A histogram is not a light meter. The histogram is of course affected by subject colors, how those colors reflect light. It only shows how the data result actually came out, but without any idea how it ought to be. Only human brains know how it ought to be, judged by how the picture looks. The histogram must be a RGB type (not a luminosity type, see more), and we are only concerned with size of the Empty Gaps at the ends of the histogram data.
Reflective light meters and Incident light meters have very different properties.
A Reflective meter is aimed at the subject from the camera position (meters built into the camera are reflective meters), and reads the degree of light reflected by the subject. This reading does represent the overall brightness of the light, but is of course greatly modified to indicate how much of it actually reflects from the subject's colors. The common problem is that a white or light colored subject reflects a lot of light, which reads high, and the meter (seeking an intermediate middle tone) compensates to underexpose it to be darker than it is. A black or dark colored subject does not reflect much light, which reads low, and the meter (seeking an intermediate middle tone) compensates to overexpose it to be brighter than it is (previously discussed). Photographers must learn to expect and adjust for this. However, subjects which do average out reflecting from near a typical middle tone will meter more nearly "correct".
An Incident meter directly reads the value of the actual light incident on the subject, instead of the light reflected from the subject. However, because of the inverse square law, it must meter at the subjects location, as closely as possible (incident meter cannot be in the camera). From the subject, the incident meter is hand-held and aimed away from the subject, specifically aimed back at the camera, to read the light incident on the subject. This makes incident metering be independent of the subject and it's colors, and therefore, directly reading the incident light is more nearly always "correct", regardless of subject (dark things come out dark, and bright colors things come out bright).
Incident meters meter the light on the subject. Reflected meters meter the light reflected from the subjects color, which reading varies with color.
Purpose below is to show the difference in how reflectance of scene colors affect reflected light meters, but the subject does not affect incident meters.
Four sheets of craft paper (red, white, black, and gray), roughly 20x25 inches, is normal stuff from the craft store. One smaller 8x10 inch 18% gray card is added (no good reason). The paper is standing up against a very black velvet background (seen at the top).
Using a Nikon D300 and one SB-800 flash in a regular white reflected umbrella, to show the difference between incident manual hand metering, and reflective TTL flash metering (i.e., how light meters work).
First picture used Manual flash power which was metered with a Sekonic L-308S incident hand-held light meter. 1/4 flash power level reads f/7, ISO 200, in this umbrella at this distance. The incident meter is held very near the subject paper, aimed at the camera, reading the incident light directly instead of reading the reflection from the subject.
NOTE: There are two very different types of histogram displays, RGB and Luminance. See samples here. This is RGB type.
First, this top section shows what incident metering can do. A hand held flash meter requires true Manual flash mode (and is incompatible with the Nikon CLS iTTL or Commander system, but an infrared commander can help the Sekonic meter work if not too close). The same manually metered f/7 incident exposure was used for all pictures in this first section, with the same 1/4 power manual flash power. It meters the actual light source, and provides the correct exposure, no matter what the subject is, no matter how the subject is changed. Then the point (in the lower section) will be to demonstrate that the reflected light meter (TTL metering) in the camera always strives to give a middle gray result. Middle gray is the only one result the reflective metering system is capable of doing. The overall scene is averaged to come out middle gray, which is in fact usually about correct for average scenes (but not every time). This middle gray result is the reflected light meter's only goal. It benefits the photographer greatly to realize this, how light meters work. We watch that result, and always stand ready to provide a little compensation, as necessary.
An incident meter (the flash meter used here) simply meters the light source itself. It does not point at or see the subject, so it does not matter if the subject is white or black, the light meters the same. Black things will come out black and white things will come out white, as we would expect (unfortunately, novices expect reflective meters to do this too, which does not happen). With an incident meter, the subject is not a factor in the metering, except that incident meters must be used at the subjects location (they meter the light on the subject), so they cannot be built into cameras. (Technically, we could stand anywhere to meter the sun at 93 million miles, but all other light sources vary with distance from subject, so we must stand at the subject).
Note this histogram is represented as being proper exposure of those papers. But if we removed just the white paper, that removes the white peak, but the exposure does not change. The histogram of course changes, but it is still the correct proper exposure. The incident meter does not even see the subject, it sees the light. So there are no absolute simple cases. Definitely also judge your picture as a picture, in the rear LCD, to be how you want it to look. The histogram can be a guide, so long as you know what it shows, and does not show.
But reflected meters are not the same, they do NOT measure the light source. A reflected light meter meters the light directly reflected from the subject (seen from the camera position), and it becomes very important if the subject is black or white (or is very reflective or not so much, etc). Bright things are bright only because they reflect a lot of light. Dark things are dark because they don't reflect much light. So the concept is that white scenes are seen brighter, but metering dims them to appear gray. Black scenes are seen darker, but metering boosts them brighter to appear gray. This is just how reflected light meters work. The reflected light meter only sees that degree of reflected light, and it does not have a brain to know what it is, or how bright it ought to be. Its only goal in life is that all scenes are to come out as a middle gray average. In great contrast, Incident meters do not even see the subject - they simply measure the intensity of the light source, at the subject.
A very useful detail to know: Note that in the Photoshop or Elements Levels tool (CTRL L), you can HOLD down the Windows ALT key (Mac Option key) while dragging the Levels White Point or Black Point slider, which will show you what pixels in the photo that you clipping (In Adobe Raw too, Exposure is White Point, Blacks is Black Point). This is how you identify pixels in the histogram. Note that relative height of the histogram data represents area in the picture, the scaled number of pixels with this tonal value. But the height is always scaled to appear full height, so height is VERY relative, not absolute.
In other words, first, what things ought to look like.
Gray paper, incident meter, with same Manual flash power and still same f/7, by removing one paper sheet from top at a time. There was only the one original incident meter reading for everything, since the lighting and distance and situation did not change. The subject is not part of the incident meter reading, only the subject's location and lighting is a factor.
Note that it would be very incorrect to "expose to the right" here, on this subject. "Expose to right" means to increase exposure so that the histogram data is shifted to almost touch the right edge. That is indeed often approximately correct for "average" scenes which actually contain something white or bright, but not on special cases like this. The histogram is NOT a light meter. The histogram simply shows where your tones have mapped, but it does not know where those tones should map.
Black paper, incident meter, with same Manual flash power and f/7.
Black paper and many black fabrics do not come out very black if there is light on them. Black velvet is one major exception which is very black (example in the top background of first picture) - the paper is standing against a black velvet background without any separation.
FWIW, to make black look jet black, we must keep the light off of it. Not done here, but to make a black background look black, the usual way is to place it way back (like ten feet behind subject), so the lights are weak on it back there. Or we can move our subject very close to the lights, within a foot or two, which requires we turn the lights down, which makes the distant background darker. Or both of course.
Again, "exposing to the right" would be fatal to "this" picture. We need to recognize abnormal subjects.
White paper, incident meter, with same Manual flash power and f/7. Needs a bit more exposure to be bright white, but the white approaches 248 here, and if trying to show detail in a bride's white dress, be careful, this is about enough.
Exposing to the right would work here, because this subject contains some white which should actually be at the right.
The larger point is that the histogram is not a light meter. The histogram merely shows you the result, where things came out. It does not know what things are, or where they ought to come out.
If this were to be white background in a high key portrait, we would adjust the background light so an incident meter at the background reads about 1/2 stop brighter than the main light at the subject, to insure the background is all totally and clinically white (to be slightly burnt out). With a Reflective meter, better make that two stops, since the meaning is quite different.
Red paper, incident meter, with same Manual flash power and f/7.
All RGB colors have three color components. This red paper has a strong bright red component, but with weaker but distinct green and blue components. For example, this box is a "pure" red, RGB=(255,0,0).
The Nikon and Adobe histograms show the three RGB channels superimposed (on top of each other), but often you can select only one channel to view, to see what each is. Multiple histogram values (like this) for the individual RGB components are common in items of one color, for example like say a manila mailing envelope color (which is three colors too).
White and Black and Gray (the pure tones of them) are unique because their RGB components are equal (no color cast).
Normally, in typical pictures of most average scenes, we often do care about how well we are "exposing to the right" (exposure compensated to push the data over towards, but not quite touching, the histogram right border at 255). However "adjusting to the right" makes assumptions about the image content being more typical - assumes subject contains some white, or some strongly saturated colors (but not all scenes do). The black and gray pictures above obviously cannot be "adjusted to the right" (they would become white). The histogram shows the count of pixels with each unique shade, from 0 to 255. The histogram shows what actually happened, it cannot show what ought to happen. Also, trying to adjust multiple lights this way would be a special problem (we need control of individual lights). This is what light meters are for. But in general, on average, "exposing to the right" is normally a good thing, for typical "average" scenes.
Ho hum.... the above is how we expect light meters to work. And it is how incident light meters work - they meter the actual light, independent of the subject. But... it is NOT how our reflective light camera meters work - they only see the light actually reflected by the subject, which varies with subject reflectivity. The fun is in the next section.
Next, the flash is simply switched from Manual flash mode to TTL flash mode (metered by the camera Reflective meter), and the same pictures were repeated, using same sheets of paper. Repeat: The difference is that the next ones are metered from the cameras reflective light meter now, instead of by the handheld incident light meter above. The idea is to show how the cameras reflective light meter responds to these same scenes. The first section above showed how it should look, and this section shows how it in fact will come out (because these are NOT the typical scenes expected by the light meter). Camera was manual mode M and the flash was TTL flash mode. These are simple normal TTL flash pictures of the paper. Center metering mode was used.
I used the D300 camera's automatic TTL flash metering, working back the other way, replacing papers one sheet at a time. Of course, this subject is about the metering of that paper. Still f/7 and ISO 200, but now the TTL metering is controlling the flash power automatically (to match the selected aperture, according to the metered light reflected by the subject), so it is no longer necessarily 1/4 flash power level.
Red paper, remetered by TTL flash automation. This is about 2/3 stop underexposed by TTL. We would normally adjust the flash power (flash compensation) to more so "expose to the right" on the histogram (on this bright color), but I did not do that here on any frame, I just went with what the camera metered (but these are NOT "average" subjects).
It is a bit underexposed, but this is what happens, because from the reflective light meter's viewpoint, it is simply more centered, striving for a middle tone result (which even if it is red, I still say it as being a middle gray result, from old B&W terminology. Meaning middle tones.)
Note that these three RGB components of this red color straddle the middle point, but the larger dark side is dominant here. The reflective light meter is trying to average the overall scene to give a middle gray result. Middle gray just meaning middle tone, between bright and dark. Meaning middle red here. Gray meaning if the scene contained a wide range of all colors averaged to gray, or gray meaning if the image were converted to B&W grayscale. If we only have red, it means middle red. The reflective meter always strives to give a middle range brightness result, about halfway between bright and dark (for the average value of any scene). I call that "middle gray", but it may well be middle red or green or blue.
White paper, remetered by TTL flash automation.
Yes, no mistake, this is the white paper, and this is simply how reflective light meters meter white objects. The light meters goal is that everything comes out averaging near middle gray, which is normally correct for mixed average scenes. But this is NOT an average scene. We need two or three stops of +EV flash compensation (overexposure, relative to the metered exposure) to bring this paper up to white. And if we are shooting some scene like this, that is our job to do it. We know what the scene looks like, and when this will be necessary.
All of these automatic TTL exposures come out a little below histogram middle point, which the camera considers to be the correct place for gamma encoded middle gray. The histogram midpoint is not middle gray (see the Gamma page).
The only one thing unique here is that this subject is 100% white (full frame is 100% white). It is not a mixed scene containing many tonal values in it, which would average out to a darker value than pure white, which would cause the reflective meter to boost it higher, so the average reaches middle gray. But if your scene is "largely" white (like a white background wall), or of course a bright sky background, or simply any highly reflective scene, you will get the same underexposed result. The photographers job is to expect this.
The light meter does NOT know this subject is white paper. It has no brain to recognize subjects, and no clue that it should come out white. The meter is just some dumb silicon chips. Repeating, it has no human brain. It only sees some reflected light, and its only goal is to make all scenes come out middle gray. The metering told the TTL flash to use less power to keep the white result down at middle gray. This blank white paper is of course nowhere near an average or typical scene. It may be an extreme case, but it is an attempted photograph, and it failed.
Preaching, but the photographer's brain knows what the scene is, and what it ought to look like, and he learns to expect this natural result, and he must plan on compensating it as necessary. Beginners unfortunately expect their light meter to always give the correct exposure automatically. But the real world does not work that way, and they are going to be very disappointed if they don't learn this fact. Middle gray is simply what reflective light meters do, in all cases. The middle gray term here just means middle intensity, it might be red or green or blue. Most typical or average scenes do come out about right (the numerical average value of all their pixels is in fact near middle gray, meaning, middle intensity). But our human brain always has to pay attention, and help it now and then, because the light meter has no brain, and no clue what the subject is, or how it ought to look. Its rule is that the average ought to look middle gray.
Black paper, remetered by TTL flash automation. Yes, this is the black paper, and this is simply how reflective light meters meter black objects. Remember, we are illuminating this black paper with a flash, and given enough exposure (flash power), we can make it appear white if we wish - and if we are using manual control. Because otherwise the light meters goal is that everything should come out near middle gray, which is normally correct for mixed average scenes. But this is NOT an average scene, and the metering told the TTL flash to use more power to move the black up to middle gray.
"Exposing to the right" would definitely be the wrong thing to do for this black scene. We need a significant -EV flash compensation (underexposure) to move this paper down to black. It is only our human brain that can recognize that. Our human brain also understands that the camera light meter will make it be middle gray.
Film was much more difficult, since we had to handle this properly before the picture was taken. Digital is much easier (instantaneous feedback), we simply look at our result and compensate as necessary to redo it.
Gray paper, remetered by TTL flash automation. Meter still tries to make all things come out middle gray. The subject need not be gray for that to be true. This is a lighter gray paper, but it comes out middle gray here.
In fact, the point here is, it does not matter what color it actually is, the reflected light meter will make the overall scene average out to middle gray. Or a middle shade of red or a middle shade of green, whatever. And this is normally very near correct for an average scene containing many different elements, but is not so great for special scenes, like the proverbial black cat in a coal mine, or the white polar bear on the snow. You need to be aware of this when spot metering too (the spot metered area will come out middle gray too, which if you are unaware of this, may or may not be your intention).
Exposing to the right would be incorrect here too. This one partially more to the right maybe, but fully to the right is only true of scenes which contain white or bright, those which ought to be fully to the right. But exposing to the right is often suitable for average mixed scenes, which likely contains some of that stuff.
Added gray card. This does not add much except to darken the average a bit, it filled in that central gap in the previous peak (a bit less than histogram midpoint). The darker average lightens the paper slightly (to keep the same average exposure). This is Center Weighted metering, and the result is still in fact near middle gray.
All of these automatic TTL exposures come out a little below histogram middle point, which the camera considers to be the correct place for gamma encoded middle gray. The histogram midpoint is not middle gray (see the Gamma page).
The meter is working right. If we had a wide scene containing a wide range of tones, like sky and mountains, or even a human portrait, this overall average middle gray result would likely be near correct. This paper is a special limited test, and is NOT a typical scene, and we cannot expect a typical result. So we photographers must pay attention to what we are photographing. Knowing this fact (that light meters aim for a middle gray result) will help you to know what to expect BEFORE you shoot.
Make no mistake, this usual middle gray average is the only hope we have, this is simply how things work. It requires a human brain to comprehend "this is white and it should come out white". Or "this is black and it should come out black". This brain power does not reside in the light meter.
The first of this set is red, at mid-level, a bit dark, but still red. The others (white, black, gray paper) all look about alike, middle gray.
The only smart that a reflective meter can have is to try to put everything in the middle. It does not know what this stuff is.
An incident meter measures the actual light, and then everything naturally falls where it should.
NOTE: My Middle Gray term here does not necessarily mean gray. It would if assuming B&W grayscale, but when discussing exposure, it could be middle red or blue or green (just a mid-tone tonal concept, between darkest and brightest), and it refers to the equivalent tone value present in the (gray) histogram. Photography and its terms used to be black and white (grayscale). Most histograms still show the equivalent one grayscale value (instead of three RGB values). The term "middle gray" usually simply means the grayscale equivalent... even if it might be red or green, etc.
Camera TTL light meters do not have human brains to know what we are pointing them at. The meter cannot recognize a snow covered mountain from a black cat. Human brains can, but the meter just sees some pixel values with no clue what it is or how it ought to look. Human brains know the expected result instantly, but the meter cannot recognize the subject, and does not know if the subject should be black or white or pink, or if it should be light or dark. They only see some degree of reflected light intensity coming back through the lens. But it turns out that most average scenes contain a range of various degrees of brightness distributed over the full width of the histogram (a shiny face and dark hair for example, or sky and mountain for example), and specifically, all the tones in most scenes typically do average out to be near a middle gray overall. Unfortunately however, not all scenes are typical. Most photographers are issued one standard human brain however, and we just need to use it.
Reflective light meters simply always assume every scene averages out overall to middle gray, and the light meter's goal in life is to make the picture exposure average out to be middle gray. Normally this is near correct for average scenes, which typically do in fact average out to be about middle gray, but there can still be exceptional non-typical scenes when this fails somewhat. Humans can recognize those, light meters cannot. I hope this point is clear.
The design of any reflective light meter is that when metering something all black, or all white - or anything, whatever it might be - all scenes are supposed to come out all middle gray. This is how simply reflective light meters work. It may not be the way humans think however. :) Humans want to imagine their light meter gives them some "correct exposure", but reflective light meters only know middle gray for all scenes. Reflective meters are designed to measure an overall composite scene which OUGHT to average out to middle gray, which is often nearly true of many things, like landscapes or portraits, but which may not always be the case. If you meter bright things, they will meter bright and exposure will be reduced to be middle gray. If you meter dark things, they will meter dark and exposure will be increased to be middle gray. It is pretty good to know this. After minimal practice, you easily recognize cases which should be compensated, and already just know by about how much. Not sure saying it a few more times is going to help. :) Try it, look and see.
Spot meters are reflected meters too. So when we take a spot reading of the subjects face, it does NOT mean we get a correctly exposed face. We get a middle gray face, every time. Not aways what you want. It can however work if you meter something that ought to be middle gray. Spot meters are not designed to give a good exposure of a scene. Spot meters are designed to let the photographer prowl around in the scene, measuring bright things and dark things, to determine the range of the scene. Center and Matrix metering have much to be said for them, averaging the overall scene to middle gray. Novices assume a spot meter gives a "correct" exposure of that spot area, but Spot works the same as other modes... Spot allows us to select the small area that will come out middle gray. But middle gray may not be the correct result for a lighter or darker area.
Incident light meters (flash meters are one example) are a big deal in the studio, and the concept is more about "correct exposure". They are instead are pointed the other way, at the camera instead of the subject. They instead directly measure the brightness of the light incident on the subject (at the subjects location), and give a corresponding exposure so that middle gray things should come out middle gray, and white things come out as white, and black things come out as black. This is a much more consistent reading but cannot be implemented on the camera. Incident meters must meter from the subject.
Once again, the point is, this is simply what reflected light meters do... tries to average every scene to middle gray (meaning middle brightness). Have you got it yet? This exposure will usually come out near correct for average scenes. But it can guarantee underexposure for bright scenes, and overexposure for dark scenes (speaking of the "color" and "reflectivity" of the scene, not the amount of light that is on it).
This example uses camera TTL flash metering, but you would get similar results with tungsten or sunlight... any reflective meter strives to give middle gray result for any scene it sees. Most typical scenes do in fact average out about at middle gray, so it normally works out fine.
We need to understand this, regarding the amount we compensate the flash. The light meter has no idea what the scene is, or how it should look, it only sees the overall average, which it assumes should be middle gray. But we humans know what the scene is, so it becomes our job to correct. When metering white things (say a wedding dress), you probably need +1EV compensation to keep it white. If metering black things, then probably -1EV compensation to keep it black. This depends, so check your results.
Seeing this here is one thing, but to really grasp it and make it yours, you should of course repeat this simple experiment yourself... to actually "see" it happen.