These days, every camera has an in-built light meter – but what actually is it, and how does it work?
In this article, I share everything you need to understand your camera’s light meter. And by the time you’re done, you’ll be able to select different metering modes, plus you’ll know how to meter your camera for great results.
Let’s get started.
What is a camera light meter?
A light meter measures the amount of light reflecting off a scene, then indicates the proper combination of settings (shutter speed, aperture, and ISO) for a well-exposed image.
In other words, light meters are all about getting a nice, detailed, balanced exposure, where the highlights aren’t too bright and the shadows aren’t too dark.
Your camera almost certainly includes a light meter, though some photographers also purchase handheld meters. The latter are just like camera light meters, except they offer increased control and greater flexibility.
Now, if you’re familiar with the basics of exposure, you know that a well-exposed image is the product of the aperture, shutter speed, and ISO working together. You might use a wide aperture but a fast shutter speed, a narrow aperture but a slow shutter speed, or a high ISO but a narrow aperture – and you’ll end up with the same exposure.
My point is that a camera meter won’t tell you the precise aperture, shutter speed, and ISO you need for a well-exposed shot. Instead, it will let you, as the photographer, balance your settings, and it will simply tell you where that balance point exists. Make sense?
How a camera light meter works
As I explained above, a light meter measures the light. These days, most cameras use a process called TTL metering (i.e., through the lens metering). Your camera examines the light coming in through the lens, then uses it to evaluate the brightness of the scene.
Next, the meter indicates the optimal exposure settings based on internal calculations. Generally, a light meter’s goal is to make the scene an average middle gray, also known as 18% gray, which is a decent approximation of most scenes.
So if you’re photographing a tree, your camera’s meter will measure the light reflecting off the tree and into your lens. It will calculate a shutter speed, aperture, and ISO combination that will make the scene 18% gray when photographed. And it will communicate (or automatically select) these settings, depending on your camera mode.
If you’re using Auto mode, your camera will quietly do its metering, then pick settings for an optimal exposure. If you’re using Aperture Priority mode, you can pick the ISO, then your camera will pick a shutter speed that produces an 18% gray result. And if you’re using Manual mode, your camera will simply indicate the ideal settings, using the exposure bar at the bottom of the viewfinder:
Set your camera to Manual mode, then look at the bottom of the screen in your viewfinder. Notice the scale with zero in the middle. That is the light meter at work.
Raise the shutter speed, and the little indicator will move to the left; this indicates underexposure. Drop the shutter speed, and the little indicator will move to the right; this indicates overexposure.
In fact, working in Manual mode is a great way to understand how your meter works, and how it’s affected by different camera settings. You can watch the exposure bar change as you point your lens at different scenes, and you can see how, with the adjustment of aperture, shutter speed, or ISO, the exposure changes.
Metering modes: what part of the scene does a light meter evaluate?
You might be wondering:
Does a light meter measure the entire scene? Or just a sliver of the scene? Where in the frame does the meter look when it measures the light?
Well, that depends on the metering mode your camera uses.
Most cameras today have a few metering modes, and each one measures the light slightly differently. Camera manufacturers have various names for these modes, but the labels used below are the most common:
- Matrix or Evaluative Metering measures the entire scene and creates an intelligent average, with weighting of different scene elements. (Nikon puts a bigger emphasis on the area where your lens is focused, for example). Nikon calls this mode Matrix Metering; Canon calls it Evaluative Metering.
- Center-Weighted Average Metering looks at the light of the entire scene and averages it, but with emphasis on the center of the frame.
- Partial Metering measures the light only in a small portion of the center of the frame (about 8-12% of the scene). This is a Canon metering mode; Nikon does not offer an equivalent option.
- Spot Metering measures the light only in a small area around the central autofocus point (about 1.5-3% of the frame).
The metering mode can have a huge impact on whether your photo is properly exposed. If your camera spot meters off a sliver of bright sky, the entire image will turn out dark – except the sky sliver, which will be well exposed. If your camera spot meters off a dark rock in a forest landscape, the entire image will turn out too bright, except the rock, which will be well exposed.
To illustrate this, here are three shots taken with different metering modes:
Image #1, taken with Matrix (Nikon) or Evaluative (Canon) Metering.
Image #2, taken with Center-Weighted Metering.
Image #3, taken with Spot metering.
Note that the Matrix metering produced an average of the entire scene, including both the figurine and the background. The Center-Weighted Average metering gave an underexposed result, because it metered off the window in the background. And the Spot metering gave an overexposed result, because it metered off a dark area inside the window.
That’s why it’s essential you choose your metering mode carefully. Meter using the wrong algorithm, and you’ll end up exposing for the wrong part of the frame!
Reflective versus incident light metering
There’s another aspect of light metering that comes into play when setting up a shot. It has to do with how TTL metering works compared to a handheld light meter.
Reflective metering
TTL metering works by measuring the amount of light that comes through the lens. But there’s a problem: unless you are pointing your camera directly at the light source, the light is actually bouncing off your subject first.
So when your camera measures incoming light, it’s looking at the amount of light bounced off your subject, not the amount of light actually hitting your subject. This has huge implications and can dramatically affect your exposure. In the illustration above, the subject is wearing clothes that absorb most colors of light except for blue, which means there is still a great deal of light being bounced off the subject and sent to the camera. However, if the child changes clothes, the light meter receives a completely different reading:
In the illustration above, even though the amount of light hitting the boy has not changed, the camera will read the scene much differently because he is now wearing a dark shirt and pants. In the second situation, the camera will think it needs more light to compensate for what it believes to be less light in the scene – when in fact the amount of light hasn’t changed. As a result, the dark-clothed subject will be overexposed.
Here’s a real-world example of how this works:
Nikon D7100 | 200mm | f/2.8 | 1/8000s
In the photo above, so much light was being reflected off the girl’s white shirt that my camera had a hard time metering the scene properly. Much of the sunlight was bouncing off the shirt and coming directly back to my camera, so it responded by using a very fast shutter speed and low ISO value to make sure the shirt was properly exposed. The rest of the scene, however, turned out underexposed.
I then took this shot a few seconds later – after my subject put on a brown shirt. Much of the light was absorbed by the dark color, so my camera created a much brighter exposure:
Nikon D7100 | 200mm | f/2.8 | 1/1500s
To summarize:
Because different subjects reflect different amounts of light, your camera meter is often fooled into underexposing or overexposing an image. In such situations, there is an optimal exposure, but thanks to reflective metering, your camera will consistently fail to find it.
Incident metering
If you are shooting a wedding, reflective light metering can cause serious problems; grooms often wear dark tuxedoes whereas brides will usually be dressed in dazzling whites, and this can really throw off your camera’s TTL metering system. The solution is to use an external handheld light meter, such as the Sekonic L-308X-U, which actually measures the amount of light falling on the subject (known as incident metering).
Handheld light meter for incident light metering (light falling on the subject).
Incident light metering will give you consistently correct exposure results, because it won’t be fooled by subject reflectivity.
Here’s how the diagram from above would look when using an external handheld incident light meter:
You will often see wedding photographers using a handheld light meter – not the camera light meter – in order to get a more accurate light reading.
Camera light meters: final words
Now that you’ve finished this article, you should understand the basics of camera light metering – and you should know how to use a meter to create top-notch exposures.
That said, it’s important to remember that there is no one correct way of metering a scene. Any of the metering modes and methods will work, as long as you know what you are shooting and what type of results you are trying to achieve.
Now over to you:
Which metering mode do you plan to use most often? Do you think you’ll purchase an incident light meter? Share your thoughts in the comments below!
