Richard L. Hess
Why the 18% Gray Card is a problem

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The question that many people ask is:

If you meter an 18% grey card with a reflective meter, it will give you the same reading as an incident type.

Well, yes and no. This has been the traditional line, but in fact the standard camera meter is calibrated more to something like 12% or 13%.

I anticipated a roughly 1/2 stop difference in exposure and got it when comparing a used Minolta Autometer IIIf and my Nikon F4s's and Nikon FA. If you factor this 13%-18% difference into account, then all four meters were with approximately 1/3 stop of each other (I say approximately because the FA's manual readout does not resolve that closely).

My 1987 copyright Kodak gray cards do not mention this, but I think the more current version suggests opening a 1/2 stop.

I wanted to understand certain things about this.

In digging into the "American National Standard for General-Purpose Photographic Exposure Meters (Photoelectric Type)" ANSI PH3.49-1971 I have come across a very plausible explanation for this.

As we know, an illuminance of one footcandle falling on a perfectly reflective surface produces a surface luminance of one footlambert. Since the reflectance is diffuse, the footcandle and footlambert are related by a factor of pi.

So knowing that and that incident light is measured in footcandles while surface luminance is measured in footlamberts we can examine the equations in the ANSI standard:

2^EV = A^2/T = BS/K = IS/C

EV equals exposure value T = effective exposure time in seconds A = actual f-number of lens diaphragm S = ANSI speed of film (using the ASA series, not the old DIN degrees) B = field luminance in footlamberts K = exposure constant (reflected light) I = incident light in footcandles (illuminance) C = exposure constant (incident light)

Now, the ANSI standard gives the value of C as 30 +/- 5. If you use a value of 30, and run through the calculations based on an ISO 100 EV value, the illuminance comes close to matching a stand-alone Minolta illuminance meter (different receptor geometries). Also, if you work backwards and forwards with these numbers, I also confirmed the Sunny-16 rule, although as one might expect in SoCal in the middle of summer that it was closer to Sunny 16.5.

Interestingly, using the value of 30 for C rather than the implied value of 27 in the Minolta book provides closer correlation to the Minolta illuminance meter that we have at work.

Then after some more math in the standard, the value of K is derived as 3.64.

After all is said and done, all we need to do is look at the ratio of K/C which is 3.64/30 or 12.3%. That is where the 12-13% comes from. It is NOT 18%. Using the 18% gray card as a metering reference will cause approximately 1/2 stop underexposure as the reflected light meter is assuming 12%.

Hope this helps!

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