Your camera uses a light meter to figure out the correct exposure settings for any scene. Like most “automatic” camera features, you do have some control over how it works. Let’s look at the different metering modes and when to use them.
Your Camera’s Light Meter
Whether you’re shooting in automatic mode, a semi-automatic mode, or full manual, your camera always calculates the “correct” exposure settings, either to use or merely display when it thinks you’re under- or overexposed. It works by measuring the amount and intensity of light reflecting off of objects in the scene.
For the light meter to do its job, it makes one huge assumption: that when you average the total brightness of a scene, it should be around 18% grey. This is how that looks.
18% grey is also called middle grey since, as you can see above, it looks to be about halfway between black and white.
Your camera’s assumption that everything averages out to a sort of dull grey is why it usually underexposes bright scenes or over exposes dark ones. The average value is either darker or lighter than middle grey, but your camera doesn’t know that.
The simplest way to deal with your camera calculating the wrong exposure is to shoot in aperture priority mode and play around with exposure compensation. On the other hand, if you want your camera to make more accurate metering decisions—or understand why it’s off—then you need to know about metering modes.
The Different Metering Modes
There are three main metering modes: Center-weighted average metering; spot and partial metering; and evaluative, pattern, or matrix metering. On modern digital cameras, you can choose between them. The process varies by manufacturer and camera, so look up your manual if you want to switch modes.
In each subsection below, there’s a photo of the same scene shot using my 5D Mark III in aperture priority mode at f/1.8 and ISO 800. I’ve changed the metering mode for each shot and let the camera use whatever shutter speed it calculated would lead to proper exposure. I’ve deliberately gone for a difficult scene for a camera to meter so you can more easily see the difference between how each mode approaches it.
Center-Weighted Average Metering
Center-weighted average metering works on the assumption that the most important part of the image is probably in the center. It measures the whole scene but places extra emphasis on the light values in the middle.
Center-weighted averaging is a bit of a throwback. It hasn’t changed a considerable amount since the first auto-exposure cameras were introduced. There are very few situations where you’d use it over one of the other two modes.
In the image above, my camera has overexposed everything a bit. The white label is roughly in the center of the image horizontally, but not vertically, so the camera is being thrown off a little.
Spot and Partial Metering
Spot and partial metering work the same way. Your camera only measures the intensity of light from a small circle in the center of the scene. The only difference between this mode and center-weighted averaging is how large that circle is.
- In spot mode, Canon cameras measure about 2% of the total image area; Nikon cameras measure about 5%.
- In partial metering mode, Canon cameras measure around 10% of the scene; Nikon cameras don’t typically have a partial metering mode.
Spot and partial metering modes are handy when you’re shooting a dark subject on a bright background or vice versa. Wildlife photographers, in particular, get a lot of use out of them.
In the image above, spot mode has given me a pretty good exposure. The label on the battle is perhaps a touch underexposed, but it’s not blown out. This was probably a situation where spot metering was the best option.
Evaluative, Pattern, or Matrix Metering
Evaluative, pattern, and matrix metering are all different words for the same kind of metering. The generic term is evaluative, but pattern and matrix are Canon and Nikon’s proprietary terms respectively.
Evaluative metering is an improved version of center-weighted average metering. Instead of assuming the center is the most important area in a photo, evaluative metering takes into account things like where you’ve placed the focus point and what else is in focus.
In general, evaluative metering is the best mode to leave your camera in. While the shot above is slightly overexposed, it’s about as good as the spot metered one, just in the opposite direction; it’s a hell of a lot better than the center-weighted average image. It’s only in extreme situations where spot metering or partial metering will serve you better than evaluative metering.
Changing the metering mode on your camera can make it easier to get a good exposure when you’re working in tricky circumstances.