What is good exposure in photography?

I like to read questions and responses in Facebook groups about photography, particularly the groups that are for beginners. Questions are often asked in these groups highlighting things that photography tutorials take for granted or just don’t cover. I’ve just come across a question in one of these groups that I found pretty interesting. The original post asked, “Should I under-expose or over-expose my photographs?” What followed was the downside of many social media groups. Someone replied that they should just get it right and not purposely under or over-expose.

Well, that reply is half correct. You do need to expose a photograph “right”, but they didn’t explain what “right” is. There are a couple of ways that you could define the correct exposure. For those who are starting to experiment with exposing photographs manually, there can only be one answer. On the back of the camera is a light meter. When the light meter is in the middle, that is the correct exposure. Unfortunately, that isn’t actually true. The way the light meter works can be influenced by several things. Firstly, there are often three or four metering modes available in most cameras. These modes determine how the camera measures light. If you get the wrong mode in the wrong situation, the light meter can read that the exposure as correct, but it could be totally wrong.

Even if you choose the correct metering mode, framing an image can cause the light meter to give an incorrect reading. Small amounts of highlights or shadows in the photograph can also cause these elements to be exposed incorrectly.

That’s not to say that the light meter will always be “wrong”. If we have good light, then the meter will usually do a reasonable job. What I mean by good light, for example, would be when we have the scene illuminated by diffused light. This shot, looking across the Princes Street Gardens in Edinburgh, is an excellent example of good light. The gardens and buildings are lit by sunlight, and an interesting sky adds to the scene. I set the light meter to what the camera told me was the correct exposure, and it’s done an OK job.  However, some might say the trees and the Scott Monument may be a little dark.

To brighten up the trees and monument, I have over-exposed the scene with about two stops of light, and the trees now look much like I saw at the scene. However, all the details in the sky have been lost in doing this, and it seems a little washed out.

To protect the detail in the sky, I took a third shot that underexposed the photograph by two stops. We now have much more detail in the sky, but the trees, monument, and foreground look too dark.

All three of these options seem wrong, so what is good exposure?

Before discussing how I define good exposure, let’s talk about how digital cameras work. We don’t capture a negative, like in the old days; instead, the camera sensor records data when we take a picture. Our camera sensor comprises lots of tiny sensors that we call pixels. They record the amount of colour hitting them and the brightness. When we transfer all this data into an editing program, each bit of information is reconstructed to form the picture we saw through the viewfinder.

However, as well as showing the image, our cameras and editing software can give a different view of the data, which is very useful. It’s called a histogram. Let me show you how they are created. The easiest way to think of a histogram is as a graph – along the bottom is a brightness level, starting with black on the left-hand side to white on the right side. When the camera creates a histogram, it reads each pixel and plots the brightness level on the graph. If a brightness level has already had a pixel recorded for it, then the height of the brightness graph increases.

I look for all the data to be recorded between the two edges in a histogram. In this very simplified example, I don’t want anything in the left-hand brightness level, pure black, or in the right-hand brightness level, pure white. We call these two brightness levels clipped shadows on the left and clipped highlights on the right, and when we have clipped shadows and highlights, we can’t change this data when we import it into our editing software. The information in the other brightness levels can be moved left to make them darker or right to make them lighter.

So, when shooting, I like to have the histogram visible. Depending on the camera make and model, you may not be able to look at the histogram before pressing the shutter button, in which case you’ll need to review the image after it has been taken.

We can also see a histogram in Lightroom. When you are in the develop module, there are two triangles in the top left and right of the histogram. If the one on the left is lit up, something in the image has had the shadows clipped, and if you hover over the triangle, it will show up on the image in blue. When the one on the right is lit up, we have clipped highlights, and we can see these areas in red.

The first shot, taken with the light meter at the correct exposure, shows a minimal area of clipped shadows. However, I wouldn’t be too bothered by where this is, as it is in parts of the image from which I don’t expect to pull back too much information.

Our second shot, over-exposed by two stops, has a large area in the sky clipped. If we reduce the highlights in this photograph, you can see that we can’t pull the details in the clouds back.

The third shot introduces much more clipping in the shadows. But again, it is mainly in the regions that I wouldn’t be trying to pull back information from. You might then think that either the first or third images are OK.

Well, let’s look at the image sizes. The second “over-exposed” photograph created a file of 55.13MB on my Canon R5. The third under-exposed image, by contrast, is only 40.43MB. The middle image with the correct exposure is 45.79MB. The reason for this is that cameras record more data in the brighter areas than they do in the darker ones. It’s why we see more noise in dark parts of photographs that have been shot using higher ISOs than in lighter areas.

This leads me to my final tip for good exposure: to use a technique called exposing to the right. What I mean by this is that we use settings that will push the histogram as far to the edge as possible without clipping any of the highlights. By moving the data to the right, we increase the size of the file, which means that we have more data to use when we come to editing our images later.

In this final image, I have over-exposed the shot by one-third of a stop, adding nearly three megabytes of data, fewer clipped shadows, and, most importantly, no clipped highlights.

If you are starting to understand the theory of using the histogram but aren’t comfortable executing it on your camera, come to Edinburgh and join my Switch to Manual session. In four hours, you’ll become comfortable changing the aperture, shutter speed, and ISO to get great results with your photography.