Two Types of Light Meters

There are two types of light meters, reflected meters (at left) aimed at the subject from the camera or from the camera direction, which reads light reflected from the subject. And incident meters (at right) aimed at the camera from the subject position, which reads the actual direct light incident upon the subject - at the subject, but totally independent of the subject. Since cameras are not located at the subject, they can only use reflected meters.

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Three ways to meter

• Metering the light with an incident meter (incident on the subject's position, because inverse square law applies.) This is independent of subject reflectivity- the meter is aimed away from the subject, toward the light source. If we measure the direct light itself, then in the subject, black things should come out black, and white things should come out white (we still might have to compensate slightly, if we want them "more white" or "more black" than the general case.) Generally, since the reading is not influenced by the subject colors, there is less question about if the incident reading is accurate or not (but our metering technique can introduce variables - we learn to be consistent.) Again, the advantage of incident metering is that it is independent of the subject's reflectivity, it measures the incident light directly.

• With a camera's reflective meter, pointing the meter at the subject, directly metering the same subject that is photographed. This is the usual case, but as shown in the case of any of the individual gray, white, or black cards, all are going to come the same middle gray. This is just what reflective light meters do (shown before.) So we might question meter accuracy here, unless we understand what the meter is telling us. The meter reading is dependent on the subjects reflectivity, and whatever it is, the camera will try to place the average of that result at a middle gray.. So... even if we might imagine 18% ought to be at histogram midpoint (it is not ), you should know it won't matter which card you used in that case (simply is a very wrong idea.) But the photographer can see this subject too, and should learn how the meter might respond, and then we stand ready to help correct it. We simply just know that white or light colored things will be underexposed, and black or dark colored things will be overexposed, and we plan for it. This is the purpose of the Compensation buttons on the camera.

  Spot Metering is just a special case of Reflective Metering. Spot Metering isolates the small metered area from the background (not influenced by including the background), so that the small spot will be made to come out middle gray, regardless of any other area. It does not mean the spot is "correct", it only means it will be middle gray. This is in contrast to averaging a larger area of the picture to come out averaging middle gray (which larger mixture generally has better odds of being "correct" as a middle gray average).

• We can directly meter the reflection off of a 18% gray card (from close, including nothing else in that view), and then use the same exposure settings for a scene in the same light (meaning: at the same distance from the light - inverse square law always applies.) Then the camera will expose the subject in the same way. This is more like Incident metering, in that it will be independent of the reflectivity of the actual subject, and will instead depend on the reflectivity of the card. Specifically, this means that any tones in the subject which match the cards 18% reflectance will also come out middle gray.

  Note that the term middle gray card (analog) and midpoint of the histogram (digital) are not the same thing - very different concepts - not related in any way. Generally, the card idea is that if we think this subject's average overall tone matches an 18% card, the result will come out about right. And it often about does, at least for many typical wide range scenes, but Kodak tells us if we meter the reflection off of their 18% gray card, and then use that exposure for a scene in the same light, we should increase that exposure by 1/2 stop. If we meter on an 18% card, and then open Kodak's 1/2 stop, that is the same as metering on a 12.7% card, all else equal (our light meters probably use 12.5%.) This method provides a known reflectivity to be metered, metering an actual middle gray subject, which the meter will make it come out middle gray, therefore striving for a more accurate result - specifically independent of the subject. Be sure to angle the card slightly to prevent any shadows or overly bright glare/flare.

  The idea is that this gray card method is independent of the subject's own reflectivity values (like incident metering is independent). Any error this way depends on the difference in the scenes actual average, and the 18% card's average. It can also be done several other ways. If in bright sun, metering from the clear deep blue sky (low, and away from sun) works about the same as an 18% card. A green grass lawn is close too. In the old days, Kodak told us metering on the yellow box the film came in (included black ink) was about the same. Don't expect great precision, things will vary every time you move your camera - we are speaking ballpark. And this logic will always still depend on your actual subject also being average.

  Or we can use the open palm of our hand as a gray card. Metering from our outstreched palm is not "average", and will be nearly one stop brighter than 18% (is more like spot metering from the face.) Then we know that we will want to set the camera for about one stop more exposure. No matter what we meter, a reflected meter will place the result at middle gray - regardless of where it "ought" to be placed. Then we compensate for how THAT middle gray differs from the actual scene's actual gray average. The metered object must of course be in the same light (and same distance from light source) as the intended subject.

  And there are other metering tricks possible (called technique.) Say the subject is sitting in the shade under a tree, or indoors with their back to a bright window. In those cases, our camera meter mostly sees the bright sunshine background behind the subject. It does not understand that the background is not our subject. It exposes for the bright sunlight, and the picture of the subject comes out very dark. Spot metering can help then, or faster, we can just aim the camera down lower, maybe as far down as their feet, specifically down enough to exclude the bright background, and then meter there. The light level is often about the same lower there, and the subject distance is essentially the same there (our brain is a great tool to evaluate situations), but this view specifically omits the bright problem outside. Half press the shutter, and hold to lock this reading (locks metering and focus), then raise camera to reframe, and the final shutter click will use that locked meter setting for this subject. Result will be greatly better than ignoring it, normally it is great. But if you instead also want to maintain the exposure of the bright outside, then you meter on that, and need to use fill flash, to equalize things.

  Old timers all understand this. Novices imagine their fancy modern camera ought to understand it too, but this belief will be disappointed. Instead, it all works as mentioned above. The meter is a helpful guide indeed, but just watch, and always do what you see you need to do. Sometimes it can be too big a job for a dumb computer, but the human brain can always help.

Ansel Adams thought his Zone System's middle zone 5 ought to be 18% reflectivity, and he promoted the 18% notion in photography. I suspect Ansel never saw a digital image or a histogram (in 1940), so don't confuse the two systems, analog film and digital. However, our meters are set closer to 12.5%, (thought to be more realistic, on average), and Kodak says we will need to open 1/2 stop if using their 18% card (this 1/2 stop more is equivalent of 12.7%.) So any and all scenes are exposed to come out about 12.5% middle tone average - regardless if they are typical 12.5% scenes or not. If photographing that card itself (as here), then no matter which card you meter, black, white, or gray, the results are always near a middle tone (as here.) BTW, none are necessarily at 128 on the histogram, which is simply the wrong idea (the histogram data is gamma encoded anyway - and the 18% card was an analog film concept.)

It should be obvious that any plan to calibrate a light meter by metering an 18% gray card is not a well thought out plan. This was popular in analog film days, but a digital histogram is something else (A related link, explaining why 18% gray is not the midpoint of the histogram we see, is at Histograms are Gamma Encoded.) Photographing the 18% card has no significance (any scene will come out middle tone, the white card and the black card too... what significance is the 18%?) The correct way the 18% card is used is that we can meter from it, and then use that reading for the exposure of some other general scene in the same light. Most wide range scenes (beach scene, mountain scene, sky and water and trees and human faces) do often average about middle tone, which is what lets reflective meters be useful, since that is what they do too.

The camera meter is a "reflected" light meter. A reflected meter is aimed at the subject, and reads the light reflected from the subject (which is very variable, depending on what it is) The reflected meter tries to make all scenes it sees to average out to be near middle tone. That is all it can do. This is more easily seen with a blank paper subject, but all scenes come out averaging a middle tone. The meter is calibrated to make that result tone be about 12.5%. Post processing can change it. This value is NOT related to the center of the histogram. This value is not related to an 18% gray card - 18% is not a factor. The meter's "calibration" value and result causes this one same resulting middle gray tone - from any subject.

Sekonic specifications (L-358 for example) specifically says their reflected K=12.5. Wikipedia explains more and says "ISO 2720:1974 recommends a range for K of 10.6 to 13.4 with luminance in cd/m². Two values for K are in common use: 12.5 (Canon, Nikon, and Sekonic) and 14 (Kenko and Pentax); the difference between the two values is approximately 1/6 EV."

Camera Reflective Metering Modes

Spot metering - Beginners beware. This is a techie tool that requires greater experience, definitely NOT point&shoot. Only a small spot in the frame is considered, at the current focus point. Spot metering isolates this small area from the rest of the frame. The idea is NOT about "correct" exposure, the only idea is that this small spot area will be made to come out middle gray (correct or not), and all the rest of the frame is ignored - the rest comes out however it does, when this spot is made middle gray. Spot has no concept of a background to match, so Nikon TTL BL flash mode will change to become TTL mode if Spot Metering. Only that small spot matters, that small area will become middle tone.

Novices mistakenly confuse Spot metering with a general purpose metering mode, and assume it means that this spot will come out correctly exposed. But Spot metering is a very special case (only about that one spot), and it just means that spot will come out middle gray, which is NOT about being "correct" - unless hopefully, we choose a spot which ought to come out middle gray (which puts the full responsibility back on the photographer). Yes, spot metering can isolate a face from the influence of a black or a white background, but should that face come out middle gray? The skill we need to know is how to compensate it, either way. That face will likely need about one stop additional exposure to make it as bright as it should be, so this is two tricks to learn. The original idea of spot metering was to meter the highlights, and to meter the shadows, to be able to determine the range between them, as a way to determine a middle compromise. Center and Matrix metering try to automate that averaging for us.

Center Weighted metering - The entire frame is used in some degree, but significantly greater importance (weighting) is given to a large center area (default diameter is roughly half frame height, but adjustable.) There is no sharp dividing line at the circle, the boundary slowly tapers, typically a Gaussian Bell Curve. Usually the center area is weighted about 75% (i.e., tones in the center count as 3x more important than at the outside edges.) All the tones in this circle are averaged to one average tone, and exposed so that this average tone is made to come out about middle tone. It is very suitable for many typical or average scenes.

Matrix metering - The focus point area has substantial weighting, but a wide area of the frame is also watched for exceptions, with the camera firmware making other judgments to factor in areas with exceptions. These rules are unknown to us, it does what it does, often pretty close.

These tests used Matrix metering (because many people do), but I normally use Center metering, to emphasize the central subject more than the edges. Center metering is very useful for portraits, or even this rose, and still works for scenes like landscapes that may not have any central subject (they do still have an average tone). We ought not to imagine any great understanding is present in any metering system. It is just a computer chip, which sees some light, and can measure it, but with absolutely no idea what it represents, or how bright it ought to look. The meter's only goal is to make whatever it is come out middle tone - all it knows, all it can do. My human brain can almost visualize and understand Center metering, to help predict the result, and my own opinion is to shun the automation of Matrix metering and TTL BL flash, simply because I never know what the computer is going to do. Possibly the automation can help sometimes, but as a personal philosophy, I simply try to avoid those unknown complications. :) But regardless of your method, you will always need to watch and stand ready to help. YOU are the photographer. No one else is going to do it. Your human brain is the only intelligent tool present that can judge "correct" or not. Your human brain is the only intelligence present that can recognize a black cat in a coal mine from the polar bear on the snow. Any reflected meter is going to make both come out to be middle gray. Realizing this makes all the difference.

In contrast to Reflected meters, Incident meters measure the incident light directly, independent of, and without reference to the subject. These can be more accurate (without that influence), but incident meters must meter the light actually incident at the subject's position (Inverse Square Law). Direct sunshine may not matter where you measure it (it is typically the same everywhere), but any incident light more local than that cannot be metered from the camera position.

Bounce Flash

You will see the same averaged middle gray result when metering bounced TTL flash. There are differences in bounce, but the same principles apply.

My notions FWIW about the differences in exposure of direct flash and bounce: We know direct light falls off with the inverse square law, so foreground objects too close overexpose. Also, any distant background is necessary darker, and preflash meters it that way (as dark). Any background that might be say 2x farther behind the subject will therefore be two stops darker (influencing metering, and dark scenes overexpose.) So to direct flash, a distant background is seen like the black paper background above, influencing exposure to be brighter.

But bounce flash more nearly lights up all of a small room evenly, because all parts of a small room are more equal distance from the ceiling. So we clearly know (within reason) that the background wall will remain much brighter for bounce, and preflash meters it that way (and we know light things will underexpose.) So bounce flash TTL is prone to underexpose (plus, white walls are common.) TTL is metering the preflash, which makes shadows to be metered if bounce, but there are no shadows (on the subject) from flat direct flash (which is why we don't like it). These are the kind of things that give errant meter readings. It may seem that bounce flash needs compensation more often, but not necessarily so - it is about these same metering concepts, about the scene in front of the lens. Compensation is NOT about the camera gear.

Bottom line is, (camera reflective) metering, and also compensation, are entirely about the scene in front of the lens. We always need to watch the TTL results, and stand ready with Flash Compensation, to get the picture that we want. You being the photographer, this is your job to do. Simply realizing this is your job is the first full step ahead.

Flash Modes

  (Nikon system)

There is more about TTL/TTL BL in Fundamentals Part 4, but will add some here.

If you plan to watch results, and then perhaps compensate the automatic flash to be more what you want, then select plain TTL mode.

If using fill in bright sun, and you don't want to bother with compensation, then this is what TTL BL mode is for. It tries to do the necessary compensation, point&shoot flash. But if you also plan to control it too, then too many cooks ruin the soup. All of the Nikon camera manuals recommend TTL mode if using Compensation (no surprises). I do not imply it is a "big deal", certainly we can compensate TTL BL Mode too, but we really do not know what TTL BL is doing, and plain TTL mode offers less interference with our own control.

If you do these tests on TTL BL Direct Flash, you may see some differences, due to D lens distance information (speaking only of direct flash, specifically, with the flash head level - flash head 90 degrees to the flash body.) Nikon TTL BL Direct Flash uses the D or G lens distance information (the lens reports the focused distance.) Nikon doesn't explain the use, but similar experiments clearly show that it can sometimes be a safety check to prevent overexposure for unusually bad meter readings. The guide number for direct flash is more of an absolute known situation computing the inverse square law... if the guide number is 32 and the distance is 4, then 32/4 = f/8 will be more or less correct, independent of subject. The lens reports its focused distance. If the TTL BL metering (dependent on the reflective colors of the subject) is saying f/4, that can't be right at 4 feet, so the camera can apply a safe correction for overexposure. That is my notion of the observable way the distance information can be used, as an override safety check. I used to have notions of how TTL BL worked, but have finally learned that I really don't know much.

The flash has a head tilt switch which prevents using this lens distance information when the flash head is tilted up (bounce for example), because then the lens distance does not match the bounce distance (up to ceiling and back down is farther than the direct lens distance.) And TTL mode does not use the lens distance information at any time. Only TTL BL direct flash does.

The effect actually seen on this rose with TTL BL here (Direct Flash) was that there was very little difference on the white background - Direct TTL BL still comes out underexposed about the same amount as the TTL. Nothing different - it is how the meter works. But the black background was quite different, in fact, it was a little underexposed instead of the overexposure expected. It seemed that the lens distance information prevented overexposure by the TTL BL direct flash - if the speedlight head was not tilted up.

My hope is that you read at least this summary part

So this is simply how reflective light meters work (meters in cameras are reflective meters.) Reflected meters only have this one way they can work. They are a dumb silicon chip, without any human brain. The meter has absolutely zero recognition about what the subject is, and has zero knowledge how it ought to look. The meter does not know if the subject is black and ought to be dark, or if it is white and ought to be bright. The human brain may recognize it instantly (in its surroundings), but the meter has no clue.... it simply sees some light. It can measure that light accurately, but it has absolutely no clue what it means. Its best try is going to come out middle tone, which is a good compromise, considering. The reflective light meter's every answer is "middle gray". It is good that we realize this, to work with it, instead of against it.

The reflective meter can only attempt to reproduce whatever it may be, to come out as a middle tone average. And this middle tone will be halfway ballpark (not too dark, not too bright, but close enough that we can recognize the subject), but if that result is correct or not depends entirely on the subject that it sees. Most typical scenes do have wide range (portraits, beaches, mountains, jungles), and do average out about middle tone, which is how the system works. But not every scene is a typical scene.

The photographer does need to keep a close watch on things. Bottom line, we can moan and complain about what the meter does, or we can simply just watch and learn to fix it. This latter way works much better. It is called experience.

We certainly need to be aware of this, so a similar experiment is one of those things which you should try at home. To become a believer, you should do similar tests as these, yourself, to see it, to believe it, to be able to use it. Just do it. If you are metering a light scene (meaning, colors which reflect much of the light efficiently), it is going to come out underexposed. If you are metering a dark scene (meaning, colors which do not reflect so much), it is going to come out overexposed. Because, both will come out averaging middle gray, which may or may not be a "correct" result for that scene. Our human brain can recognize this and plan ahead. The least we can do is to look at the result and compensate. What we ought not do is to act surprised. :)

The meter is a big help, and it is still pretty easy. The idea is, you have eyes, you have a brain, and if you use them, then with just a little experience, you quickly come to know that a light scene with enhanced reflection (a white dress, a beige wall, the snow covered ground, etc) will need additional exposure, more than TTL thinks. You quickly come to know that a dark scene with relatively little reflection (black tuxedo, dark colors, dark open background, etc) will need less exposure than TTL thinks. Pretty soon, you will just already know about how much more. A little experience goes a long way, just think a little about what you see in front of your camera. Digital is easy, it allows us to see the first exposure result, and to react.

Did I mention? Flash Compensation is how we control TTL flash, to get the result we want. Just do what you see you need to do.

Continued - A Histogram is Not a Light Meter