Metering Light 102 – The Medium Grey

A Bit More on..., Advanced, Concept, Video

obvious fact

To start metering light you need a light meter!

There are two kinds of light meters. They either measure incident light, i.e. the amount of illumination that actually falls onto the subject or reflected light, i.e. the amount of light reflected into the camera from the subject.

You can recognise incident light meters by the translucent half-dome at the top. Underneath the half-dome is a light receptor. By its very nature, the half-dome functions as a tool that averages the light coming from all directions. When you use an incident light meter, you need to go to your subject and hold the light meter towards your camera. (There are more advanced uses for these meters: you can also measure the relative intensity of light hitting your subject from various directions, as well as getting a simple and fast reading for the amount of ambient light.)

Light meters measuring reflected light are more common these days. Every single camera with a built-in light meter, be it an 40-year-old film camera or a digital one has one of these! You measure the light from the position that you’re taking the picture from and the meter collects the light that is reflected from the subject into its sensor. These light meters can be highly advanced and they might measure the amount of reflected light in your whole viewfinder or just parts of it.

DSLRs have numerous settings the will influence how much of the image gets measured. One very special setting is the spot metering method and the dedicated spotmeters above are special, extremely precise instruments that do exactly what your camera does when you select this metering method. They look at a tiny portion of your image, usually a 1º circle and measure how much light is reflected from that single spot.

The Medium Grey

The medium grey is the only strange concept that you need to grasp to become a master of exposure. And to do that you need to understand a simple fact:

the camera (i.e. the light meter) doesn’t ‘KNOW’ what you’re looking at

Only you know that. The light meter only measures the amount of light but it cannot “KNOW” and “TELL” how bright your subject should be. Imagine that you’re shooting the nighttime sky with nothing but lots of tiny stars. Only you, the human behind the camera knows that the sky should be very, very dark. Now picture shooting a bride in a white wedding dress against a white wall. Only you know that most of the image should be bright.

Okay, so how does the light meter determine what your settings should be?

Simple. All light meters are calibrated so that they gather light and find settings so that the average brightness of your scene would be medium grey.

Medium grey is a standard. It’s a surface that reflects 18% of the light that falls onto it. On the above exposure targets it’s in the middle.

Us, humans see the medium grey as halfway between black and white. In other words, the medium grey is the average between black and white.

What does your camera's light meter actually do?

When you’re using your camera’s built-in light meter, you probably know that you should ‘get the needle/bar in the middle’. (Most of the time.)

The ‘middle’ is the medium grey. In Manual exposure mode, you find your all your settings. In any other semi-automatic or automatic mode (A, S, P or Av, Tv, P) your camera will do it for you.

But keep in mind: no matter what method you’re using, ‘the needle in the middle’ is a statement. And you are making that statement. What you’re saying is: this scene, in front of my lens is a scene which is on average medium grey. So if I find settings so that the light meter would point to the middle it means that it will be reproduced correctly.


Metering light is both a technical and a creative process

The technical side of metering light is about getting your exposure right, making sure that your scene is not overexposed or underexposed.

The creative side is about making informed decisions and understanding the exposure is a creative tool that allows you make your images brighter or darker, to change the mood of your shots.

And this is what it all boils down to. As with any tool, practise will allow you to get your exposure right both technically and creatively. To get it right technically, you need to understand what dynamic range and the medium grey are. To control exposure creatively you need to understand that the light meter is just a tool telling you what you should do, if you wanted to get your image medium grey on average. It’s not telling you, however, that you MUST get it medium grey.

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