It's been argued and proven scientifically that middle gray is not 18-percent as we've been lead to believe over the years, it's in fact 12- to 13-percent. (source, The National Geographic Photography Field Guide
by Peter K. Burian and Robert Caputo, National Geographic Society, Washington DC, 2003)
18% grey is in fact a gray area--
18-percent gray came about from way back when Kodak pushed it as a standard, some rumor because of heavy influence by Ansel Adams, though Kodak now admits that 18-percent gray doesn't accurately mimic a middle-tone and includes instructions in their new
gray card kits to adjust your exposure + 1/2 F/stop, or when you do the math, 12.75 percent reflectance when an 18-percent gray card is held at a 45-degree angle reflectance to the camera.
The key here is the angle at which you hold your 18-percent gray card, it must be 45-degrees to the axis of the camera, unless you're lighting an image with a ringflash
The camera sees the reflectance at an opposite angle, cos(45degrees) = x/18. which gives you approx. 12.72 degrees.
Someone mentioned Zone 5 in reference to Ansel Adams Zone system is 18-percent gray and the proper way to expose--that's only true for darker skins. Ansel's definition for Zone 5 included it's use to accurately reproduce "dark skin" whereas Zone 6 was for "average Caucasian skin value." Even Adams knew Zone 5, or 18-percent gray was not used for "average" skin.
Even Wikipedia states, "With a slightly revised definition of reflectance, this result can be taken as indicating that the average scene reflectance is approximately 12%," for light meter calibration
Many sources state that camera meters in fact work at 12-percent reflected light based on averaged
metering and depending on the make and model, some cameras come in 10-, 11-, 12-, 13- and even 14-percent flavors while still others use the 18-percent standard, but compensated by software after the reading is taken.
All very technical and if you scour the web you'll find some interesting articles on it, my favorites is from the Photographic Society of America Journal
One other point, if you really want to check your meter's cabilities, take a solid piece of white card and an identical piece of black card, then go outside in an open shade area. Place your camera on a tripod, set the exposure to "program" or automatic, then photograph each card where it totally fills the frame of the camera--no outside influences and have your meter set to average or matrix. Take a mental note of the exposure values. Then take the same two cards, place them side by side and photograph them and see what you get? All three photos should be gray!
Why, because your camera meter is a reflectance
meter and is calibrated to see only middle-gray (11- to 18-percent) and will take that black card and give you the correct exposure to make it gray, same with the white card, to make it grey.
If you took Roger's black, gray, white card combination he posted, and metered through the camera
for the black area only (making sure you fill the entire camera frame with black only) then while holding the exposure stepped back and photographed the entire card combination, the gray and white portions of the will reproduce as white while the black is reproduced as middle gray--an overexposed
image would look like this on a histogram.
If you did the exact opposite and your reflected meter reading was from the white portion only, white will reproduce gray and the black and gray portions will reproduce black, or the equivalent of underexposed
in a histogram.
In the end, it's a very gray area and technical. I highly recommend you throw the technical out, learn to "feel" how you camera operates and "know" your equipment. Even the same make and model cameras and meters will be different from each other as they used a lot of algorithms and "canned" calibrations or programming, making no two cameras or meters alike. Learn what histograms can do for you as they are the "fingerprint" of the scene. Learn to judge your scene and how that should replicate on a histogram pattern--don't be fooled that a histogram has to look like a nice mountain range from end to end. For the record, 11- to 18 percent gray is in the middle area of a histogram.
Note to Roger:
The McBeth ColorChecker™ Color Rendition Chart you posted is misleading when you lable it as "...Custom White Balance set with a Gray Card." When I first look at that image with that label on the top it makes it look like you're saying that is a gray card. It's not, the McBeth is from the film days with densitometers
. While it's easily adapted for today with a known white-point
reference for Photoshop, it's not a gray card, it's a color scale with gray patches and I would never use it for white-balance. Keep in mind, even though you can get gray to be gray if you photographed that card and ensured your densitometer in Photoshop showed equal RGB values (128,128,128) for gray, depending what color space
your shooting in and what color space is your working space in Photoshop, the other colors will reproduce differently, or show different color values in the RGB densitometer.
Here's to clear skies, wishing everyone a Happy Holiday season, rg sends!
13% not 18% & a Ringflash? (grin)