There are two uses of the gray card below, metering exposure and controlling White Balance (below).
Some significant metering ideas first:
Photographers should learn to recognize this, and automatically evaluate this when we first walk up to the scene. Since we aim the meter at the scene, and it sees only the light reflected to us, possibly affected by the colors that reflect differently, to be different than the correct reading for the actual direct light. Even Spot Metering is reflected light, and is certainly affected by subject colors (so it definitely matters greatly what color of spot that you aim it at). Spot Metering is more difficult, you need to know that whatever color you Spot Meter on will come out about Middle Gray intensity (Not gray color, but that intensity ... it's a B&W film era term).
Novices are shocked by this idea, beginners usually imagine the camera meter should always get it right. But it shouldn't take long for them to start realizing it doesn't necessarily happen that way. See How Light Meters Work for more. Average scenes containing an average mix of colors can work pretty well, but scenes with color exceptions often need manual compensation. A skilled photographer learns to instantly recognize the subjects color exceptions. Meaning, try taking a couple of seconds to first think about the scene you see in front of you, what it is, and what it will need you to do. Actually look at the scene, meaning, actually see what it is and needs.
So we might use 18% gray cards in two different ways.
Why? The issue is that just normally metering the scene with a standard reflected meter (a camera meter) is affected by the reflectivity of the colors in the scene, and is not always correct. This is because scenes with too much of light colors (like white, on walls maybe) will read high, and will be underexposed. Too much of dark colors (like black) will read low, and will be overexposed. Because the reflected meter will try to make the average of the scene color come out as middle gray, regardless if it ought to be more black or white. That's just how reflected meters work, a fact of life. Beginners who assume the camera should always get exposure right still have much to learn. It is nothing new, every photographer has always had to learn to deal with this, since the first reflected meters in the 1930s. It is a fundamental of photography, something we need to learn.
So the idea of using the gray card is to meter exposure independently from the subjects colors. Examples of reasons which could consider metering on a gray card are anytime that light or dark colors (or even any one color) dominates the scene. Like for your dog in a field of snow, the meter mostly sees white snow, which will read high and the reflected meter will underexpose the picture and the dog. Or a group of people all in dark suits will read low and will be overexposed. Other reasons to use the gray card could be to avoid a bright window or sky or a light seen behind your subject, that would affect the metering aimed at it. In such situations, you cannot trust your reflective meter to get it right. That snow or the black suits will be made to show as middle gray tone, which then likely is not correct. The experienced photographer will immediately recognize those troubled situations, and will realize that exposure compensation is necessary. Or in today's digital world, everybody can at least look at the photo result preview on the camera rear LCD (if they will bother to look), to know it is wrong, and to realize another try with exposure compensation is necessary. Preaching, but the skill is in the photographer, not in the camera.
The idea of metering on the 18% gray card (in the same light as our real subject) is that then metering is independent of the subject colors. The exposure of the gray card will be made to be middle gray too, which happens to be about correct then, because the card IS middle gray. It is not 50% intensity, but 18% which is how the human eye and brain decides middle gray. And most typical scenes do average about middle gray overall, but the response of different reflected colors can still be a problem. So by metering on the standard 18% card, this meters the light directly and about correctly, so that by using that same metered exposure, the usual real scenes will come out about right too (with some exceptions). Then whites will be white, and blacks will be black. The result is more consistent, like using an incident meter would give, metering the light itself, independent of the scene colors. But if we don't have an incident meter, the gray card can be an inexpensive substitute for one. More about metering.
How? If we use a known neutral white or light gray White Balance card in a test picture (in the same light), we have very good white balance tools to click it to tell the computer: "We know this spot is neutral color, so make this spot be neutral". Then it removes any color cast there on that neutral color spot of the card, and also in the whole image (so that the card appears neutral again). This works if the card is in fact actually neutral (meaning no color cast in its real color). The 18% card is supposedly controlled to be 18% reflectance, however we may not be certain about the color accuracy of an 18% gray card, but most are usually close enough to help greatly.
A neutral color is defined as having equal RGB components (no color cast), which is a pure white, or black, or any gray tones in between (that has equal RGB components). Many grays have blue or pink tones, but some are neutral. The 18% card works, but 18% is pretty dark for White Balance. An actual White Balance card (which are white or light gray) may be preferred for White Balance. We can always go overboard, but a Porta Brace White Balance Card (about $6 at B&H or Adorama) works wonders, like magic (see more about WB cards). FWIW, that card is very white, but the ad pictures often shows it gray (the white paper label is gray too).
We can either use the WB card to set a Custom WB in the camera, or easier, just place it in the scene, and click it later in processing. Clicking it later implies one test picture, then we can remove the card and continue with our session, and later use that same WB value on all session pictures in the same light. Raw software makes that trivially easy to do this, and the result is normally perfect. When we click a neutral card in an image, we are saying "computer, I know this spot is neutral, so make it be neutral", and then it does, removing any color cast found there. More about white balance.
The idea for the 18% gray card became popular in the early days of B/W negative film because it was perceived to represent 'middle gray' for a film. And it has caused us so much confusion since. 😊 The plan on this page is to help with "what does middle gray mean?" Or not mean?
Actually, the 18% card was earlier than light meters. There is a history.
It was believed that 18% reflectance was the gray tone level that the human eye recognizes as "middle gray", meaning our brain thinks this tone looks like it should be about middle of the range between black and white. It reflects only 18%, not 50%, but our brain tends to change things, in some cases to be what the brain thinks we should see. Look up optical illusions. (Here's an amazing one). Fun, and about what eyes and brains see and don't see, and sometimes that the brain just makes it up. But the reflectivity of our gray card ink is 18% (which is not 50%, but the response of the eye is a different subject), and our histograms are instead actual data... sort of (the histogram is modified actual data, due to gamma).
Specifically, history in older days (halftone screens date from about 1880, before electronics or sensors or anything), so back in the day, the 18% gray card was developed and used by ink press operators to judge their halftone ink flow. Ink flow was adjusted so that a 18% halftone was judged to match this middle gray tone card. Halftone patterns could easily design the number of black dots to be 18% of the total area, but then dot gain will affect its printed result. Dot gain is the spread of the ink dots (to be larger), as the ink soaks into uncoated paper (and spreads out), but this is less so in coated papers. And too much ink was an issue. The ink flow to an 18% halftone was adjusted until it looked about the same darkness as the 18% card. So 18% is a concept older than photography.
Then around 1939-1940, Ansel Adams developed his then popular Zone System about controlling photographic grayscale tonal values. Early light meters existed then (Weston 1932, General Electric 1935) using selenium cells (but no transistors yet). The 18% card already existed, and after all, it was called middle gray, and Ansel declared the 18% card was Zone V at the middle of his system, eleven tone levels, 0 (zero, black) to X (10, white), even if the Romans had no number for 0. The system and the 18% gray card were widely used then, and Ansel described details of the use of the card in his book "The Negative", 1948, and several later improved editions. One goal was that middle Zone V will look about middle gray to our eye. The bigger intention was that this proper exposure of the middle tone would then capture both black zones 0 and white tones X (full photographic range could be utilized, centered in the film range).
18% might be seen as Middle Gray by the human eye response, but 18% is not necessarily what an average colored scene reflects (scenes and colors have many reflection differences, like from green foliage, blue sky, red barns, human faces, orange sweaters, etc). Colors reflect different amounts of light. Specifically, pigments absorb different colors, and thus have less to reflect of the remaining colors. A yellow taxi absorbs the complement blue. And other problems, like in a B&W photo, should red lipstick come out looking black or gray? So for example, since 1940, television converted a RGB color image to B&W luminosity by RGB Luminance value = 0.3 R + 0.59 G + 0.11 B = 1.0 Total (green was brighter than red, which was brighter than blue). The point is, reflectance from various colors is a serious issue to measure necessary photo exposure. And that is what a reflected light meter sees.
It was known back then that Kodak disagreed with Ansel's 18% value, but even though Kodak knew some science, they thought it was good business to avoid any public argument, because Ansel was so popular back then. Ansel Adams thought his Zone System's middle zone 5 ought to be 18% reflectivity, and he promoted the 18% card notion (in the 1930s). I cannot imagine that Ansel could have ever seen a digital image or a histogram back then, so don't confuse the two systems, analog film and digital. However, our reflected meters today are set to *or near) 12.5%, thought to be more realistic for an "average" scene.
Incident meters however, simply view only the light intensity from the light source (the Sun or flash, etc.), used at the subject, in the light incident on the subject, (and does not even see the subject), so Incident meters generally work well for any scene. However incident meters are used at the subject, so cameras cannot use incident meters, but they are extremely handy in the studio. And of course, the same light as hitting the subject is what should be metered. Just realize that your card may or may not actually be 18% reflectance, and your scene may or may not be an "average" subject, and 18% may not be quite the exact number, but it should be pretty close, allowing a slight compensation for your second shot.
Anyway, Kodak always said we will need to open 1/2 stop if metering on their 18% card (this 1/2 stop more is equivalent of 12%). So any and all scenes are (reflective) metered to come out about 12.5% middle tone average — regardless if they are typical "average" scenes or not (the reflectance of the scene can vary with its colors). But Kodak has not made an 18% gray card in over 25 years. Kodak sold all their printing business to Silver Pixel Press around 1995, who sold it to Tiffen in 2000. Both sold the "Kodak" cards, and Tiffen still owns Kodak Wratten filters and these cards, and publishes Kodak books, still available. My point of that history is this: Old timers knew well that Kodak instructions always told us if we meter the reflection off of their 18% gray card, and then for that exposure of an average scene in the same light, we should increase that exposure by 1/2 stop (for a front lighted scene) — see these older Kodak Gray Card instructions, the "very first words" at top of page 5, continued from page 4 (NOT the mention of backlighting). We don't see these instructions with today's cards, but it will still help today to fudge a bit on the 18% card exposures. FWIW, cameras had no half of third stops until near the 1970's, but half stops could be approximated by setting aperture between the two click stops on the lens. Film could always make minor brightness corrections in the printing process.
If photographing that card itself (as previous page or at another here), then no matter which card you meter, a black, white, or gray card, all these cases do always cause results always near the same middle tone, regardless if that is correct for whatever scene or not. Assuming the one card fills the camera frame, a white card will meter as gray, a black card will meter as gray, and a gray card will meter as gray. That is simply what meters do. No matter what you meter, a meters goal is to produce an (average) middle gray tone (not too bright, not too dark), because it does not know what tone it ought to be. None will be at the 128 middle on the histogram, which is simply the wrong idea. The histogram data is gamma encoded anyway — Middle gray of 50% (one stop down from the 255 end) is at about 3/4 full scale (73%) in the histogram gamma data, but the 18% card was an analog film concept, no histogram then. But metering on an 18% card does assume an "average" scene, and many scenes are about average if the view is wide enough.
The actual plan of the Zone System was about grayscale contrast, specifically a major belief of Ansel was that his B&W photos should end up containing at least some small area of ACTUAL BLACK and also some ACTUAL WHITE, and Ansel's prints certainly do specialize in doing that. Contrast is extremely beneficial to grayscale work, however too much contrast may be detrimental for average color photo work.
Ansel Adams achieved his work in the 1930s with extreme darkroom manipulation (dodging and burning in, see Wikipedia). But in digital, we can achieve this contrast in any adjustable histogram tool, like shown here in the Adobe Levels (CTRL-L in Photoshop or Elements) at right. The Auto button in this Adobe Levels tool will make a similar adjustment which is appropriate for grayscale, filling the full histogram range to create both full black and white, however this act of clipping the weak tails (to near the base of the tall peak, as shown) can add objectionable color casts on color images. Or in grayscale, you might make even tighter clipping than this, more to the actual beginning of the actual outer vertical steep peaks of the data, clipping away the weak edge slopes which areas will then become pure black or pure white.
Setting the Black and White Points specifies the new marked black point becomes the new 0 point, and specifies the new marked white point becomes the new 255 point for white. This clips so that those new levels marked become 255 white and 0 black, and all pixels outside those points are clipped (to remain at the 255 and 0 limits, losing any detail in them). Which can cause a color cast in color images, but grayscale generally loves it, to a degree. A little clipping in bright sky areas or dark shadow areas generally don't matter much in grayscale.
Important: In Adobe, hold the Windows ALT key down (Option key in the Mac) while nudging the sliders slightly, and Preview will show which pixels are being clipped, and then you know where to look to judge how important that detail might be (clipping gives up detail in those pixels... but if typically like in the sky or in the shadows, there likely wasn't any, so no big concern, the contrast might well be worth it). This adjustment is done "by eye", judging the Preview. The point is that blacker blacks and whiter whites is greater contrast, which dresses up grayscale to make it sparkle. Try some things, do see some range of it, and back it out if you don't like it, but odds are good you'll like it (speaking of grayscale images). But only save it to a file copy. Don't ever rewrite any edit to your archived original image file, you may want it back again some day.
Generally, the intended clipping part of this treatment is speaking of grayscale. For color, you can adjust Levels, but stop where the data begins, and don't clip, which can change color a bit, causing color cast. But Grayscale, there is no color cast, and a little clipping can often improve contrast, perhaps dramatically. Individual photos may vary.
Then the center slider (which is technically Gamma) also adjusts overall brightness level very effectively and extremely satisfactorily, without any risk of clipping. Most brightness tools work by clipping the data more, but this center slider will never clip anything (gamma never clips).
Generalities: Setting the end points tighter, to where the major data peaks actually start up is clipping (of the thin trailing tail), normally a minor thing in grayscale, because part of the goal is at least some smaller areas of pure black and pure white. But it depends on where this clipped image detail is, and how important it is. Clipped areas lose their detail. Clipping can change colors in color work, but in grayscale, there is no color for clipping to change.
Then color film came along, and really complicated the Zone System. 😊 Clipping grayscale for contrast only slightly changes the gray tone, lighter or darker, but still gray. But it won't turn blue or pink like color might. But correct color requires there be no clipping. Color differences are many, luminance values, different colors, different subject reflectivities, different contrast goals, a different ball game. Contrasting colors also give contrast in color images, color has more working than just tonal variations. One opinion is a little more contrast might sometimes be good, but that excessive contrast is often considered a bad choice for color. Depends on the subject, if you want
glaring color vivid and vibrant, or smooth and real. But grayscale needs and thrives on contrast.
Then decades later, digital came along. I suspect the dates mean that Ansel never had opportunity to see a digital histogram, it's a modern digital thing. Gray cards were a film thing, about "looking middle gray" to the eye, but primarily trying to center the exposure so that both ends can register correctly. Digital histograms instead analyze the specific binary tonal data, the real data. Histograms do not indicate correct exposure, histograms only examine the existing exposure by counting the existing pixels with each tonal value. We might assume the histogram data should reach near 255, but that is only true if the scene has actual data that ought to be that bright. Cameras can have no knowledge of how it should be (the photographer's brain might know, but the camera does not). We like to assume our "normal" subjects ought to fill the full histogram, which might be often true, but the "black cat in a coal mine" will always need our help. We tend to get confused when we try to numerically combine the two concepts (below). We might even imagine that 18% must somehow mean 50%, but 18% means 18%. I fear we often get confused when looking for a gray card in the histogram, because 18% raised to gamma 2.2 in the histogram does coincidentally normally reach around 46% (but the camera is also busy doing other things too, like exposure, contrast, white balance, etc). However the 18% card can have two valid uses in photography, for metering on it to simulate an incident meter, and for white balance correction (however there are better white balance cards that are not so dark).
Metering on the scene and subject, containing random colors and such, can sometimes give skewed results. I do use the camera meter (except for studio flash), and do compensate it as necessary. We eventually learn a few things, to do and to avoid. 😊 But instead, metering on the 18% gray card in the same light is a plan to substitute a stable situation, independent of the scenes colors. Which is what an incident meter does too, metering the light directly, at the subjects position, but aimed toward the camera. The incident meter can be substantially more accurate more often, but it can be awkward for walk around shooting (it's wonderful in a studio situation).
Why? How Camera Light Meters Work shows that if we meter on a white card, it comes out middle gray. And if we meter on a black card, it comes out middle gray. And if we meter on a middle gray card, it comes out middle gray. The (reflective) meter is always seeking a middle gray result (meaning the subject colors will be averaged to a middle tone). And in this last case, when middle gray comes out middle gray, then white will come out as white should, and black will come out as black should. That's the idea of metering on an 18% card that is in the same light as the desired subject. We hope to place the middle correctly, and then the rest will be correct too.
See Wikipedia about light meter calibration constants (like K).
Reflected light meters today (Sekonic, Nikon, Canon) meter to K = 12.5. Gray cards and the "middle gray" term may be 18% reflectance, but the common opinion is that the mix of colors in many scenes average closer to K=12.5.
Because, the RGB Luminance is Luminosity = Red x 0.3 + Green x 0.59 + Blue x 0.1. Brightest Green reflects about 59% of the light on it, and Red 30%, and Blue only 11%. So the mix of the scene colors affects what the light meter can see and read, but of course, it depends on the scene.
About using reflective meters (like the camera meters) on the exceptional scenes (aren't they all?), there was another oldie but goodie: "How To" Kodak Tech article (reprint dated 1998), Accurate Exposure With Your Meter. Kodak may be gone now, but this classic Kodak article about accurate exposure is available (and photographers have always had to learn about light meters). When we know how things work, our results go much better.
One thing that article says about reflected meters is:
For the reason, see first paragraph at top above, which is same as just said down here. The purpose of metering on the gray card is for the reading to be both mid-range and independent of the subjects colors that could fool the reflected meter (and it works out rather well). A reflected meter metering on a gray card gets results very similar to an incident meter, metering the incident light, independent of subject's color reflections.
Light and dark colors (as used by Kodak) means the reflected color, not the lighting intensity (is not about a dim room). A white wedding dress is a light colored subject, which reflects a lot of light, and so the meter will read high, and the exposure is adjusted back to middle, and the dress will be gray. A black tuxedo is a dark colored subject, which does not reflect well, and so a meter will read low, and the exposure will be adjusted up to middle, and the tux will be gray. However, both standing together may be an averaged gray subject seen together, and that exposure probably comes out correct. We do have to pay attention to what we are metering. This has always been true, from the day of the first reflected light meter. We do need to learn how the meter works. And it is why cameras provide a Exposure Compensation control.
For Incident meters, Kodak reverses the words "increase" and "decrease" from what it says for Reflected meters. That reversed concept would also include the gray card, in addition to the first 1/2 stop, but the gray card simulates use of an incident meter. Both of those methods meter the light more directly, independent of the subjects colors (not aimed at the subject).
Reflective meters measure the reflections from subject, from the camera position, affected by the subjects colors.
Incident meters measure the light source directly, from the subjects location, but independent of the scenes colors, and is usually about right for most scenes.
Reflective meters will read light colored scenes too high, and then drops that value back to middle (causing underexposure of light colored scenes, or maybe a white wall background, etc).
Incident meters (and gray cards) read the light directly, usually about right, but sometimes light color detail may show better if less bright.
Reflective meters will read dark colored scenes too low, and then raises that value back to middle (causing overexposure of dark colored scenes, maybe dark colored backgrounds, etc).
Incident meters (and gray cards) read the light directly, usually about right, but sometimes dark color detail may show better if brighter.
Our reflective meters today (Sekonic, Nikon, Canon) do K=12.5 (it is not a debate, it is ANSI standard, and K=12.5 is printed in specifications in all Sekonic manuals, and also matches Kodak's "open 1/2 stop" for those scenes other than gray cards). The first meters in the 1930s may have been 18%, but they are lower today. K=12.5 is NOT called reflectance or percent, but Luminance is metered light Intensity, directly corresponding to reflection percentage. According to Wikipedia, the ISO 2720:1974 specification states:
The constants K and C shall be chosen by statistical analysis of the results of a large number of tests carried out to determine the acceptability to a large number of observers, of a number of photographs, for which the exposure was known, obtained under various conditions of subject manner and over a range of luminances.
So this value for light meters is a judgment about some average mixed tone of typical scenes (if possible exists), and is NOT related to any value of middle gray, nor to any one specific scene or image). Because real life doesn't work that way. No rule says the average scene is or should be middle gray. The shadows in a green forest or a bird in a blue sky will vary. It depends on the reflectance of the specific scene colors, however the color mix in many usual scenes is in fact close enough to the light meters, so it often sort of works out (which is the basic idea, but it is also often not as great). Which is why we have compensation buttons on our cameras. The best possible advice about exposure in photography is this: If the photographer would simply use their head (learn a bit about how things work, and first consciously look at and actually SEE the situation, and think a second about what it is going to need), it sure can work better. And now that we have digital cameras, we can also simply check our first result to determine what it will really need.
The meter is a dumb device. It can accurately measure the intensity of the blob of light it sees, but it cannot recognize the scene to know what it means, or how it ought to be. Our brain is our best tool, so try to use it. Humans can recognize things, and have experience to know what it means. The meter's only goal is to place the average of whatever metered scene at a middle tone, hoping for greatest range either direction. This does work pretty well for many average scenes, but some unusual scenes will require manual compensation (when reflective metering is affected by too much dark or light colors, meaning low or high reflectivity of colors like black or white, Not meaning in dim or bright lighting). Our human brain can recognize the unusual scenes, then we can help the meter.
General guide lines for metering exposure on the 18% gray card (or White Balance would be very similar):
If the card does not fill the frame, at least use Center Weighted metering for exposure on it, but better, this would be the ideal situation to use spot metering (for the card to fill the meter view, and if the card is in fact medium gray). But if the very common case of preparing for use of a later White Balance tool, full frame is meaningless, the tool can simply click on the card.
For Custom White Balance, follow the camera instructions (and maybe consider a real white balance card).
For clicking White Balance later, just put the card in the same light as the subject. If flash, not much closer than the subject, we must avoid clipping the card (and do check it, but a portrait subject holding a white card near their chest should not be a problem).
18% cards can work for this, but they vary, and they are pretty dark. Incidentally, we can still buy "Kodak" gray cards, but Kodak has not manufactured gray cards in over 25 years. Kodak sold their printing business (includes books and 18% gray cards) to Silver Pixel Press around 1995, who sold it to Tiffen in 2000. Both have sold "Kodak" branded cards, and Tiffen also makes Kodak Wratten filters now. But any newer "Kodak" 18% gray card was not made by Kodak, and 18% cards do not have specs to necessarily be exactly neutral (they can vary, but most are usually pretty close, unless they do not specify 18%).
If your goal is white balance instead of 18%, actual white balance cards are available, with the specific goal to be neutral color, white or light gray. A color tint can show better on a light card. I like the Porta Brace White Balance card (about $6 at B&H or Adorama). It is not rocket science, it is just white without a color cast. Or the Whibal brand white balance card is possibly technically better, as they claim to test each card, and it costs more. Not a thing wrong with either, but I have two of each, and I tend to prefer the Porta Brace card.
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. Wall and trim paint are usually off-white, not very good for White Balance). But normally, these things can be much better than if no other planning was done. If no other choice is available, then is this choice better than it was? Often it is real good. However better planning is always a good thing. There are actual White Balance card targets.
But White Balance can actually be quite easy. Simply clicking a good WB tool on a good WB card (in the same light with subject) is trivial to do. The philosophy is that we know the card is neutral (equal RGB, so no color cast). Then after the computer makes that spot actually be neutral color, therefore no color cast in our image. Couldn't be any better, or easier. Lots more about White Balance.
There is a similar 18% gray card discussion at the bottom of the gamma page
We are aware of a few specific known and certain facts, which don't always match our simplest assumptions about "middle". Two facts are:
The definition of a "stop" is a 2x change, so we do know that one stop underexposure is 50% of the previous proper level.
This part is a bit tedious perhaps, but we also know our image is linear analog light in the lens and at the camera sensor. Then it is digitized to ones and zeros binary data, but raw data is still linear for example. But when the image is converted to RGB data for use, then the exponent gamma modifies all of the RGB data we ever see. The histogram shows this gamma data (for raw images too). "Linear" in math means that functions graph on a straight line (a 2x change has a 2x result, i.e., not exponential), but in computer systems, the word linear is also used to say "not gamma encoded" (and is also my use here). Gamma is sort of a no op for us — our eyes will hopefully see the linear data again later, as the reproduction of the original linear scene. Later when the image light is in the air, sent traveling to our eye, the reproduction will necessarily be linear analog light again, suitable for our eye (same as when we saw the original linear scene).
This Test Case to find the middle point: Suppose we intentionally expose an image so that the brightest white tone reaches histogram 255 level (specific value for this example). Then we intentionally underexpose by one stop to drop this data to 50%, which would be from 255 to be at 128 level, if we assume linear data at the digital sensor. However, all of our RGB picture data we see is always gamma encoded data, which boosts the data numeric values. And histograms only have the gamma encoded data to show, so the histogram data we see is gamma encoded. This means that in our histogram, we see that one stop lowers this 255 end to only about 3/4 scale in gamma data. You'd think that ought to be called "middle". 😊 Nevertheless, it is handy to know when viewing the histogram and contemplating a one stop compensation, this is the degree of change you can expect (relative to the 255 end).
You can and should easily check this test yourself: (seeing is believing)
So, to check what we see when we create 50% by intentionally underexposing by one stop, then we see 255 fall to about 186, around 3/4 scale. It won't be exactly 186, because the digital camera is also making other tonal changes, white balance and color profile and contrast, etc. But certainly we do NOT see one stop go down to 50%, because gamma data boosts the histogram value. One stop down may be 50% in linear, but we don't see linear data, our RGB images only have gamma data. So point is, we are mistaken if we imagine 128 is the middle of any histogram data that we see. The histogram is semi-log data, so to speak. The elementary articles we see discussing histogram 128 either oversimplify or don't know, but virtually none mention gamma. That's not much help, because gamma in RGB histograms is all that we can see.
Anyway, 18% is 18% and is not 50%, and is not half of anything in digital. And 18% of 255 is 46 in linear data, however gamma raises it to 117 at 46% in our photo data, which is due only to gamma, it is NOT because it is half of anything. Yet then we see that 46% is nearly 50%, so we imagine 18% must be middle of something. We heard it was middle, but we're not sure what middle means (could be it's what the eye might imagine it sees later, but 18% is the actual data now, before gamma changes it to 46%). Likewise we know 128 is half of 255, so 128 must be middle of something too (however remember middle 50% at 128 went to 186 after gamma). So therefore (?), we assume middle must mean middle, so 18% is 50%. I'm just having fun teasing, but some accounts of it are a bit messy. 😊
The facts might confuse placing the histogram middle numbers somewhere, but otherwise should not interfere with our normal uses of the gray card (top of this page). Except my peeve with the "middle" thing is that we even hear people telling us to calibrate our meters so a 18% card is at histogram 128, which turns out by coincidence of gamma to be not far wrong (about 0.3 stop, and gamma 2.5 could hit it). But they clearly have no clue about what any of this means. They say "yeah, yeah, but it works for me". But I'd rather leave it alone, because it doesn't make sense, since gamma has nothing to do with determining exposure. And worse, it was NOT even about the 18% card, because a white card or a black card would give exactly the same results (examples here, maybe start at the top of that page), because reflective meters simply put stuff in the middle. It's hard to defend the unique meaning of 18% when a sheet of white copy paper will give the same result (different numeric exposures, yes certainly, but which produce the same photo result). We often don't really know what we mean by "middle", and we do make crazy assumptions. The 18% card might seem different in an incident meter, but here is Sekonics incident calibration procedure. No gray card is used, and they do know a thing or two about meters, and according to Sekonic (and the ISO standards organization), the right idea to calibrate your meter is that IF it repeatedly and continually gives a consistent error on a wide range of scenes, THEN adjust the meter to compensate. Any one single reading is not fail-safe anyway, too many variables.
Metering a precise exposure is a hard problem. Incident meters work well, easiest (if not always convenient). Reflected camera meter accuracy can sometimes be a close ballpark, but reflective meters are really just a guide, not an absolute (depends on scene colors). So after we avoid clipping, then what looks right on the camera rear LCD is really about the best plan. Meaning, if we need to compensate, then we need to compensate.
But when checking for clipping, we always have to look at each of the three RGB histograms. The single gray histogram is NOT valid color data, it is a mathematical manipulation (called luminance, about how grayscale can reproduce brightness of color values). It is NOT true data, and it CANNOT show RGB clipping, usually not at all. The colors register differently, and then White Balance shifts red and blue oppositely. To see clipping, we must look at each of the three RGB histograms. More about the three channels in RGB histograms.