What is Middle Grey?
Middle grey is the middle shade between white and black. Middle grey is also referred to as 18% grey. 18% represents the amount of light that middle grey reflects. Your camera considers an average of 18% reflectance as correct exposure.
Why 18% Grey and Not 50% Grey?
You are probably asking yourself why do we refer to it as 18% grey rather than 50%? The simple answer is that our visual perception of brightness is not linear.
Though white objects reflect 100% of light and black objects reflect 0% of light, objects that are ¼ as bright will not reflect 25% of light and objects that are ½ as will not reflect 50% light.
This means we cannot use the arithmetic mean when calculating the average brightness level; instead, a geometric mean is used. To calculate a geometric mean you multiply two numbers and then find the square root of the result.
This is why the middle grey is referred to as 18% grey. At the midpoint between white and black that shade of grey only reflects 18% of light.
How Light is Measured?
There are two types of light that you will find in your scene: incident and reflected. Incident light is light that falls directly on an object. Reflected light, on the other hand, is the amount of incident light that reflects off the surface of an object.
Cameras have a built-in light meter designed to read the exposure of a scene. It does this by measuring the amount of reflected light that reached the camera’s sensor.
The Light Meter
To understand what metering for 18% grey means, you first need to understand how the light-meter in your camera works.
A light meter is a built-in tool inside your camera that reads the exposure of a scene. Unlike you and me, camera light meters can’t see color; it sees the world on a greyscale. That is, when measuring exposure, it converts the colors in a scene into tones of grey.
Light meters measure exposure by reading the amount of light that is reflected by objects within the scene.
The brightness level of each color determines its shade of grey. The brightest colors reflect a range of tones from white to light greys; the darkest colors reflect tones between black and dark greys. The colors between bright and dark colors reflect mid-tone grey.
After the light meter converts colors into tones of grey, it measures the average amount of reflected light for the entire scene. Scenes reflecting more than 18% light are read as overexposed, while scenes reflecting less than 18% light are read as underexposed. Scenes that are reflecting exactly 18% light are read as correctly exposed.
The light meter will assign exposure values ranging from negative three to positive three. A negative value represents an underexposed scene, while a positive value represents an overexposed scene. The zero value represents the middle grey or 18% grey.
Each metering mode will weigh areas within your photo differently. The exact calculation of your image exposure differs based on the metering mode in your camera. For example, when calculating exposure, a matrix mode will weigh your scenes evenly. A center-weighted mode, on the other hand, will place a higher weight on the center of the image.
Faults with 18% Grey
It is important to note that the light meter has some limitations.
Your camera meter turns colors into tones of grey. But our world is not grey; it is full of colors. Thus, there will be instances when the camera will provide an incorrect interpretation of reflectance.
If you take a photo of a black wall, for example, your camera meter is likely to interpret your image as underexposed, even when its adequately exposed. The misinterpretation is because a black wall is twice as dark as grey. But your camera’s light meter doesn’t intrinsically know that. So, it will read your image’s overall reflectance as less than 18% and tell you that your image is underexposed.
Similarly, if you take a photo of a white wall, your camera meter will interpret your image as overexposed, even if its properly exposed. This is because a white wall is twice as bright as grey; it will have an overall reflectance higher than 18%.
When determining exposure using the light meter, consider the elements within your scene. In general, using 18% grey as a baseline works, but is not suited for every situation.
18% grey or middle grey is a measurement of the amount of light objects reflect. Your camera meter reads objects that reflect exactly 18% light as correctly exposed.
Your camera meter measures exposure by first turning colors into tones of grey. Thus, it is important to remember that there will be instances when the camera will provide an incorrect interpretation of reflectance.