Why Your Camera’s Meter gets Exposure Wrong

This is the second in a series of four articles about exposure by Andrew S Gibson – author of Understanding Exposure: Perfect Exposure on your EOS camera. You can read the first lesson, which explored the reasons for using program, aperture priority and shutter priority modes, here.

It doesn’t matter how advanced your camera’s meter is, or which metering mode you use, all camera meters work the same way – by measuring reflected light. If you understand this, you will see why your camera may get the exposure wrong, and how to put it right when it does.

If exposure were easy, your camera would have just one exposure mode, and it would get the exposure correct 95% or more of the time.

The truth is that arriving at the optimum exposure isn’t always simple. If you rely on your camera’s meter, it may get the exposure wrong, leaving you with under or overexposed images.

So, why is this? It’s down to the way your camera’s meter works. It measures the light reflected from the subject, and then calculates what it thinks the best exposure is accordingly.

While this is the best method that camera manufacturers have come up with so far, it’s far from perfect.

Imagine that you have a black cat and a white cat sitting next to each other. The ambient light level, and the optimum exposure, is the same regardless of which cat you take a photo of.

But if you move in close and take a photo of the white cat, your camera’s meter will give you a very different suggested exposure setting than if you had chosen the black cat. That’s because the fur of the white cat reflects more light.

Why does the camera’s meter get it wrong? The reason is that the camera expects all the tones within the scene that it is metering to average out to a mid-grey tone (also known as 18% grey).

If you’ve never come across this concept before, it may seem a little hard to believe that the tones in a typical scene average out to mid-grey. However, apparently they do, and as it’s the basis on which all built-in camera meters are based, it’s an important one to be familiar with.

In practice, you may often find yourself photographing subjects that aren’t typical. That’s when the camera gets the exposure wrong.

If your subject has a lot of light tones, your camera will underexpose the image. The camera’s meter gives an exposure reading that renders the light tones as grey, and this results in underexposure by up to two stops.

Take a look at these two photos of a white flower. The one on the left was taken at the camera’s suggested settings, and it is underexposed. The flower is grey, not white. The camera’s meter is doing its job – it’s given an exposure reading that records the entire subject as grey. The problem is that the subject isn’t grey – it’s white.

For the second photo I increased the exposure by two stops. This is the optimum exposure, and it has rendered the flower as white.

How do you tell if your camera has underexposed the image? You can’t rely on the camera’s LCD screen, as the appearance of the image depends on the brightness settings and ambient light levels.

The best way is to look at the histogram (there’s an excellent article explaining how to use the histogram here). If you are photographing a light toned subject, then the majority of the tones should be in the right hand side of the histogram.

I opened the flower photos in Lightroom to look at the histograms (above).

The histogram on the left is from the underexposed image. You can see that the majority of the tones are in the centre and the left of the graph. It shows that the photo is comprised mainly of mid-tones and dark tones.

The second histogram belongs to the correctly exposed photos. Most of the tones are on the right hand side of the histogram, which is where they should be.

If your camera has a lot of dark tones, it will overexpose the image. I don’t take photos with a lot of dark tones very often, but the one above shows the sort of scene that will give your camera’s meter trouble. The camera’s meter will give a suggested exposure reading that renders the dark tones mid-grey, and overexpose the image.

Having said that, you will be fine if the camera overexposes a dark image, as long as no highlight detail is clipped. It is easy to darken the image in post-processing without losing detail.

The advice you often read about cameras overexposing dark subjects is a legacy of the days when many serious photographers used slide film, which doesn’t tolerate over or underexposure.

You may be wondering if exposure errors matter, as you can correct them in post-processing. The answer is that exposure is very important, and that’s regardless of whether you use JPEG or Raw.

If you underexpose the image, and brighten it in post-processing, you increase the amount of noise in the image. You also lose shadow detail.

Take a look at the above samples. Both are taken from the centre of an image of a flower. Both were taken at ISO 1600. The first was correctly exposed, and the other underexposed then brightened in post processing. There is much more noise in the second image than the first.

In the next lesson I’m going to explore the difference between your camera’s exposure modes, and explain how to use exposure compensation.

Andrew S Gibson is a writer and photographer. He’s the Technical Editor of EOS magazine and writes for Craft & Vision. The techniques in this article are explored in more detail in his ebook Understanding Exposure: Perfect Exposure on your EOS camera.