Reflective vs. Incident Light Meters
A Histogram is not a light meter
A histogram is not a light meter. The histogram is 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 when looking at a histogram, we are mostly only concerned with size of the Empty Gaps at the ends of the histogram data.
This page is about the difference in reflected and incident meters, and hopes to make the one big point below, about histograms and reflective meters, which is also repeated here in this box:
Your own experiments can easily verify this, and this fact should be the starting point basis of understanding use of reflective light meters. We may initially imagine all results ought to be like incident meters, but reflective meters simply don't do that.
Even if in a hurry, you ought to slow down a bit and see some of this. It's about how light meters work.
- Normally, the main thing of interest in camera histograms is how close the image data approaches the right end. If it actually touches the right end, that is overexposure, because brighter data might be pushed off the 255 end and be lost — since all brighter tones are clipped to 255, causing an unnatural spiked peak right at 255. If for example, the Red component is clipped, but the green or blue components are not, that changes colors. Our main goal is to expose so as to prevent any clipping at the right end. A large peak slightly below the right end may be fine if it has a reason (see first histogram below, because it is white), but the same large peak RIGHT ON the right end is probably clipped data, and a bad thing. Back off a little. This is not to say that slight intentional clipping cannot improve contrast sometimes, but this needs to be seen and judged later. Because clipping in the digital camera cannot be reversed.
- A second concern is if the data is too far below the right end, representing underexposure — it could be exposed more. A black or dark colored subject OUGHT to be well below the right end, but a good guide is that bright things ought to approach the right end (but never actually touch it). We assume most average scenes contain some white or bright area, and normally ought to extend high, but this is not always true. NOT all things should approach the top end. Where it actually should be depends on its color and brightness. A little practice makes it become clear, the skill called experience. How the picture actually looks on the camera rear LCD is the final guide (you may want to compare the same image on this LCD, to your calibrated processing computer monitor, and maybe set the camera display to be a step less bright).
- A third concern about histograms in general (but we cannot do this while in the camera) might be that our "typical" data probably also extends to near the dark end at zero. For the opposite example, a picture in a strong haze or fog situation will have no actual black (everything is gray, near the same, but not black at zero — the contrast is dismal.) Images are better enhanced with greater contrast, more full range. Especially B&W photos, the main key is they specifically need to contain at least a few very black tones, and at least a few very white tones, a range for greater dramatic appeal. Blacker blacks and whiter whites is more contrast, greater range. Color, maybe a little less so, black is less appealing, color is preferred.
- Normally, we can have no knowledge about how other subject objects ought to appear in the histogram. We just ignore the middle, and only care about the top end, and in lesser degree, the bottom end. Specifically, the ONLY concern is about the size of the Empty Gaps just past those ends. The first histogram below is a very special case, containing only a few objects and colors, and carefully studied to identify some middle peaks. That is NOT the normal situation. Normally, we could not care less between the ends, and we are only are concerned with the location of the ends of the data range, specifically the size of the Empty Gaps at the ends.
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 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 handheld 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 directed onto 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 colors do not affect incident meters.
Incident Metering
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 handheld 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.
The histogram shows pretty much perfect and ideal exposure of both blackest black and whitest white.
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. Because incident meters the actual light source, at the subjects location, and provides the correct exposure, no matter what the subject is, no matter how the subject is changed (incident light exposure reads the incident light directly, and is independent of the reflected subject colors).
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 handheld 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).
Then the lower section will 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 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.
Using Incident metering, with different subjects
In other words, first what things ought to look like.
The 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.
The 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 dress 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.
Again, "exposing to the right" would be fatal to "this" picture. We need to recognize abnormal subjects.
The 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 ensure 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.
The Red paper, incident meter, with same Manual flash power and f/7.
All RGB colors have three possible color components. This red paper has a strong bright red component, but with darker 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).
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 and its color. The fun is in the next section.
(Incident Metering above, Reflected Metering below)
Now Shifting to TTL flash metering by the camera Reflective meter
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 (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. 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, metered 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).
This red is a very special case. It's a bit dark, but still not too far from how it should look. It is very different than either white or black in that regard. It is a bit underexposed, but this is what happens, because from the reflective light meter's viewpoint, the metered tone is simply nearly 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.)
The White paper, metered by TTL flash automation.
Yes, no mistake, this is the white paper, and this is simply how reflective light meters meter white objects. The black paper next below is essentially the same result. And in the long run, it will help tremendously to learn to recognize this.
White and Black are simply the extreme shades of gray, so their variations come out gray (where red does not, and remains at least pinkish). The reflective light meters goal is that everything comes out placed near this mid-point, 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.
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 place it more correctly, so the average nearly reaches middle gray. But if your scene is "largely" white (like a white background wall), or a bright sky background, or simply any highly reflective scene, you will get the same underexposed result. The photographers job is to expect this, and can compensate in advance.
An Incident light meter meters the actual light incident on the subject, without seeing the subject to be affected by the reflectance of its colors. This lets all colors reproduce appropriately, so blacks will be black and whites will be white. But in reflected meters, an all black or an all white scene will both be middle gray.
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.
The Black paper, metered by TTL flash automation.
Yes, again, no mistake, this is the black paper, and this middle gray 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. Or less exposure will show it as black. But otherwise the light meters goal is that everything should come out near middle gray intensity, which is normally correct for mixed average scenes. These are NOT average scenes, so the black and the white paper look about alike here (as metered), because the goal of reflective meters is that everything is rendered as middle gray (the average of the overall scene). The metering told the TTL flash to use more power to move the black paper up to middle gray, and told it to use less power with the white paper to move it down to middle gray. That is simply how camera meters work (reflective meters).
"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. But our human brain can see these scenes first, and should know what will happen, and can take corrective action to make it come out better.
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.
The Gray paper, metered 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. Meaning, or a middle shade of red or a middle shade of green, whatever, if applicable. And this is normally very near correct for an average scene which contains 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.
The 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.
Your own experiments can easily verify this, and this fact should be the starting point basis of understanding use of reflective light meters. We may initially imagine all results ought to be like incident meters, but reflective meters simply don't do that. Reflective meters are affected by how the various colors absorb or reflect the light colors.
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.
