How Light Meters Work

First, the fast ten cent quickie tour.

Novices always assume their camera meters ought to always give the correct exposure, but alas, they need to learn that life is not that simple. Meters are pretty dumb, and metering is an art done by the photographer.

Camera light meters (reflected meters) expose to make all pictures average out to be a middle tone. The only possible goal is not too dark, and not too bright. The beach scene, the macro of the spider, the studio portrait, or whatever... the reflective meter's goal is that overall, the metered area averages out to a middle tone (I call it middle gray, but it could be colored.) And fortunately, a middle tone is often near about correct (most scenes are near average range). But photograph something all black, or something all white, and it comes out near middle tone too. You need to be aware of this, it is simply how life is, how things work. Shown next below.

A Gray card, a Black card, and a White card
photographed with automatic TTL flash (reflective metering in camera)

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) - 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. 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 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 (from craft store, or any dark and light objects), and 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. Now here are the same cards, repeated in a lawn chair outdoors in bright sun (no flash), same thing. 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.

Three cards, f/8, 1/1250 second Nearly exactly Sunny 16	Black card, f/8 1/200 second Result is 2.3 stops overexposed, from first one.
White card, f/8 1/5000 second Result is two stops underexposed, from first one.	18% gray card, f/8 1/800 second Result is 1/3 stop over first one.

Any and all scenes will be metered (average value of the metered area) to come out middle tone (not too dark, not too light). This is simply what reflected light meters do. It is very good to know and expect this. The exaggarated plain card background scene is used here to simply make it trivially easy to see this. These cards are not typical scenes, and any real scene will have mixed brightness areas (mountain and sky and green trees, etc), so that even unusually light or dark real subjects will rarely need as much as the two stops these two "unreal" all white and all black subjects needed. But it is routinely true that using refelective 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.

Typical or average wide range scenes do often average out about this same middle tone the meter expects, so it often works. It is the basis of the system, on average, it can often work. Just not always, and in varying degree even then.

NOTE: When I often mention result is middle gray here, I merely mean middle tone... it might have a red or green or blue tint sometimes, but the average of a wide range scene actually is often near neutral color. 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, peak will likely be a bit lower. I just mean 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.

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. Black cat in a coal mine, or polar bear in the snow storm? The meter has absolutely no clue what it is. 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 probably recognizes 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 is, or what it means. Its best try is going to come out middle tone, which is a good compromise, considering. It is good that we realize this.

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. 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.

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 (1/2 stop 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.)

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.

Handheld "incident" meters also exist, which stand at the subject and aim back at the camera, and directly read the incident light falling on the subject, totally independent of the color of the subject (next page.) These are less convenient to use (except in studio), but are greatly more consistent (accurate.)

Don't give up here, more is coming, at Three Ways To Meter, below. Meanwhile, how "accurate" can a reflected meter be?

A sample approximating a real picture subject - Reflective metering

Here is a quick rose, intended to mimic some real photo scene. It is always photographed here with the same parameters. The ONLY difference in these pairs is that the background is switched between the white and black paper (both are shown first here - ordinary white and black craft paper.) All else is the same. The camera is aimed at the subject, and TTL automation takes over to expose the picture. The camera's TTL reflective meter sees different scenes differently. This is something worth knowing.

Nikon D300, ISO 200, f/8, 1/200 second, Matrix metering at about three feet. No adjustments were made, except as noted. These are all full DX frame (no cropping.)

SB-800, direct flash, off camera, on a hot shoe cord, but at same distance as the lens. Automatic TTL metering mode.

This first one meters halfway reasonably, because the white and black do average out closer to middle gray, and the reflective meter is going to expose to make all scenes come out averaging middle gray. Most average or typical scenes do contain a wide range of tones, and do in fact average out near middle gray. Which is good, because that is how reflective meters work. However, not all scenes are "typical", so we simply pay attention, and do what we see we need to do.

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. This is all also true in sunlight too, but flash differences seem more pronounced. The trick to know is that Flash Compensation is how we control TTL flash. Simply watch, and do what you see you need to do. It will greatly improve your flash pictures.

TTL flash mode

White paper background

White is more middle gray (underexposure)

Black paper background

Black is more middle gray (overexposure)

Notice that this automatic TTL metering gives different results, depending on the scene in front of the camera.

White +1.0 Flash Compensation

Black -1.0 Flash Compensation

I am not saying 1 EV is sufficient or correct compensation in this case. I am merely showing the effect, and a little compensation sure does look like a good start. Note again though, this article is not even about flash - it is about how reflective light meters work - to explain why the metered TTL flash needs compensation. Flash Compensation is how we control TTL flash (how we correct what the light meter wants to do.)

To make it be real clear and obvious, let's adjust the background a little at time, all else the same (but still with automatic TTL metering.) At each frame, the ONLY difference was that the black paper was shifted left a little, and then the shutter button was pressed again. Then the TTL metering reacted to the new scene. This is how reflective metering works. This is the expected result. You can count on it.

See?

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 no clue what any of this is, or how it ought to be. The meter cannot recognize a rose from a polar bear. Its method is, if more dark area, make it brighter. If more light area, make it darker. Its only capability is to make the metered area of all pictures average out to be middle gray, which is "correct" by its rules, but it may or may not be the result you want. Repeat after me: "There is absolutely no possible way a dumb computer can recognize the scene, and have any clue about the result that your human brain knows to expect." The reflected meter can only assume we want everything middle tone, not too dark, not too bright. The rest is up to us, and Flash Compensation is how we control the automatic TTL flash.

The effect of a large area of light color is that the light colors become middle tones (underexposure.) The effect of a large area of dark color is that the dark colors become middle tones (overexposure.) You can repeat this same type of test too, and you should repeat it, so you can definitely believe it, and understand it, and be able to use it. This is indeed a LOT to know.

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 how things 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 influences your procedures.

We hear novices complain about such things, thinking that their meter is misbehaving, when they simply don't understand how reflective light meters work. They have a lot to learn. 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, 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 middle tone is precisely "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. Therefore, the TTL automation exposes each picture of a different scene differently, to make that middle tone result be true (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 sense 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.

Details get in the way of the big picture. This subject is not about the background as such. It is more likely about the subject's own colors, but could be anything in the scene that the meter is metering. And it is not about "gray" as such. I say middle gray, from the B&W film days, but I simply mean any middle tone, usually near gray, but which sometimes might be red or green or blue. My middle gray term also does not mean center of histogram, and it does not mean 18% (which are not the same thing anyway.) It just means a middle tone.

The meter's "averaging the picture to come out near middle gray" means something like this (these are the same five rose pictures):

The Photoshop Blur - Average filter numerically averages all pixels to all be one average value, and its results on these last five are shown. The camera light meter exposes all so that each comes out middle gray level, in a similar way.

But there is a difference this time. These five averaged roses are close, but not quite equal, because the Photoshop filter averages the entire picture frame, whereas the light meter has its own rules, and only its own lesser metered area is involved. The image content in that lesser area will be a factor. This filter's full frame area includes more black or white pixels here, and overshoots what the meter will do. So this result shown is only an approximation, but you get the idea. The point is that reflective meters expose to cause all images to come out near a middle tone in this same way, which is what I mean when I say a middle gray result. I don't necessarily mean the color gray, but it is often close to gray. And this average may or may not be the "correct" exposure, which still depends entirely on the subject content in front of the camera. Meters simply don't know what the scene is. They simply always try to create a middle tone result. The scene may or may fit that model well. It's OK, the photographer is there to help.

There is more detail below, but the point is, assuming the common reflective meter, what reading that you meter depends on what you are metering. You, as the photographer, can see the subject, and are supposed to realize this. The reflective light meter will try to make everything average out to about middle gray. "Average" or "typical" subjects generally have wide tonal ranges which do in fact average out to about middle gray. 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 a typical subject, then you better pay attention, and stand ready to help with Flash Compensation.

I say Flash a lot, but do not misunderstand - Metering in daylight works exactly the same way (middle gray result). But flash does seems more fussy about it. The rapid falloff with inverse square law emphasizes differences (affecting flash, but not sunlight.) And of course, the TTL preflash creates dark background shadows, metered as dark areas, which reduces the bright area that reflects and is metered. Flash is a little different.

Bounce Flash

You will see the same thing when metering bounce flash, but sometimes it can seem more so. TTL bounce flash does not necessarily need compensation, but it does seem more likely to need it. The same principles apply.

My notions: We know direct light falls off with the inverse square law, so the background is necessary darker, and preflash meters it that way. Any background that might be say 2x farther behind the subject will therefore be two stops darker (influencing most metering, and dark things overexpose.) Direct flash TTL might be prone to overexpose. 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 (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. Regardless of any corny guesses at reasons, 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, then too many cooks ruin the broth. All of the Nikon camera manuals recommend TTL mode if using Flash Compensation. We really do not know what TTL BL is doing anyway, but plain TTL mode offers less interference with your 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.

Camera Reflective Metering Modes

Spot metering - Only a small spot in the frame is considered, at the current focus point. Spot metring isolates this small area from the rest of the frame. This small spot area will be made to be middle gray, and all the rest of the frame is ignored - the rest comes out however it does. 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 may or may not be correct. Hopefully we choose a spot which ought be be middle greay. Yes, spot metering can isolate a face from the influence of a black or a white background, but should that face be middle gray? Compensation may be needed either way.

Center Weighted metering - The entire frame is used, 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 gradually tapers. Usually the center area is weighted about 75% (i.e., center counts as 3x more important than the outside edges.)

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

These tests used Matrix metering (because many people do), but I normally use Center metering, to emphasize the central subject more. Which is really important for portraits, but less important for landscapes where there often is no central subject. I can almost visualize and understand Center metering, but my own opinion is to shun the automation of Matrix metering and TTL BL flash, simply because I never know what the dumb computer is going to do. Possibly it can help sometimes, but as a personal philosophy, I simply try to avoid those complications. :) But regardless of your method, you will 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.

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. 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 (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.

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 direct light incident upon the subject (independent of the subject.) Cameras are not located at the subject, so they can only use reflected meters.

Reflected meters read the light reflected from the subject. A black dress will read much lower than a white dress (so, we must realize what we are metering.) Incident meters read the actual light source, and are usually close to correct, because incident measures the actual light, instead of the variable reflection from the subject. Meters like the Sekonic L-358 or L-308S offer both metering methods, for continuous light or for flash.

Incident meters tell you how bright the light is. Reflected meters tell you how much of it is reflecting from the subject's colors. The article on next page compares a reflected meter in a camera, with a handheld Sekonic L-308S incident meter. Incident meters are essentially required for manual studio flash.

Three ways to meter

• Metering the light with an incident meter (from the subject's position, because inverse square law applies.) This is independent of subject reflectivity. If we get the direct light itself right, then in the subject, black things should come out black, and white things should come out white. Generally, 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, incident metering is independent of the subject's reflectivity.

• 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 above.) 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.

• We can directly meter the reflection off of 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 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 card 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. This card method becomes more like the incident meter. 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. 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 our open palm as a 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 and hold to lock this reading (locks metering and focus), then raise camera to reframe, and use that meter setting for this subject. Result will be greatly better than ignoring it. But if you also want to maintain the exposure of the bright outside, then you 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 often be disappointed. 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.

A related link, explaining why 18% gray is not the midpoint of the histogram we see, is at Histograms are Gamma Encoded

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."

And here is an old but good Kodak Tech article about using light meters. All still very true and valid and fully applicable. All of this has been well known for many decades. Good info, simply about how things work, and you can believe it.

Continued

Menu of other photo pages here