Now, when photo-cameras are literally typed by the latest electronics many beginning photographers have the impression, that the camera is capable to define illumination of the shot and appear of re-exposure or under-light, there are feelings that somewhere the vendor of the camera deceived us…
And it is partly correct. I will tell in this article how exposure metering works and as it is correct to define the exposure.
There are a lot of articles on this subject, so I will try not to describe well-known, and to introduce something new. If who has questions on elements, you can always ask a question to this subject.
For a start we will decide on terms.

Correct exposure

Exposure in the modern understanding is a combination of sensitivity of a photo-camera matrix (ISO), value of a diaphragm (F) and an exposure (T).
What is the correct exposure? If to speak a simple language, the correct exposure, is that illumination of a picture which you wanted to receive. I avoid standard determination as it misleads me.
Classical determination is that it is desirable for us to inscribe the range of brightness of a picture in the range of brightness which can accept a light-sensitive material, in our case it is a photo-camera matrix.
But your picture just shan’t be inscribed all in a photo-width of the camera matrix and you not always need details in shadows and suites. Everything depends on your creative idea. That is good for the person shooting on digital camera, it isn’t suitable for the world, but not to do photodocumentary frames shooting with the reflex camera and trying to transfer the vision.

Methods of exposure determination by the reflex camera

Normal mode

Light goes through a lens, gets on a mirror, from a mirror it is reflected up, on a pentaprism, and from it the part of the light gets on the exposure sensor, and part to the view-finder. As on the way of rays of light there are a lot of hindrances, measuring accuracy depends on many parameters, plus it is foreseen, but not measured from a finite sensor.
For us in this case in respect of measuring accuracy by this method only the focus screen as it is the single removable element on the way of light rays which go on the sensor and capable strongly to influence measurement matters.
If we use standard focus screens, it is not a problem, we select suitable settings from the menu and the camera itself does the correction. If the screen is non-standard (as, for example, the focus screen with Doden’s wedges for the Canon 5D Mark II), you have to calculate the correction of exposure experimentally and most to enter it.

1 — a lens1
2 — a mirror
3 — a lock
4 — a camera sensor
5 — the focus screen
6 — the collecting view-finder lens
7 — a pentaprism
8 — the view-finder
9 — the sensor of exposure metering

LiveView mode

The light gets directly on a camera matrix through a lens, exposure is determined by the image. The same method is used in all non-reflex cameras.
Plus there is no especially exact sampling of exposure as the camera itself is set up under the finite image. Look as the screen on the camera is gradually clarified or it is darkened when you only included LiveView.
Minus — setup goes with some time delay, as the camera needs some time to process the information removed from a sensor. In case of average illuminating this time delay is imperceptible, and in case of the strong changes of brightness of exposure in case of the fixed diaphragm arise with a small time delay in the AV mode.

29 — the sensor is in the normal mode (with the lowered mirror)
10 — the sensor is in the LiveView mode (with the lifted mirror)

Now, I hope, you understand why exposure in LiveView is defined though more slowly, but is more exact. For the same reason and focus on LiveView is set up more precisely. You set up the image directly on a matrix.

The exposure metering of the reflected and incident light

There are two types of exposure sampling, on the reflected light and on the falling light.
Sampling of the reflected light
On the reflected light the SLR camera uses sampling of exposure. Light is reflected from the shooting subject and gets to a lens. There, according to the scenario described above it reaches the light sensor, the sensor transfers data to the camera, and the camera according to the microprogram calculates correct from her point of exposure.
Sampling of an incident light
The second type of sampling is sampling of an incident light. It is especially useful in difficult conditions of lighting when the camera can’t cope or with separate elements of a subject shooting or with overfall of brightness. Provide that your model is illuminated from the different sides by different light sources, and point-wise. To measure illuminance in these small sections you should rotate a lens, remembering all digits, and then to count a certain average exposure to contain all over-falls of brightness.

But the key problem consists that all subjects have the different reflecting ability, and the camera doesn’t know with what reflecting ability is the object. It is considered to be that the average reflecting ability of subjects in a scene is 18%. And therefore the camera tries to give all your pictures to these 18%. In 80% of cases the camera appears right, because all pictures were made on the basis of the analysis of a huge number of photo-plots.
But these remained stories though meet less in normal life (a landscape), in portrait shooting etc. Each beginning portrait photographer tries to shoot on a black or white background. And here is the problem. The camera tries to tighten a black background to 18% of illuminance and it becomes gray, and it tries to darken a white background to 18% and it is too gray..
Here example. In the foreground at me the photographer’s tool — Xrite ColorChecker (a set of targets for creation of a color profile, I will tell about it in the following articles), on which in the upper part a light there is a gray field, and lower is white, but with black texts.

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We will look as such bright object will be perceived by automatic equipment of the camera measuring the reflected light.

4Exposure by the camera is measured point-wise on center, but got on the black frame. The result — a tree on a background has quite good illuminance, and ColorChecker is overexposed because the camera measured the correct exposure only for the black frame, having tightened its illuminance to average.
The tree was clarified for the company.

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The histogram shows us as everything wonderfully became average-gray (the big equal mountain at center) and on the right it is slightly noticeable that the insignificant part of a frame is overexposed. Such generally it is possible not to note on a tiny screen of the camera. For this reason include the blinking indication of relight in the camera.

Now I will measure illuminance of a gray map of ColorChecker too point-wise. The matter is that at Xrite ColorChecker the gray scale is not 18%, and is much lighter (59%).
Pay attention as the judgment of the camera on the correct exposure changed though the lighting of a scene didn’t exchange.6

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The histogram of brightness shows under-light. Here that small “a grass bundle” on the histogram, which approximately is in the middle — the information on our principal object of shooting — Colorchecker.

We will try the operation of automatic equipment. Whether the camera will be able to guess the correct illuminance in the most automatic modes?
We use evaluation sampling which analyzes all image and Canon is better for portraits and objects with back illumination.8

As you can see, the tree is normal, but our object — Colorchecker, is overexposed.
In this case the portrait would turn out a little more brightly, than it is necessary for the reason that the plot at us is more dark than in general.

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Pay attention as we receive not enough information on our principal object of shooting from the histogram. These are two small cloves on a graphics on the right. The first clove is a gray map, the second clove is white, with relight.
After all the camera doesn’t know what exactly we shoot and assumes that we shoot that occupies the big space of a frame. And the big space is occupied by a tree. Here it will also work on the correct exposure of a tree.

Other automatic mode is the partial sampling. It uses about 8% of a frame on view-finder center for calculation. It is recommended if the background is much brighter than object. It not our case, but after all we will try.

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Here the information on a tree occupies the left half of a frame, and about our object of shooting — some cloves are closer to the right edge. Nevertheless from the histogram it is visible that despite a tree (in our case it is the correct exposure, it is visible), ColorChecker is correctly exposed.

Now we put the real 18% a gray map and we measure on it.
In general the exposure is correct.
I.e. as it was required to confirm — grey plots are perceived by the camera and exposure in general is measured correctly.
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Look how looks the histogram of brightness of a picture. First the histogram doesn’t occupy all range of brightness and some will want to expand it on all range. But where you see white objects?
Tree on brightness from black to the average-gray. A gray map — dark gray.

Think over that our task in most cases is to transfer illuminance of a place as it is, but not to pull out artificially that brightness which is not visible to our eyes.

And how will behave the sampling on the basis of an incident light?

The light meter Sekonic 758D (the model isn’t basic) is F2.8 and 100, an exposure it is 1/125s.13
Sampling of exposure on an incident light in this case was very exact.

Here you can see, that we managed to push it. We saved a maximum information on a tree and even our Colorchecker got to the range of brightness, without enlightening. It is ideal option.
Of course, it has restrictions and the main is that it isn’t always possible to bring the light meter to object of shooting and not always there is enough time. But it is quite justified, as it can help out in many situations. Plus to everything, many light meters are equipped with spot-meters, i.e. measuring instruments of the reflected light. It is also convenient to use them as sampling of the camera, but allows to leave the camera on a mount, aimed at a plot, and to take measurements on specially intended instrument (conveniently during the shooting a landscape).

In case of exposure correction it can be entered on a constant basis into the light meter. Also it can be calibrated on other reflecting ability (by default 12.5%).
The modern light meters allow to remember the last measurements and clicking on one button gives the value of exposure in case of which you will have the maximum from the measured range of brightness.
It is also possible to build profiles of the camera and to skid them in the modern light meter, like Sekonic thanks to what you will see directly, whether the range of brightness gets into dynamic range of a matrix of your camera.

And if all of you decided to use camera with exposure metering, I recommend to remember the useful button of sampling fixing of exposure.

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We will provide a situation, you have a bright sky and the dark earth. For alignment of illuminance you have no adaptations (filters). Forget about a breketing too. You want to loose a minimum of picture details. You aim a lens at the sky, you press the descent button to a half. Thus the camera will measure exposure. The sky will be great, and the earth will go to darkness. Retaining the button of descent clicked to a half, you press this button with an asterisk (it is successfully located). Sampling of exposure is fixed. Now you can release the button of descent and quietly set up frame composition.

Why we measured exposure on the sky? The matter is that picture details in case of overexposure of a picture and underexpose are lost with a different speed. In case of relight they are lost much quicker. Therefore it is always better to under-light — then you will be able to pull out more details from shadows and you will try to return details from the overexposed area.

A little bit about the correct exposure and the histogram of brightness

About the histogram I at first didn’t want to tell everything, seems to me, it is already known how to use it, but the subject seems insufficiently enveloped without mentioning of this method, including its pluses and minuses.
Histogram pros generally belong to average plots (an equal hill in the middle of a scale). For example, photographing in cloudy weather can be such a plot. But once you appear in twilight or on a bright sun with brilliant subjects, it begins…
The histogram walks that to the left, to the right and doesn’t give any information on the correct exposure. Here already automatic equipment of the camera won’t help and you should use also the intelligence. Look for grey subjects which reflecting ability can be approximately same as at 18% of a gray map. It can be both gray asphalt and a gray wall of the house. It is good to have a gray map, but it is inconvenient as it is easily rumpled. Instead of a gray map you can take a piece of a gray studio background, it isn’t a pity and it develops somehow. After measurement of exposure I recommend to record values with the button described above and to use them, you won’t transfer in other conditions of lighting yet. We will allow some plus or minus in illuminance which is drawn out in the RAW converter.
If the histogram has peaks, it means that in these values of brightness there are a lot of information (on the area of a frame).