Maybe this title sounds a bit strange, but I am pretty sure that it will become clear soon why I chose this heading for the article below. Before we get into anything, let’s get back to basics for a few moments.
As we discussed earlier if we would like to take great shots, one of the most important things is to master the exposure. It means that our image should contain shadows, mid-tones and highlights that have to be balanced for the largest range of tones. When we select the settings for our exposure, actually we give instructions to the camera how to measure the intensity of light. In order to be able to determine the correct values, most of the cameras are equipped with built-in light meters nowadays.
Hereinafter, I would like to demonstrate what camera metering is and the way it works, what the different metering modes are and how to know which one to use.
What is a light meter in photography?
Light meter is a built-in camera device in most of the DSLR’s nowadays, which helps photographers to control the exposure by measuring the amount of light in the scene. However, many years ago photographers had to use handheld light meters to decide the optimal exposure.
There are two different types of light meter exists and each one measures the light in a different way. One is called reflected, while the other one is named incident.
- Incident light metering measures the light that falls on the subject from the source instead of its reflection, and because of that it provides more precise values.
- Reflected light metering means that the light source lightens the object and that light reflected in the direction of the camera through the lens. Most of the DSLR cameras have this kind of TTL (through the lens) light meter. It measures the amount and intensity of light reflecting off of the object in the scene.
How does the light meter work?
If we look through the viewfinder we can find a scale at the bottom part. We can see that the central point is zero and there are positive and negative numbers on the sides, like 1, 2, 3 and -1,-2,-3. And we can also find some small dots between the numbers. (Occasionally we can see small rectangles as an alternative of the numbers.) But what does it all mean?
If we are facing our camera to different things, there should be a moving line or triangle, etc. under the diagram. If the line stays at 0, it means that our image is correctly exposed. Furthermore, if the line goes towards the negative side, it means that the picture is getting darker, and if it goes to the positive side, it’s getting brighter. So, if the line is going towards the negative side, we will underexpose the image, and overexpose if it’s going towards the positive side. Please be aware that some cameras may have + sign on the left and – sign on the right, while the rest is other way round.
Problems with metering
Basically, the camera meter works fine when the whole scene is equally lit. However, most scenes have areas with difficult light with a large range of brightness levels from dark shadows to very bright highlights which gives hard time to the camera to determine the correct values.
18% grey the middle value
The light meter sees our scene in black and white with all the different brightness levels. Middle grey is about halfway between absolute white and absolute black, and we call it 18% grey. The reason why it’s 18% instead of 50%, because that shade of grey reflects 18% of light only. Consequently, we have to say that brightness and reflectivity are non-linear. To make things easier, objects that reflect 18% light are correctly exposed, but anything below 18% is underexposed and anything above 18% will be overexposed. Let’s see an example with some snow in the background:
If we set up the light meter to zero, the image will look grey. As we know the light meter tries to keep the exposure in the middle, so the snow will look grey instead of white. In that case we should overexpose the image by 1-2 stops depending on the conditions.
It works the same with dark scenes, so we need to underexpose our dark image for the optimal exposure.
The different metering modes
There are some different ways how the built-in light meter can measure the light. It means that we can choose from several metering modes for different shooting conditions. Most of the cameras have at least 3 different metering systems. Please note the names of the metering modes are different with each brand! Let’s have a look at the basic metering modes in more detail!
Matrix (Nikon) / Evaluative metering (Canon)
The most complex metering mode is the matrix / evaluative metering, which is the default mode on most of the DSLR’s. This kind of metering works by splitting up the whole image into different segments and reading the light individually from each one. Therefore it gives priority to the selected focus point and marks it more important than other areas, and also considers colours of the scene and distance to object to find the greatest exposure for the image.
We should use this kind of metering at most of the time, as it usually does a good job. I would say only use the others in extreme circumstances. But keep in mind, it works best when the whole scene is equally lit.
At this mode, the camera checks the light of the whole scene and averages it, but gives priority to the tones in the central area of an image.
This kind of metering mode is mainly used for portraits when the subject is in the centre. Also works well when the background is very bright or with backlit images.
This kind of metering system only evaluates the light around the selected focus point and ignores the rest of the scene. Nikon cameras measure about 5% and Canon cameras measure approximately 2% of the image.
Spot metering is very useful for bird photography and for backlit, high contrast objects and silhouettes too.
How to change metering mode?
Changing the metering mode on the camera can be different on each brand, model. Many cameras have a button or a dial with one of the metering mode’s images on it. In addition, we can find more details in the camera’s manual how to switch between modes.
Don’t forget that the light meter is only a guide and it’s not always right, so don’t rely on it! Understanding how metering modes work will definitely improve our photography skills. Over time, after a lot of practice, we will be able to set our exposure independently without checking the light meter. My advice is, change modes, adjust exposure and keep experimenting!