the reality that this standard is meant to "codify" (but not normalize) the diverse existing practices of exposure meter manufactures as to the "calibration" of their meters. Thus a meter manufacturer could adopt that value of K (or C) for their meter they have found gives results that suit most of the users most of the time.

The standard then says: "The criterion of overall accuracy of the meter shall be the deviation, from the stated value, of the constant K [or C, as applicable]."

The standard then gives a quite detailed procedure for determining the actual value of K [or C] exhibited by a meter under test.

Yet nowhere does the standard express (or even suggest) a degree of the deviation of the value of K [or C] from the stated value that is "permitted" under the standard.

So for some meter model, the manufacturer may state that it uses a value of 12.5 for K. In a laboratory, we determine by scrupulous and tedious testing the actual value of K practiced by a specimen meter and find that to be 11.2. Does that meter conform to the standard as to its accuracy? That is not defined.

Best regards,

Doug

Hello Doug,

Sekonic for the L-398 states K=12.5 and also gives a fractional accuracy of "0.3 EV (1/3 stop)". So perhaps a measured K of e.g. 15.4 is just OK according to them.

Your 11.2/12.5 = -0.158 EV for whatever that's worth, inside Sekonic's spec. Not sure if that's relevant to your post.

best,

Ted.

 

[Doug Kerr]

Hello Doug,

Sekonic for the L-398 states K=12.5 and also gives a fractional accuracy of "0.3 EV (1/3 stop)".

Yes.

Interestingly enough, ANSI PH3.49-1971, the US ancestor of ISO 2720, in fact gives specific target values of C and K and also states an "accuracy tolerance" on each. The value of C is stated (in non-SI units)as "30 ± 5". (In SI units that would be, to integer precision, 323 ±54.) The value of K (for use with film larger than 8 mm) is stated as 12.5. (The value of K takes into account assumed values of the lens transmittance and the spectral response ratio of the photocell.)

Here is the passage relating to C:

******
Calibration Accuracy. In determining the overall
accuracy of the meter, the deviation from the stated
value of the constant C shall be considered the criterion
of accuracy. Indications over the angular center half of
the scale shall be within ± 0.333 Ev of the constant C
and within ± 0.5 Ev for all other usable points on the
scale. If two scales are used, the multiplying means shall
not introduce error exceeding ± 0.167 Ev.
******

Of course here "Ev" is used to mean "stop", a usage that is slightly justifiable but not attractive.

The reference to "the stated value" [of C] of course is consistent with the "choose to taste" doctrine of ISO 2720. In that regard, PH3.49 says:

******
The exposure constants K and C may be assigned
values in accordance with Table I [the values I quoted above] unless the design
parameters differ from the assumptions explained in
Appendix C. In this case, new values may be computed
as outlined. The value of K or C, or both, shall be
marked on the nameplate of the exposure meter or
given in the instruction manual furnished with the
meter by the manufacturer. The manufacturer may
choose the value which results in optimum exposure
for the most critical film, usually color film.
******

Appendix C is quite complex, and I ill not attempt to summarize it here.

The reference to parts of the "scale" presumably allude to the meter movement scale proper.

Best regards,

Doug

 

[Ted Cousins]

Yes.

Interestingly enough, ANSI PH3.49-1971, the US ancestor of ISO 2720, in fact gives specific target values of C and K and also states an "accuracy tolerance" on each. The value of C is stated (in non-SI units)as "30 ± 5". (In SI units that would be, to integer precision, 323 ±54.) The value of K (for use with film larger than 8 mm) is stated as 12.5. (The value of K takes into account assumed values of the lens transmittance and the spectral response ratio of the photocell.)

Here is the passage relating to C:

******
Calibration Accuracy. In determining the overall
accuracy of the meter, the deviation from the stated
value of the constant C shall be considered the criterion
of accuracy. Indications over the angular center half of
the scale shall be within ± 0.333 Ev of the constant C
and within ± 0.5 Ev for all other usable points on the
scale. If two scales are used, the multiplying means shall
not introduce error exceeding ± 0.167 Ev.
******

Of course here "Ev" is used to mean "stop", a usage that is slightly justifiable but not attractive.

The reference to "the stated value" [of C] of course is consistent with the "choose to taste" doctrine of ISO 2720. In that regard, PH3.49 says:

******
The exposure constants K and C may be assigned
values in accordance with Table I [the values I quoted above] unless the design
parameters differ from the assumptions explained in
Appendix C. In this case, new values may be computed
as outlined. The value of K or C, or both, shall be
marked on the nameplate of the exposure meter or
given in the instruction manual furnished with the
meter by the manufacturer. The manufacturer may
choose the value which results in optimum exposure
for the most critical film, usually color film.
******

Appendix C is quite complex, and I ill not attempt to summarize it here.

The reference to parts of the "scale" presumably allude to the meter movement scale proper.

Best regards,

Doug

Well found and most interesting! I've never found or read a full copy of the ANSI std. "Parts of the scale" seems quite relevant, considering the cramping up at each end:

In my previous link to Spielman's "Myth of 18%" based on that Standard, accuracy was not mentioned.