Reflected and incident light
Understand reflected and incident light metering
To get a more advanced understanding of how metering works in a camera we need to take a look at reflected and incident light metering. A reflected light meter measures the amount of light being reflected across the scene. This is how the built-in light meter within your DSLR or mirrorless camera works. An incident light meter reading is taken with a handheld light meter and means you are measuring the light falling on the subject. It’s a subtle change but in certain lighting situations it can have a big effect. Reflected light metering reads the light from the entire scene and then uses an averaged reading based on 18% per cent grey. This is also known as mid or neutral grey, and is a standard in photography designed to produce a predictable exposures for most subjects. Reflected light meters assume that the subject is going to reflect 18% of the light that falls on it and calculates the exposure settings from this.
This is fine for a typical scene in daylight, but if you are faced with a high-contrast scene or one with a large area of dark or light areas, you might run into problems. For example, consider photographing someone in a dark suit or outfit. The reflected light meter in the camera sees very little light being reflected and will calculate a higher-than-needed exposure setting. This will overexpose the scene and produce an image with a much lighter suit. Conversely, if the scene is extremely bright, such as a mountain covered in snow, the reflected light meter will see too much light and produce an exposure calculation that will underexpose the scene. This results in grey snow. Both of these situations are caused by the reflected light from the scene, instead of reading the incident light falling on the scene.
Although the light meter in your Nikon reads the reflected light, the algorithms within the Matrix metering system are now so advanced that you can confidently rely on it to produce good exposures in most situations. But even if the light meter produces an over- or underexposed image, all you need to do is apply a small amount of exposure compensation.