[dms]

Here is data I took in Sept 2011, meteorology class compared to theory. Not done super carefully--just well enough to show trend is reasonable. To do much better would require a considerable outlay of money or time. For some reason I couldn't get the jpg with more details/more data to appear as an attachment.
Of course in a darkened room w/o smoke. That is how one applies the science. When you use it you make the secondary corrections as need be.

angle..measured......theory (%)
(deg)...(%)...............=cosine (angle)*100%
0........100................ 100
20........94...................97
40........85...................76
60........62...................50
80........24...................17
____________
measured (%) = (measured intensity at angle/ measured at 0 degree)*100%

Attached Images

PentaxForums_CosineLAw314.pdf (49.2 KB, 99 views)

[lmd91343]

As for gray card readings outdoors, close to two thirds of the light that reaches the surface of the earth is from light scattered in the atmosphere. Think about ambient light just before sunrise or after sunset. Also think about open shade. That ambient light will swamp the one third of the direct light on the card as modified by the angle of that card to the sun.

[stevebrot]

No myth--light reflected (from a target) is proportional to cosine of the angle of light source-to-the target normal.

...and that is why you place the card in the same orientation as the subject and meter from the direction of the camera. At least that is the current user instructions on the Kodak Web site. In practice, it is more useful to follow the older instructions as in imd's post above.

As for the 18% value. It is commonly misstated that it represents the average illumination of common scenes. Rather, it is as Adams states, the midpoint in the range from white to black. That is why it is sometimes called middle gray. Adam's uses the analogy of middle "A" on the musical scale. It is a well-known point and a useful standard of reference. Anyone who has done B&W darkroom work will understand where the gray card fits in to the general puzzle of exposure. Quite simply, it provides exposure that will allow the photographer to attempt to utilize the full dynamic range of the film.

The whole 12% vs.18% discussion continues to puzzled me. It is a phenomenon of the Internet from what I can tell. The meter makers (Sekonic, Gossen, and company) appear to be ignorant of the excellence of the 12% card and continue to make meters whose incident readings correspond nicely to reflected readings from an 18% card. The vendors of photo gear are also apparently ignorant since 12% cards are not on the shelves.

I know that Thom Hogan (whom I respect despite some of his brain-dead advice regarding gear) strongly advocates the 12% card, but a quick Google search indicates that he pretty much stands alone.

On a related note, has anyone actually seen a 12% card? Is it one stop darker than the 18%? How does it fit into Zone System thinking?

Edit: According to Thom Hogan, a 12% card is 1/2 stop darker than 18%.


Steve

---------- Post added 05-04-14 at 08:37 PM ----------

Here is data I took in Sept 2011, meteorology class compared to theory. Not done super carefully--just well enough to show trend is reasonable. To do much better would require a considerable outlay of money or time. For some reason I couldn't get the jpg with more details/more data to appear as an attachment.
Of course in a darkened room w/o smoke. That is how one applies the science. When you use it you make the secondary corrections as need be.

angle..measured......theory (%)
(deg)...(%)...............=cosine (angle)*100%
0........100................ 100
20........94...................97
40........85...................76
60........62...................50
80........24...................17
____________
measured (%) = (measured intensity at angle/ measured at 0 degree)*100%

The cosine law is well-known and easily demonstrated. I learned about it in General Physics and did a similar exercise in lab. Unfortunately, daylight (and most studio light) is a fairly diffuse light source and not strictly unidirectional. There is fall-off with angle, but the amount varies with atmospheric conditions. As noted in the posts above, field measurements usually don't detect significant differences based on angle. It is also good to note that the surface of the better gray "cards" (some are fabric-based) is designed to mitigate the effect of angle such as to provide uniform reflectance and spectral composition regardless of orientation.

In any case, I interpret your comments to mean that a 45 degree measurement would yield a measurement equivalent to a 12% card straight on for a unidirectional light source (no smoke in the room). The problem is the assumption of the 45 degree angle. Common practice is to either ignore angle or orient the card the same as the subject and meter from the direction of the camera. The latter is how it is done in cinematography where the card is used both as an exposure reference and also as a neutral point for color correction. (The card is actually included in the scene for production purposes.)


Steve

[stevebrot]

I just did a quick look-see on both the Sekonic and Gossen Web sites and both suggest the 18% card for use with their meters for reflectance readings. Both also strongly suggest using incident measurement for general use with most subjects and educated use of a 1 degree spot meter for finer control in placing values with some subjects.

Gossen provides an interesting case for traditional meter usage that points out the weaknesses of even the most sophisticated in-camera meters.

http://www.gossen-photo.de/pdf/Pro_Externe%20Belichtungsmesser_GB.pdf


Steve