The lightmeter: what is it and how do I use exposure metering modes?
You must surely have found yourself in a situation where your camera was stubbornly overexposing or underexposing your images.
So often, it suggests the correct exposure for your pictures, why didn’t it do so that day? The answer lies in the mode in which the camera’s flash meter works and how it establishes a good exposure for the shot.
Because of the obvious importance of making a good exposure of the image, cameras generally offer different light metering modes to help you overcome this problem.
Knowing how to read the scene to be photographed, understanding how the flash meter or light meter works, knowing the exposure metering modes and determining which one is most appropriate at each moment can make the difference between an incredible picture and one that is unusable.
Read on to learn how to find the right exposure on all your photos.
The flash meter: what is it and how does it work?
The flash meter is the instrument the camera uses to measure light.
It is a light-sensitive cell capable of measuring the light reflected by the scene, in other words it “sees” the scene in black and white and perceives different tones of brightness depending on the light they reflect.
The light sensor is usually placed behind the lens so that the effect of the lens and any filters or other accessories that may have been installed can be taken into account.
Hence the name TTL (Through The Lens) measurement.
Based on the measurement made by the lightmeter, the camera proposes the right exposure.
And how does it decide that this is the right exposure?
In a very schematic way, we can say that today’s cameras have been “shown” that a standard scene reflects 12% of the incoming light.
Based on this information, our camera’s lightmeter suggests that we take an exposure for any image so that the final picture will have this degree of brightness.
This 12% is an empirically established average value and therefore applies in many situations but not in all.
In fact, the 12% reflectance value has been estimated for outdoor scenes at specific latitudes and times of the year.
As one changes scene, latitude and time of year, if one photographs something other than a landscape, etc., there will be a variation in this value such that it may invalidate it.
What would happen, for example, if the scene to be photographed had an average reflectance of 7%? The scene would be very dark, but the camera would let the light through until the picture had a brightness equivalent to that 12%.
In other words, the photo would be overexposed compared to reality.
Conversely, if you have a scene with a high reflectance of 28%, the flash meter will recommend to quickly close the light input so that the image shows a final reflectance of 12%.
The image would be underexposed.
Therefore, every time the reflectance of the scene to be photographed deviates from this 12%, the exposure meter will give an erroneous proposal.
This happens especially depending on the scene you want to photograph, on low key or high key scenes, for example.
A photo of a white owl on a snowy landscape will not have the same reflectance as a photo of a black horse on a dark volcanic landscape.
How does the lightmeter communicate with us?
We also need to know how the camera informs us of the exposure it considers correct.
On the lower part of the camera’s viewfinder (and very often on other screens), you can find a scale similar to this one :
Camera Exposure MeterThe numerical values refer to EV (Exposure Value) or light stops.
Each time you add a light stop by opening the aperture, which extends the exposure time or increases the ISO, you double the amount of light reaching the sensor.
For our camera, a good exposure (the one where the captured image will show 12% of reflected light) displays a value of 0 EV.
Therefore, in order to expose correctly according to our flash meter, we will have to set the shutter speed, aperture and ISO values so that the slider is set to 0 (as in the image above).
If the black cursor stays on a negative value, it will mean that the photo is underexposed and if it stays on a positive value, it will be overexposed, still according to the light meter.
To find out the value of our flash meter, it is usually sufficient to frame the scene to be photographed, then half press the shutter release button in manual mode.
The measurement will be taken and the cursor will move to indicate whether to increase or decrease the light input.
When shooting in auto mode, the camera directly sets the shutter speed, aperture, and ISO values to make the exposure fall to 0.
Once you have mastered the operation of the flash meter, a short exercise can be made to check and understand its operation.
Take your camera and photograph a white wall, then a black wall (or a black canvas) that occupies the entire frame and follow the recommendation of the lightmeter so that it reads 0.
What results do you get? You can see them in the images below.
As the lightmeter always tries to adjust the brightness of the pictures to 12% reflectance, both images will show a color very close to this color, known as mid gray.
The first picture (black canvas) is overexposed, while the second (white wall) is underexposed.
Overexposed black canvas
Under-exposed white wall
How to read a scene well to prevent variations in the measure?
With all that we have seen so far, we can start to deduce interesting practical applications to display our photos.
In practice, in order to take a picture, one must be able to determine whether it is a high or low reflactance image to anticipate the response of the flash meter.
If you are facing a scene with high reflectance (snowy landscape, backlit sky…) you know that your flash meter will tend to underexpose the scene.
Therefore, you will have to trigger at values higher than 0 EV to correct this.
If you take pictures of a snowy landscape, for example, where the snow reflects about 36% of the light, you will need to shoot with two stops above the exposure your camera offers, if you want the snow to appear white and bright.
Observe below the difference between a photo taken with the value indicated by the light meter and the same photo overexposing by two stops.
Photo of the snow taken with the value indicated by the lightmeter
Photo of the snow taken with the value indicated by the lightmeter
Photo of the snow taken in overexposure of two stops
Photo of the snow taken in overexposure of two stops
On the other hand, a black cat reflects nearly 8% of light.
Thus, when photographing a black cat on a dark background, we will obtain an image with low reflectance, less than the 12% that the flash meter estimates.
Therefore, when the shot is taken, the lightmeter measurement should be corrected by underexposing the photo by 1 or 2 stops from the proposed value to avoid it being overexposed.
What if the scene is predominantly dark or bright but not the object to be photographed?
How can you make sure that your photo stays perfectly exposed the first time?
We know that if we take an average measurement of the scene, the photo will be under-exposed or over-exposed.
You can then overexpose or underexpose it with a stop to correct the excess or lack of light.
With a bit of experience, you can aim right without having to take too many extra measurements.
Another option would be to analyze the scene and take advantage of the fact that cameras currently have different light measurement modes.
So, one will be more suitable than the other depending on what you want to immortalize, and you will choose the most adequate one.
The types of measurements the camera offers us
We will now look at the most common types of measurements that can be found on cameras with some manual trigger capability.
In the options menu, symbols or numbers similar to the ones shown below usually appear.
This exposure metering mode takes into account the brightness of the entire scene, so it aims to achieve an average reflectance of 12%.
If you look at the scene to be photographed and find a relatively even brightness (or reflectance), you can use this mode without concern.
Matrix metering icon
For what has been explained, it will work particularly well in outdoor situations, with a sky that will not be predominant, but also in other situations with a medium (neither low nor high) and homogeneous reflectance.
It will be equally adequate when one wants to balance the light between light and shade, although it will be impossible to obtain information on the whole image if the difference is large.
For backlighting, high key or low key photos, etc.
the result will probably not be what you expected.
In any case, it is a very practical mode that works in many situations.
This is the measurement the camera uses when shooting in automatic.
See below for an example where this mode has found the exposure perfectly.
Because the tones were fairly even throughout the image, the matrix metering had no problem giving the image the correct exposure.
The exposure meter: what is it and how do I use the exposure metering modes? Matrix metering
To take the measurement, this mode takes into account the brightness of the whole scene but gives more weight to the central part (30% for the periphery, 70% for the center, approximately).
It is indicated when the background can distort the measurement.
The most typical example to illustrate the use of this metering mode is the portrait of a face.
We want it to be well exposed, before the background, so we give it more importance during the measurement.
The center-weighted measurement will be very appropriate when you want to make sure you have a well exposed model.
Centre-weighted measurement icon
As an example, a model was photographed against the light of a window in the image below.
Since we wanted the face to be black, for an exterior landscape appearing in the well exposed window, we then opted for the centre-weighted measurement by taking the measurement on the window since the surface it occupies is significant, in relation to the totality of the image.
Triggering with the raster measurement, since the dark area occupies a large area, the contents of the window would have been overexposed and the shadows would have presented some detail, something we did not want on this occasion.
The lightmeter: what is it and how does it work? Centre-weighted measurement
With this measure, only one “point” of the scene is taken into account.
A point is defined as a very small area in relation to the entire image, in the order of 5% of the total area of the scene.
The closer you get to the object, the more accurate the measurement.
The measurement point is the next point chosen for focusing.
Spot metering icon
This is highly recommended if you have the time and want to work in zones.
It is also a good measure to apply a rectification technique that consists of underexposing the brightest point of the scene by one or two stops to generate an image with the maximum information.
With this technique, the final exposure setting is made during digital development.
And spot metering is especially appropriate to ensure that an element of the photo will be perfectly exposed, no matter how small it is on the composition, such as the light from a candle, the white of a wedding dress, etc.
In the example photo below, where a large part of the image is very black, the best option is to take a spot measurement on the narrow band that shows the body of the model if you want to get a good exposure.
This will ensure an adequate value that will properly expose the skin and you can forget about shadows.
This mode is most often used by many expert photographers because it allows maximum control and measurement accuracy.
In addition, once the brightest area of the scene has been captured, it also prevents overexposure of any one element.
But it is also the mode that requires the most experience, time and attention to what is being measured.
So it’s probably not the most recommended mode if you want to take snapshots without a lot of preparation time, as is the case with street photography, for example.
Another typical situation where the light meter can give you problems is when the sky is cloudy or backlit and occupies a large part of the image.
In these cases, the sky is usually much brighter than the earth.
If you choose the raster mode, the flash meter will generally underexpose the picture at the ground level since the sky is much brighter, so the ground remains very dark.
It can also happen that the lightmeter overexposes the sky, which then remains completely white (overexposed).
In this case, you must keep in mind the elements that you want to capture in detail, as can be seen in the images below:
If it is the sky that interests you, you will take the measurement on the sky and you will forget the earth, which will be completely black.
If it is the ground that interests you, you can take the measurement by framing only the ground, then cropping while capturing part of the sky.
Another option is to take the point or center-weighted measurement at ground level, so as to neglect the information from the sky, which will then appear completely white (overexposed).
Finally, if you want to get maximum brightness on the earth but not overexpose the sky, one option is to take a spot measurement of the sky on the brightest part of the frame and then raise the exposure value by two or three stops (the number of stops will vary depending on what the camera’s sensor can handle).
You will then try to get the maximum information about the shadows (the earth) without overexposing the lights (the sky).
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A famous final trick, to take the right measure of light, is to use your hand as a chart of gray.
To do this, place the hand -your hand or that of an assistant- with the same light source as the one that will be used to take the picture.
You will frame it so that it occupies most of the viewfinder and then measure the light.
In general, the palms of the hands have a reflectance of more than 12%, so you will have to raise the proposed exposure by one stop.
This way, you will have a portable gray chart that will help you take a fairly accurate measurement in complicated conditions.
Best Online Classes to learn more about Light Meter
We saw in this post that making the right exposure was not done like that, that it was necessary to take into account the luminosity and its distribution, in particular, in the composition to be photographed.
You will then be able to choose the most adequate metering mode or to correct the value indicated by the flash meter as you wish.
I hope that these explanations will make it easier for you to obtain the desired exposure in the most demanding situations.
As is often the case in photography, the best way to reinforce one’s knowledge and apply it, so we invite you to put it into practice and leave us your comments and experience on the subject, below or on social networks.