How to Understand Your Camera’s Light Meter and Get the Exposure You Want

By: Simon Ringsmuth

Regardless of how you shoot, and whichever shooting mode you prefer to use, there is one item that remains constant – the light meter. Somehow, either you or your camera has to know how much light in on your scene in order to determine the optimal combination of aperture size, shutter speed, and ISO sensitivity to get the photo you want. This tool, which may not seem all that relevant to new photographers, is called a light meter.
Understanding what your camera’s light meter does and how it works is critical to advancing your skills and helping you get the shots you really want. Hopefully, this article will help you get a grip on it.

An analogy to help you understand the light meter

Before I get into a discussion about how the light meter works, think about the last time you cooked some meat on the grill. Whether it was a steak, some pork chops, or even just a couple hamburgers – you likely had a vision in your mind of what the finished product would be.
For backyard chefs like me who aren’t very good at this sort of thing, we have to use a meat thermometer to make sure our food is properly cooked. There’s always the question of where to put the thermometer to check and see if the meat is done. Or, in photography terms, check to see if the meat is properly exposed. You can touch it to the surface, poke it through to the middle, or insert the thermometer at various points around your dinner in order to get a good overall reading.
Each method would work for a different scenario, but it all depends on what you are cooking and how you want the finished food to turn out.

Your camera’s light meter is like measuring the temperature using a meat thermometer. Placement 

is crucial for an accurate reading.

How the camera light meter works

When you point your camera at a scene you also need a way of measuring the incoming light so you know how much of it there is and what settings you (or your camera) need to control in order to get the shot you want. It’s just like measuring the temperature of your food with a thermometer to make sure it’s done properly.
Most cameras today use a process called TTL Metering, which stands for through-the-lens. It means that your camera examines the light coming in through the lens and evaluates the brightness of the scene. Then you, or your camera, can adjust the settings in order to make sure your photo is exposed how you want. You may not ever notice the light meter at work or even see that it’s there at all unless you shoot in Manual Mode. But trust me, it’s constantly monitoring the light whether you know it’s working or not.

View the metering scale in Manual Mode

To see the light meter doing its thing, put your camera in Manual Mode and look for a series of dots or vertical lines at the bottom of your camera’s viewfinder.

In Manual Mode, 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.

The number scale at the bottom of the image above is an example of a camera’s light meter, and the tiny little triangle shows whether the picture is properly exposed or not. In this case, the triangle is at 0, which means the image is neither under or overexposed, but changing the aperture, shutter speed or ISO would make the triangle move up or down the line accordingly and result in a picture that is either a little too bright or a little too dark.

What part of the scene is the camera measuring the light from?

While that is all well and good, it’s only part of the story because it doesn’t explain how your light meter actually functions. Is it looking at all the incoming light or just some of it? Where in the frame is it looking as it measures the light? Understanding the answers to these questions is the key to unlocking the power of your camera’s light meter, and it all has to do with what’s known as metering modes.

Measuring the Light

Most cameras today have a few basic ways of measuring the incoming light:

1. Matrix or Evaluative Metering – the camera looks at the light in the entire scene and averages it, (Nikon puts a bigger emphasis on the area where your lens is focused as well). Nikon calls this Matrix Metering, Canon calls it Evaluative.
2. Center-Weighted Average Metering – looks at the light of the entire scene and averages it, but with emphasis on the center of the frame. Nikon and Canon both call this Center-Weighted Average Metering.
3. Partial Metering – this 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 have one similar.
4. Spot Metering – measures the light only in a small area around the central autofocus point (about 1.5-3% of the frame). Nikon and Canon both call this Spot Metering.

Other camera manufacturers have different names for these modes, but suffice it to say the way in which your camera measures incoming light can have a huge impact on whether your photo is properly exposed. As an example, here are three shots that were 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.

Reflective versus incident 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 as opposed to a handheld light meter.

Reflective metering

The former, (the type of metering used in DSLRs), works by measuring the amount of light that comes through the lens. But the problem with that is that unless you are pointing your camera directly at the light source, the light being measured is actually bouncing off your subject first.
All the colors we see in the world around us get their hues and tonal values by absorbing every color of light except for what is bounced off of them. As many of us learned in grade school, light is made up of a spectrum of colors including red, orange, yellow, green, blue, indigo, and violet. A green tree leaf absorbs every color of light except for green. A red car absorbs every color except for red, and so on.

When your camera measures incoming light, it’s looking at the amount of light being 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 him and sent to the camera. However if the child changes clothes things can change a great deal.

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. The camera will think it needs more exposure to compensate for what it thinks is less light on the scene, and the overall image will be overexposed as a result.
Here’s a real-world example of how this works:

Nikon D7100, 200mm, f/2.8, 1/8000th of a second.

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 in an effort to make sure the shirt was properly exposed. Unfortunately, the rest of the scene was underexposed as a result.

Nikon D7100, 200mm, f/2.8, 1/1500th.

This was a few seconds later in the exact same spot, and all I did was have her put on a brown shirt. With much of the light from the sun being absorbed by the dark color of her outfit, my camera created a much brighter exposure by using a slower shutter speed. Not as much light was being captured by the TTL metering system so the camera thought more light was required for a good exposure.

Incident metering

This phenomenon can be particularly troublesome if you are shooting a wedding; grooms often wear dark tuxedoes whereas brides will usually be dressed in dazzling whites, which 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-308S-U, which actually measures the amount of light falling on the subject.

Handheld light meter for incident light metering (light falling on the subject).

In the image above you can see that the meter shows you need an aperture value of f/16, shutter speed of 1/125th of a second, and ISO 100 in order to get a properly exposed scene. These numbers will likely be different from what the camera’s TTL system measures because some light will invariably be absorbed by the subject, which is why an external system like this can be so useful.
Here’s how the diagram from earlier would look if the setup involved an external handheld incident light meter.

You will often see wedding photographers using a light meter such as this in order to get a more accurate reading of how much light is hitting the wedding party during formal photos. This is especially true if they’re using a system of flashes or external speedlights because they need to know how much extra light the scene will require or tolerate.
When shooting a wedding it is quite common for the bride to wear a white dress, which reflects a great deal of light, and the groom to wear a dark tuxedo which absorbs almost all light. This can wreak havoc with a TTL metering system, and an external light meter is a great way to address the problem.

The overall goal here is to understand how the light meter in your camera functions. This, in turn, will help you know how you will need to alter the exposure settings to get the shot you want.
I hope this article has been helpful in explaining how the light meter works, how light is reflected off your subjects, and why your camera may not see a given scene quite like you expect it to. Ultimately 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.
Knowing the difference between the various metering modes and types, and understanding how light is measured as it hits your camera can help you get the shots you want. None of these methods are any better or worse than the other, but each one has its own strengths and weaknesses. The more you know about how all of this works the better equipped you will be to get precisely the photographs you want.

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