The idea for the 18% grey card arose in the days of B/W negative film because it was intended to represent 'middle grey' for a film..
And it has caused us so much confusion since.
Actually, it was earlier.
It was believed that 18% was the gray tone level that the human eye recognizes as "middle gray", meaning we think it looks like around middle of the range (between black and white). The brain changes an amazing amount of things that we think we see. But the ink is 18%, not the middle of anything, and the response of the eye was a different subject.
Specifically, in older days (no automation, no electronics, no sensors, no nothing), the 18% gray card was used by ink press printers to judge their halftone ink flow (ink level adjusted so that a 18% halftone was judged to create this middle gray tone we think we see).
Then in the 1930s when Ansel Adams developed his popular Zone System, the card existed, and he declared the 18% card was Zone V, and middle gray again. The system and the card were widely used, and Ansel described details of the use of the card in his book "The Negative". One goal was Zone V will look about middle to our eye. It was less about exposure, but the actual key was his belief that photos should contain some actual black and some actual white, and Ansel's prints certainly do (contrast is extremely important to grayscale work, much less so for color, even a little detrimental for color).
However, Kodak always said if metering on their 18% card, we should increase exposure 1/2 stop to come out proper (which converts t0 12%). Ansel himself (same book) said 1/3 stop increase (meters of the day, metering on 18% cards).
Our meters today do 12.5%, but note that this is the judgment about the average mixed tone of typical scenes (if possible exists), and is NOT related to any value of middle gray. Nothing says the average scene is middle gray. However, the meters only goal is to place the scene average at the middle, hoping for greatest range either direction (which relates to that 1/2 stop boost).
Then decades later, digital came along. I doubt Ansel ever saw a histogram, it is a modern digital thing. Gray cards were a film thing, about "looking middle gray".
We do know that one stop underexposure is 50% level (the definition of one stop is a 2x change).
If we have some fully exposed object in a scene (I am trying to say meaning at 255 level of digital), we know that intentionally underexposing one stop puts it at 50%, or from 255 to be at 128 level. That is in linear data, at the digital sensor. However, all of our RGB pictures are gamma encoded, which boosts the data levels. Histograms show the gamma encoded data. We should expect to see gamma encoded data values in the histogram. The 128 linear level we call middle goes to about 187 in gamma. The digital camera is also making other tonal changes, white balance and color profile and contrast, etc, but when we intentionally underexpose 1 stop, we see 255 fall to about 3/4 scale, close around there, but probably not precisely 187 (73%). Certainly we do NOT see one stop down go to 50% (because gamma boosts its histogram value).
Anyway, 18% is 18% and is not 50%, and is not half of anything. However, gamma raises 18% to about 46% in our photo data, which is due only to gamma, it is NOT because it is half of anything. Yet we see that 46% is nearly 50%, so we imagine 18% must be middle of something, so we call it middle (however remember middle 50% at 128 went to 187). Likewise we know 128 is half of 255, so 128 must be middle of something. So therefore (?), 18% is 50%.
The eye may imagine it "looks" 50%, but it measures 18%. But gamma makes it Not even look that way. We even hear people telling us to calibrate our meters so a 18% card is at histogram 128, which by coincidence is not far wrong, but they have no clue about what any of this means. We never really ever know what we even mean by "middle", and we make crazy assumptions.
Regarding white balance, 18% cards vary. Kodak sold that business and has not manufactured a gray card for 20 years (other companies bought right to use their name). Most cards are somewhat neutral (gray color that has equal RGB components). Gray colors often have pink or blue color casts, but if they look very gray, most cards are pretty close, but they do vary. They are only intended to reflect 18%. They have no specs about neutral color. And while they can work, they are pretty dark for white balance purposes. White balance cards are either white, or very light gray.
But white balance cards are available, with the goal to be neutral white or gray. It is not rocket science, it is just white without a color cast. I like the Porta Brace White Balance card, $5 at
B&H. The WhiBal card is good too, using gray pigments which then has to be tested, but they claim to test each card, which costs more. I have two of each, and I use the Porta Brace.
Use of a good WB card is the plan, and a good plan, but actually, another skill we should learn is this: Many of our photo scenes already contain white objects (white paper or envelopes, signs, porcelain dishes, T-shirts, white collars, church steeples, picket fences, even the white polka dots in the kids pajamas). Many of these are intended to look white (not off-white), and they mostly work rather well too (maybe a few exceptions when we must Undo), but normally much better than if no other planning was done. See this:
Easy White Balance Correction, fix with or without Raw
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