Novices to photography always assume their camera meters magically should always give the correct exposure, but alas, they need to learn that life is not that simple. Meters are pretty dumb. They can be a good guide, but proper metering is an art done by the photographer. It's not difficult, but sometimes requires a bit of thought, about the subject.
Next is a quick rose picture, intended to mimic some real photo scene. Photographed below five times, all with the same settings (automatic point&shoot TTL flash).
To make that be real clear, let's adjust only the background scene a little at time, all else stays the same (but still with automatic TTL metering of the flash). At each frame, the ONLY difference was that the black paper was shifted left a little, and then the shutter button was pressed again. The scene the meter sees is different, different ratios of dark and light colors. Then the TTL metering reacts to the new scene it sees. The exposure you get depends on what the meter sees. This is how reflective metering works (it sees the light reflected from the colors in the scene).
Direct flash, on cord off camera. Automatic TTL, matrix metering mode. No cropping, ISO 200, all f/8, 1/200 second, TTL.
No adjustments were made. The ONLY change in each frame is that the black background was simply moved left after each picture, so that the camera TTL metering saw more white paper.
See? Both the predominately dark scene and the predominately bright scene are exposed to be a gray scene. The scene that does actually average to a middle gray is metered more accurately. This is the reason some users meter on an 18% gray card (in the same light as on the subject).
This is what happens, how reflective metering works, on your camera too. Reflective metering exposure depends on what you aim it at and how well it reflects light. The general rule is that low reflectance dark colors tend to overexpose, and highly reflective light colors tend to underexpose. It's just how things work. It does seem good to expect this. It does require a quick look and thought about the picture you're about to take. Not everyone can be bothered to think, but photographers learn this. See the Kodak article just below.
Light colors reflect light well to meter a high value. The exposure sets that to be a middle result.
Dark colors reflect less light to meter a low value. The exposure sets that to be a middle result.
The meter does not know what anything is, or how it should be. A reflected meter puts (the average of) everything in the middle.
The flash TTL system meters on its fixed central area. Which includes the rose, and some of the background. Our human brain and experience can immediately recognize the scene, and we know exactly how it ought to look. But the meter has no brain, it just sees a blob of light with no clue what anything is, or what it means, or how it ought to be. It can measure the light, but it sees the averaged tone blob in the area it sees. This average tone of all such properly metered scenes can only go near the middle range.
In the first picture, the black is overexposed towards middle gray. As the background becomes more white, the blob of light appears brighter, so reflective meters underexpose it, still trying to achieve the same overall (middle gray equivalent) average result for the area (the averaged colors of rose and background). We would think that a dark scene ought to look dark and a bright scene ought to look bright, but the meter cannot distinguish them. So the only possible goal is a result not too bright, not too dark. That's all a meter can know to do, without a human brain and experience to be able to recognize the scene, to know how it should be.
Incident meters instead work very differently (examples on the third metering page), more correctly metering the actual incident light on the subject instead of the reflected light (which is affected by the subject colors). Metering on an 18% gray card (in that same incident light) makes a reflected meter mimic an incident meter (reading the incident light, independent on subject colors).
A reflected light meter (camera meters are reflected meters) cannot try to recreate the correct tones, because it only sees a blob of light, and it cannot recognize anything to know what it means, or how it ought to be. It cannot differentiate between dim white vs. bright black, it just sees a level. It is a silicon chip, not an experienced human brain that can think about what it sees. Reflective metering merely just tries to keep the average of all the tones more in the middle, not too dark, and not too bright. There may be very bright or dark areas, but the overall average should come out about midtone level. This middle tone average means that bright areas will meter brighter, and so will be adjusted to come out darker. Dark areas will meter darker, and so will be adjusted to come out brighter. If a scene is placed at the middle, then both extremes might be within the total range of the exposure now. That is all the reflective meter can do. It doesn't have a brain to know what the subject is or how it should be, but the photographer sees and does, and can help if paying attention.
All of this has been well known for many decades, and reflective metering has always worked this way, and camera meters are reflected meters. It is very good info, simply about how things work. Since it is obviously true, I promise it will help you greatly if you realize how it works. Give the scene a seconds worth of thought — use your eyes and head to help the camera when you realize it will be necessary.
So the rule is, a mostly light-colored reflective subject will come out gray (underexposed), and a mostly darker-colored less reflective subject will come out gray (overexposed). The result goal is "Not too dark, and not too bright". I'm speaking generally, and don't mean exact technical "middle gray". Reflected light meters work to 12.5%, not 18%, about 1/2 stop difference.
Which is the reflective meters only capability. It does not have a human brain to recognize the scene and understand "correct" exposure — it can only give an automated averaged middle tone exposure. And this often does work out pretty well, for "average" scenes (containing a wide mix which probably ought to average near middle gray brightness). The meter is a very helpful guide, but there are many exceptions, to which we must pay attention, and think a little ourselves. We must learn to expect that a white or light colored subject will be underexposed, and a black or dark colored subject will be overexposed.
However, there are bright colored scenes (like white) that meter high, so the system drops them back to middle, appearing darker. Or dark colored scenes (like black) that meter low, so the system boosts them up to middle, making them brighter. So the average of all scenes goes towards the middle (not too bright, not too dark). The light meter does not have a human brain to know what it is, or how it ought to be. However the photographer can recognize it, and can often supply proper compensations.
All photographers can look at any result, and think "that's too dark or too bright". The trick is to learn to look at the scene FIRST, and think about this FIRST. We can expect and correct this result in advance with Compensation (Exposure Compensation for ambient, and/or Flash Compensation for flash). Now we can see our rear LCD result, but this advance thought was quite necessary when using film, and just a few tries is sufficient experience to really help. Most general scenes are easy, but we need to learn to recognize the exceptions which will need attention. This is how exposure is properly considered. A little experience makes this be an automatic thought when you first walk up to a scene.
The exposure varies with the scene. More black or dark color causes overexposure (trying for a middle tone result). More white or light color causes underexposure (trying for a middle tone result). The meter's goal is to always create a middle tone. The picture above that is about 50/50 actually averages near middle gray, and so comes out about correct. The variations are the expected result. You can count on it. Simply how reflected meters work, all of them.
Reflected meters are aimed at the subject from the camera, and meters the light that the subject's colors reflect.
A white background or subject reflects a lot of light, which reads high, so the meter underexposes the picture.
A black background or subject reflects little light, which reads low, so the meter overexposes the picture.
The expected reflected goal is that all metered results (the average of each scenes areas) come out middle gray brightness, not too dark, not too bright. This is all the meter can do (it cannot recognize anything). Fortunately, many typical scenes contain a random mix of dark and light colors that probably will average out about middle gray, then the middle gray result can often be about correct. When otherwise, we can recognize it, and compensate it, to correct it to come out as we want, brighter or darker, as needed by this scene.
A Spot meter is simply a reflected meter, so the small spot should come out middle gray. Meaning, the user better consider the reflectivity of that spot they selected. The spot will not necessarily be "correct" exposure, it will simply be middle tone (because that's the only way reflected meters work). If the selected spot ought to be middle tone, then it might be a correct exposure.
Incident meters are the reverse, aimed at the camera from the subject, which reads the incident light from the light source directly. An incident meter never sees the "subject", it directly meters the actual light incident on the subject (independent of the subjects colors). So then any subject tone, be its colors light, dark, or middle, is shown as it is. Kind of a big deal. Point&shoot where it counts. :) Incident meters have the accuracy that newbies imagine their reflected meters ought to have (but can't). This is not possible at the camera, since incident meters meter the actual light level incident on the subject (from the subjects position). We might make exception for situations in bright clear direct sunlight outdoors, if the meter and subject are in actually the same sunlight, but specifically, the incident meter needs to be at the same distance from the exact same light that is actually on the subject. In a portrait situation, you'd meter from under the subjects chin.
Said again, speaking of reflective meters (camera meters), their method is, if more dark area, make it brighter. If more light area, make it darker. This is simply because — its only capability is to make the metered area of all pictures average out to be a middle tone, which is "correct" by its rules, but it may or may not be the result you want. Whatever the scene, the reflective meter's overall goal is that the metered area will be exposed to average out to a middle tone (I call it middle gray, meaning the brightness equivalent, but it could be any color tint). The metered area of this is a factor too (next page). Incident meters are a different story (second next page).
Back in the negative film days, the standard saying then was "Expose for the Shadows", suggesting intentional ample exposure of shadows, and allowing overexposure of highlights (which did not hurt negatives much, they had wide latitude in the dark room). However, that applied only to negative film (which will be inverted). "Expose for the shadows" was a bad plan for positive slide film, and is also bad for digital, because of the overexposed highlights, and digital clipping. So the rule for positives was "Expose for the Highlights" (meaning "don't burn out the highlights, and don't clip anything in digital"), and is still very applicable to our digital cameras. Today, positive digital images are more demanding than negative film, due to less latitude, and digital clipping, etc, so clipping due to overexposure becomes a Real Bad Thing now.
We know that a scene with a good degree of white or light colored content will reflect light very well, so a reflected meter will read too high — and also a dark colored scene will read too low. But the reflected meter does not place these high or low as we would hope, it places everything in the middle. The meter is a dumb chip that cannot recognize the scene, and does not know the difference. A high reading might mean the light was bright, or it could mean the scene colors reflected unusually well, like white. The simple chip can only assume all scenes are an average scene (it cannot contemplate things). So according to a reflected light meter, all scenes should and will go to the middle, which does take care of bright or dim lighting. But the camera meter is fooled by scene colors.
Preachy here maybe, but intended as hopefully helpful. This fact about scene colors is Photography 101, perhaps not obvious, but basic and clearly evident, and one of the first things we should learn. Our best tool is a human brain that can see and actually recognize the scene. Brains and photographers have experience to know the difference. We should think about how we work. If we can see there is a white background, we know to expect underexposure, so we would compensate to boost metered exposure a bit. Or maybe a stop or two if the scene is mostly all white. For example, most pictures in the snow probably need +1 EV, and if the scene is entirely snow in bright sun, maybe consider dialing in +2 EV exposure compensation. (Always do what is seen needed, because you will be disappointed if you imagine the camera should always get it right.) Experience lets us "already just know" when we first walk up to the scene. It does require we look, and think a little about we're doing. Do Not turn off the brain while the camera is engaged. :)
Or easier, an incident meter directly meters the light itself (independent of the subjects colors), and in that light level, light and dark scenes will seek their proper high and low levels then, same as we see them.
And metering on a gray card is about the same deal as the incident meter (standard reflection from the gray card, representing the light, and independent of the subject colors). If we did not have an incident meter, this would be the reason we might meter on a gray card.
The need is to SEE the scene and use your head. You could use + EV exposure compensation, but the bright background might make that extreme. You might plan to provide proper fill flash for the dim subject. Or easier, perhaps reposition camera to avoid including the bright background. If these are not possible, the popular and easy classic work-around in that type of situation (bright background is making picture too black) is to instead aim the camera meter at the ground at the subjects feet (same focus distance), and then hold shutter half press to lock that darker reading (and focus), and then holding that, re-aim at the subject. This is not precisely metered then, but it will be a vastly better result of the subject. Try that, you'll like it when needed. Remember it, it can very often be very valuable to easily save your picture in this situation (bright background is making picture too black). The bright background will then go overexposed, but the subject should come out great. Center Weighted metering mode will do this better than Matrix metering mode.
General camera reflective metering can be noticeably affected by the varying reflectivity of the subject's colors.
Exposure methods that are independent of variable subject color reflection:
If not yet a believer, then two more cases of additional obvious proof. These are closeups, as shown. You can and should repeat these simple tests yourself, to understand how the meter works. This IS the basic principle.
These three cards (two are fun foamies) really are black and white and gray, here propped up on a couch background. This combined overall scene more nearly averages out to actually be middle gray (averaging a middle tone). This average is more what light meters expect to see, so its exposure is relatively correct. Reflective light meters simply try to make every scene average out to middle tone. The overall average tone is not too dark, not too light.
Black card with TTL flash (result is not black).
The metering used much more flash power (exposure) to make the black card be middle gray (tone). It makes couch and pink paper be white.
In extreme lopsided cases like this, we must know to manually apply a couple of stops underexposure (-EV compensation) to make black appear black.
White card with TTL flash (result is not white).
The metering used much less flash power (exposure) to make the white card be middle gray (tone). Makes couch black, and pink paper dark. In extreme total cases like this, we must know to manually apply a couple of stops overexposure (+EV compensation) to make white appear white.
Gray card with TTL flash
Approximately correct. The small paper was in fact pink, and the couch looks normal. Middle gray is made to be middle gray (tone), which is merely coincidence here. All images are always made to be middle tone average — that is all a reflective meter can do.
Get some black and white paper (craft stores have it), or any dark and light objects — items around the house — white walls, or bed sheets, or refrigerators. Maybe a black suit jacket and other dark stuff. Try this yourself, to see it, to believe it, and to understand and expect it.
This is simply what reflected light meters do. It is very good to know and expect this.
That was flash above, but now here are the same cards, repeated outdoors in bright sun (ambient, no flash), same thing. It works the same way. This subject is not just about flash, it is about reflective light meters (like in cameras). This set is all automatically metered (without adjustment) by D300 camera in A mode (aperture preferred), f/8 ISO 320 (full frame shown, not cropped). The first scene, of the three cards, does actually average middle gray overall, so it comes out correct. All four come out middle gray, which is what reflected light meters do.
Any and all scenes will be metered to come out middle tone (average value of the metered area, not too dark, not too light). This is simply what reflected light meters do. The results may not match your goal, but these are "correct" results, doing exactly what the reflective meter is designed to do. It is very good thing to know and expect this, and then you will know how to achieve your goal.
The exaggerated plain card scene is used here to simply make it trivially easy to see this. These cards are not typical scenes. Any real scene will have mixed brightness areas (sky and trees and shadows, etc), so that even unusually light or dark real subjects will rarely need as much as the two stops compensation these two "unreal" all white and all black subjects need (to actually be white or black). But it is routinely true that using reflective meters, light colored subjects (higher reflectivity) likely will need 1/2 stop or 1 stop more exposure to make them be light. Dark colored subjects (less reflective) likely will need 1/2 stop or 1 stop less exposure to make them be dark. The closer to middle gray the subject averages, the more accurate the reflected meter reading.
Newbies seem to imagine the camera ought to always be correct (imagining that our attention is not required). And maybe the meter is "correct" on all of them, but the meter may be doing something different than you expect. It only does what it can do. The meter has absolutely no clue what any of this scene is, no human recognition of what it is or means, and certainly no comprehension how it ought to be. The meter cannot distinguish a rose from your Aunt Martha. Our human brain does have smarts and experience to know immediately, what it is, and how it ought to be, so that is our natural expectation. But the reflective meter only sees a blob of light, which it can measure, but without any understanding about what it is, or what it means, or how it should be. So necessarily, what the result will be is a middle tone average, not too dark, not too light.
A reflective meter is aimed at the subject, and only sees the light reflected from and affected by the subjects colors. In contrast, an incident meter is aimed away from the subject towards the camera, and it can measure the actual direct light level, at the subject, but unaffected by the subject or its colors. That's a big advantage to accuracy (next page), but is necessarily more awkward and inconvenient to use.
The meter is an excellent guide and aid, but those who imagine their camera meter is recognizing and evaluating details in our picture, and should always give the correct exposure, are simply in for big disappointments. Instead, the trick is in learning what the meter actually does, so we can use it as a good guide. The meter just gets it into the ballpark, often about correct, but from which we make relative adjustments, as we see needed. We humans can see the scene too, and in many cases, learn to recognize how it will come out, and can compensate in advance. That was a required skill with film, and is very handy with digital too, but digital shows us the result that we get, giving us another chance to fix it. Now is the time to realize that the one in charge of your camera's exposures is YOU.
Said again, the exposure you get (underexposure or overexposure) will depend on what you aim the camera at. I am trying real hard to get your attention. If you want to resist, then imagine me shaking you by your shoulders shouting "wake up". :) We really ought to know how the light meter works (and this is how it works). This is all also true in sunlight too, but flash differences seem more pronounced (due to inverse square law falloff). The trick to know is that Flash Compensation is how we control TTL flash. Flash Compensation will greatly improve your TTL flash pictures. Simply watch, and do what you see you need to do.
Again, if any doubts, then it is absolutely necessary that you repeat this, do and see this kind of test in your own situation. It is real. It is the big overall view of how things actually work. All you need to repeat this is a sheet of black paper and a sheet of white paper, and your camera as a light meter. Then you can see, and believe, and will understand how it must influence your procedures.
We hear novices complain about exposure, thinking that their meter is misbehaving, when they simply don't understand how reflective light meters work. They may have a little more to learn, but it is easy when we know. The meter's goal in life is NOT to give "correct" exposure. The dumb meter has absolutely no clue how to do that. It has no clue even what the scene is, and certainly not how it ought to be, so there is never any concept of a "correct" reflective meter reading. The meter's goal in life is simply to expose all scenes to create a middle tone average, however much that takes, for whatever it is, regardless if we will agree that middle tone is "correct" or not. The five rose scenes above have five different contents, with five different average values. The reflective meter's goal in life is to make all pictures average out to be a middle tone (speaking of the central metered area). Therefore, the TTL automation exposes each picture of a different scene differently, to make that middle tone result be true (the average of the metered area). So dark scenes are exposed more, and light scenes are exposed less, to achieve the same middle tone every time.
This is simply how reflective meters work (and it is a good thing to understand and expect this). Note that the photographer's job is to see and realize if this current scene is darker or lighter colors than normal, i.e., if this exposure is going to need our help to come out darker or lighter as it should. This is easy today, the digital camera shows the result to us, then and there, while we can still correct it. When you know why, you also know how, so this is easy, no big deal at all. You will soon already "just know" before you take the picture, what to expect, and what to do about it... it quickly becomes second nature. It does involve looking and thinking.
NOTE: When I often mention result is middle gray here, I merely mean middle tone... as in B&W, but if color, it might have a red or green or blue tint sometimes. Middle tone has been called middle gray for decades, B&W film habit I guess, please forgive my quirk. In these cases, I don't necessarily mean the color "gray". And I don't mean center of the histogram either (another story). I just mean for any scene, the meter's goal is to create a middle tone (average of its metered area), not too dark, and not too light. Which is often reasonable, about right. But the problem comes when the subject ought to instead be dark or light — it won't be. It will always be more middle.
The point is, assuming the common reflective meter, in the same light, what your meter reads depends on what you are metering.
The reflective light meter will try to give an exposure that will make everything average out to about middle gray. "Average" or "typical" scenes/subjects generally do have wide tonal ranges which do in fact average out to about middle gray, but which is not true in all cases. But this is the basis of the system, and you, as the photographer, can see the subject, and are supposed to realize this.
If your subject is a more typical one, with typical wide range, and it actually does average out about middle gray, then great, easy as pie.
If not an "average" subject, then you better pay attention, and stand ready to help with Flash Compensation or Exposure Compensation.
In contrast, handheld "incident" meters also exist, which stand at the subject's position, and aim back at the camera, and directly read the incident light falling on the subject, totally independent of the color of the subject (page after next). Again, incident meters read the light directly from the subjects position, so cannot be built into the camera. This may be less convenient to use (except is wonderful for studio flash), but are greatly more accurate and consistent.
Bottom line: When the exposure does not come out right, it cannot help to cuss the meter or bemoan your fate. This is simply how meters work, and have always worked. The way we learn to use them is to study the picture, and figure out what the meter was metering to cause that middle tone result... why it happened? (usually, dark or light colored subjects or backgrounds are expected to cause mid-tone results). Then we learn to recognize those situations, and then we easily just know ahead of time what to do to prevent it next time we see a similar situation . There is no other way but to learn.
I say Flash a lot, but do not misunderstand — Metering in daylight or other continuous light works exactly the same way (middle gray result). But flash does seem more fussy about it (more on next page). The rapid falloff with inverse square law emphasizes differences (affecting flash, but not sunlight). And the TTL preflash creates background shadows, metered as dark areas, which reduces the bright area that reflects and is metered. Flash may seem a little different, but the meter merely reads what it sees.