Why cameras get exposure wrong

Exposure Lesson #2: Why Cameras Get Exposure Wrong

Your camera’s built in exposure meter is amazingly advanced and complex, but there are still lots of times when your camera gets exposure wrong.

To understand why it’s important to appreciate how your camera’s exposure system works. Once you do, you will know why it gets exposure wrong, and what to do to put it right.

Exposure isn’t easy

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

The reality is that measuring light and calculating exposure isn’t easy. There are a number of factors that contribute to this. Let’s take a look at them.

1. Your camera’s meter reads reflected light

Your camera’s meter measures the light reflected from the subject, not the amount of ambient light falling on the scene.

Imagine that a black cat and a white cat are sitting next to each other. Take a photo of them together and your camera will probably give a good exposure.

But if you move in close to take a photo of the white cat, then the camera’s recommended exposure settings will change, even though the ambient level is the same.

These photos show how it works.

In the first, the girl is wearing white and the area around her is dark. Your camera would probably handle this okay.

Photo of woman laying in a stream

But move in close and what happens? The girl’s white clothing takes up much more of the frame and starts influencing the camera’s meter. Your camera would probably underexpose this scene.

Photo of woman laying in a stream that demonstrates principles of exposure

Why does the camera’s meter get it wrong?

Your camera’s meter gets exposure wrong because the it expects the tones within the frame to average out to mid-gray (also known as 18% gray).

If this is new to you it may seem hard to believe that the tones in a typical scene average out to mid-gray. But they do. More importantly, all built-in camera meters use this principle, so it’s essential to be familiar with it.

Problems start to arise when you photograph subjects that don’t reflect the average amount of light (such as the white cat in the earlier example). That’s when the camera gets the exposure wrong.

If your subject is white, your camera will underexpose the image by as much as two stops.

Remember, the camera thinks everything is gray. When you photograph a white object, it simply calculates the exposure settings that will make the photo gray.

These photos of a white flower show how it works.

Photos of a white flower showing how cameras get exposure wrong

The photo on the left used the camera’s suggested settings. It is underexposed. The camera’s meter has done its job. It’s given an exposure reading that makes the flower gray. The problem is that the flower isn’t gray, it’s white.

I overexposed the photo on the right by two stops. This is the optimum exposure. The flower is white.

2. The file type matters

There are situations when the optimum exposure for the same scene changes depending on the file type.

With Raw, you can expose to the right to create a Raw file that contains as much detail as possible (I’ll cover the topic of exposing to the right in a future lesson).

With JPEG, you need an exposure that records the scene accurately. There is far less leeway for changing brightness levels in Lightroom, so precise exposure is essential.

3. Light meters don’t take contrast into account

Some scenes have more contrast than others. The camera measures the light reflected from the subject, but it’s not good at assessing contrast and making decisions based on that.

A good example of this is landscape photography, where the sky is often much brighter than the foreground.

There are a number of solutions for this problem, including using graduated neutral density filters or exposure blending techniques. Both of these require action by the photographer.

In other words, it’s up to you to recognize that you’re dealing with a high contrast situation and to find the most appropriate solution.

4. The camera isn’t creative

Imagine that you are taking a photo of somebody who is backlit by the setting sun. Do you want to expose for the sky and create a silhouette? Or do you want to increase exposure so that you can see the person but the sky is burned out? The camera doesn’t make creative decisions like these. Only the photographer can do that.

These two backlit photos are a good example of this principle.

In the first, I exposed for the sky, which turned the statue and cactus in the foreground into silhouettes.

Photo of a silhouetted cactus and statue showing how exposure works

In the second, the cathedral and statue are also backlit. The camera’s suggested exposure settings would have resulted in another silhouette. I choose to override them to make this photo.

Photo of a cathedral showing how exposure works

Metering modes and exposure

Your camera has several metering modes. You might think that selecting the most advanced metering mode would guarantee more correct exposures. And perhaps it does. I’ve never tested one against the other to see which works more effectively.

But the point is that whichever metering mode you select the camera is still measuring reflected light. It doesn’t know what to do with a high contrast scene, and it can’t make creative decisions.

Only you, the photographer, can do that.

Histograms and exposure

How do you know if you your camera has under- or overexposed the photo? You can’t rely on the camera’s LCD screen. If you’re looking at the screen in bright light then every photo will seem dark. If you’re looking at it in low light your photos will look brighter than they really are.

The only way to know for sure is to look at the histogram.

If you’re photographing a light toned subject, then the majority of the photo’s tones should be in the right hand side of the histogram.

Here are the histograms for the two photos of white flowers shown above. They are the histograms from Photoshop. They are not exactly the same as what you would see in the camera, but they are close.

Photos of white flowers with histograms

The left histogram belongs to the underexposed photo. The majority of the tones are in the center and the left side of the graph. The right side, which shows light tones, is empty.

The right histogram belongs to the correct exposed photo. Most of the tones are on the right side, where they should be.

I’ll expand on the topic of histograms in a future lesson.

If the subject has a lot of dark tones the camera may overexpose the photo. The image below shows the sort of scene that may give your camera’s meter trouble. The camera’s meter gives a suggested exposure reading that turns the black fence and clothing gray.

Portrait of Asian woman leaning against black fence

This is not a problem as long as you use Raw and don’t overexpose any highlights. You can make the photo darker in Lightroom if you need to. The advice you often read about cameras overexposing dark subjects is a legacy of the days when many photographers used slide film, which doesn’t tolerate much under- or over-exposure.

Why exposure matters

You may ask how much errors in exposure matter, if you can correct them in Lightroom. The answer is that exposure is very important. If you use JPEG it has to be spot on.

You have more leeway with Raw. But, that can become a trap and shouldn’t be used as an excuse to be imprecise with exposure.

If you underexpose a photo, and make it brighter in Lightroom, you also increase the amount of noise in the photo and lose shadow.

The photos below tell the story. Both are taken from the center of a photo of a white flower. I exposed the first correctly (at ISO 1600). The second is underexposed then brightened in Lightroom. There is much more noise in the second image.

Photo of correctly exposed white flower

Photo of under-exposed white flower

It should be noted that some modern camera sensors handle underexposure very well. They are known as ISO invariant sensors. I’ll write more about this interesting topic another day.

This is the second in a series of lessons about exposure for digital cameras. I’ll add links to new lessons below as I publish them.

Exposure lessons

Exposure Lesson #1: How to Choose an Exposure Mode

Exposure Lesson #3: Does the Metering Mode Matter?

Mastering Exposure ebook coverMastering Exposure

Have you ever wondered why your digital camera has so many exposure modes, and what each one does? Or why it’s so easy to under- or overexpose your photos even with the latest cameras and most advanced evaluative or matrix metering modes? Learn the answers to these questions and more in my newest ebook Mastering Exposure.

 

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About Andrew S. Gibson

Andrew S. Gibson is a writer, publisher, traveler and photographer based in the UK. He started writing about photography while traveling in Bolivia, and has been published in many prestigious photography magazines including EOS magazine, where he worked as a Writer and Technical Editor for two years. He currently writes for The Creative Photographer, Digital Photography School and Craft & Vision. He is inspired by meeting new people, seeing new places and having new experiences.

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