How Does the Camera Meter Work Exactly?

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Jo Plumridge
Jo Plumridge

When you point your camera at a subject, it needs a way of measuring the light. Whatever shooting mode you’re using, the camera (or you, if you’re shooting in manual mode) needs to know how much light there is to correctly set the Exposure Triangle (ISO, shutter speed, and aperture). This tool is called the light meter, and understanding how it works is crucial to your ability to get perfectly exposed images. Most modern cameras use TTL metering, which stands for ‘through the lens,’ and this means that your camera looks at the light coming through your lens and evaluates the brightness of the scene.

In this guide, I’m going to explain exactly how the camera meter works so that you can feel confident shooting in manual mode at all times. Here’s what we’ll cover:

  • How does your camera measure light?
  • Reflected versus incident light
  • Understanding metering modes
  • How to view and understand the metering scale (exposure meter)

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How Does Camera Measure Light

Your in-camera metering is extremely clever; however, it can be challenged by scenes such as this snowy vista! Photograph by Pixabay

In-camera metering measures reflected light, which is the light reflecting from your subject (more on that in a bit). It’s also standardized so that it sees your subject and focal point as 18% gray (which is the midpoint between pure white and pure black) and balances the surrounding scene accordingly. Whilst this usually works very well, there are occasions when the in-camera metering can get things wrong or needs a little help. So, understanding your metering options gives you more opportunity to get your exposures right in-camera.

Key Lesson: To help develop your understanding of what 18% gray actually means, it’s worth reading up on and understanding The Zone System. The Zone System was invented by Ansel Adams, who was undoubtedly the master of black and white landscape photography. Adams developed The Zone System, which divides the photo into eleven zones: nine shades of gray along with pure white and pure black. 18% gray is in the exact middle of these zones. Of course, the system is more complicated than this, but it helps to have a brief overview.

Reflected Versus Incident Light

As I’ve already mentioned briefly, in-camera metering measures reflected light – the light reflected from a source. But there is also incident light, which can be read with hand-held light meters. I think it’s important to understand both types of light for a full understanding of your camera meter.

Reflected Light

The camera measures the reflected light from around your scene to provide an average exposure (which can be controlled further by different metering modes, which we’ll discuss later). By looking at the metering scale (exposure meter), we can then ascertain whether the image is correctly exposed. ISO, shutter speed and aperture can all be adjusted to achieve the correct exposure. But there are issues with this method. Your camera is taking a reading of the whole scene, including shadow and highlight areas so that your focus point (your subject) is represented at 18% gray, as already discussed.

I think it’s important to understand both types of light for a full understanding of your camera meter.

Measuring reflected light works very well until your camera encounters a scene that isn’t ‘average.’ White clothes on your subject need particular care and attention. Photograph by Damir Spanic

This works well in many situations but becomes a problem when your subject is not ‘average’ (e.g. they’re wearing a white or black dress that is towards the extreme ends of The Zone System and therefore nowhere near 18% gray!). These situations can cause a lot of issues for your camera’s metering system and lead to under-or overexposure. Why does this happen? It’s because the light being measured is actually bouncing off your subject and it’s not the light actually hitting your subject. So, the colors of your subject (or what they’re wearing) can have a huge effect on exposure. It’s easily fixed by manually dialing in a little positive or negative exposure compensation, but it’s important to understand why it’s happening.

Incident Light

Hand-held light meters measure incidental light and are commonly used in studio situations. Photograph by Jacob Mejicanos

Hand-held light meters give the ability to read incidental light, which is the amount of light falling on a subject. This undoubtedly gives the most accurate readings and is why many pro photographers will still use a light meter. Hand-held meters work by using ISO, shutter speed, and aperture. The ISO is always set to the one chosen on your camera. You can then set either your shutter speed or aperture and use the light meter to get the correct setting for the one you’ve not set. Because you are measuring the light before it falls on the subject, you can achieve consistent results regardless of what your subject is wearing.

Understanding Metering Modes​

Modern cameras come with a range of metering modes, which give you the ability to expose a greater range of subject lighting and reflective combinations more accurately. The metering options work by assigning a weighting to different areas of the image, thus helping to calculate a more accurate exposure. Let’s look at the different types of metering.

Evaluative/Matrix Metering

This is the default metering mode on nearly all digital cameras and is usually the only option available if you’re shooting in auto mode. It works by measuring light across the whole frame and then averaging out an exposure (although it will still bias the reading slightly towards the area around your focus point). Evaluative metering will consider the brightness of the general scene, front, and rear lighting conditions, and even the orientation of your camera to reach its final suggested exposure.

Recommended Reading: Want a step-by-step guide on photography skills with your DSLR? Grab a copy of our best-selling guide, the DSLR Crash Course.

Center-Weighted Metering

Center-weighted metering allows you to direct your camera’s metering bias towards the center of your shot and is ideal for portraits (or quirky portraits!). Photograph by Kyaw Tun

Center-weighted metering is actually the mode that will give the most predictable results – measuring light across around 80% of the scene, with a strong bias towards the center of the viewfinder. You should be aware that it doesn’t take focus into account, using the same averaging pattern for each shot. It’s particularly useful for shooting backlit subjects, as you can make sure that your camera meters for your subject rather than the strong light. I also find it very useful when shooting portraits outside, as it allows you to make sure your metering is biased towards your subject.

Spot Metering

Spot metering allows you to meter for a small circular area in the center of your image, giving very precise results. Photograph by Ali Kazal

This mode measures the light over an extremely small circular area in the center 5% of the viewfinder, allowing for a lot of control. It can, however, be hard to master and definitely rewards a little practice! It’s particularly useful when there’s a relatively small object in your scene that needs to be perfectly exposed, or that you know will be the closest match to 18% gray. (You can fool the camera by placing the spot metering circle over your subject, focusing, and then recomposing your shot with the shutter still half-pressed). Some advanced DSLRs might also give you the option to take several spot readings from
around one scene, which is then used to take an average reading of the whole shot.

Partial Metering

Partial metering is generally found on Canon cameras. Nikon cameras do not have anything similar. It basically meters for a slightly larger area than spot metering of around 15%. Again, it’s commonly used with backlit portraits and outdoor portraiture.

How to View and Understand the Metering Scale (Exposure Meter)​

Key Lesson: When you’re shooting in manual mode, you need to know if your image will be correctly exposed. The camera will display this on your metering scale (also known as the exposure meter). This scale is usually displayed as a number scale going from -3 through 0 to +3. There will also be a small triangle that sits above the numbers. When the triangle is on 0, your image will be correctly exposed. If it was on -1, your image would be one-stop underexposed, and if it was on +1, it would be one-stop overexposed (and so on). In between the numbers you will find small dots that represent 1/3rd of a stop. The ideal is obviously to have the triangle settle on 0 for perfectly exposed images, but you’ll probably find that you can get away with a third of a stop to either side and still have a well-exposed image.

Conclusion

I really feel that understanding how the meter in your camera operates is key to improving imagery. A lot of people don’t bother to learn about metering modes and leave their camera on evaluative/matrix metering at all times. Just by experimenting with the different metering modes, you can open up a world of possibilities and options with what you’re shooting. And, of course, if you start to see how different types of metering affects your images, you may even consider adding a hand-held light meter into the equation for complete control.

Just by experimenting with the different metering modes, you can open up a world of possibilities and options with what you’re shooting.

What’s important to remember is that there’s no absolute right or wrong way to meter an image. Any of the methods I’ve discussed here will work as long as you consider what you’re shooting and the results that you’re hoping to achieve. As with all photography, the more you know about how the camera meter works, the better equipped you’ll be to get the images you want. 

Self-Check Quiz:

  1. Who invented The Zone System?
  2. Does in-camera metering measure reflected or incident light?
  3. What is reflected light?
  4. Name the three major types of metering modes commonly found on digital cameras.
  5. What percentage of the center of the viewfinder does Spot metering measure?
  6. What do the + and – numbers on the metering scale represent?

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