06 February 2010

Understanding exposure

Tricky lighting situation This is the second article in my series on taking control of your camera. Here I introduce the concept of exposure and explain the importance of getting it right.

Exposure is the most fundamental principal of photography, and it's all down to the light. It doesn't matter whether you're taking a photo with a digital camera, or with 35mm slide film, or if you have your head under the hood of a huge old plate camera. The camera still has the same job to do. It has to focus the image and expose a sensor (or film in what now seems like ancient times) to the right amount of light. The sensor (or film) records the light, and hey presto, we have a photo. That's pretty much it. The rest is just bells and whistles.

Take a look at the three images below. The centre image is correctly exposed: there is a good balance of light and dark shades, the colours are bright, and the scene looks natural. The image on the left, however, is underexposed. In other words, not enough light has been captured by the camera, so everything is too dark. By comparison, the one to the right is overexposed, i.e. too much light has been allowed into the camera, the colours are washed out, shadows lack depth, and the highlights are completely white.

Correct exposure

The centre image of Sacre-Coeur, Paris, is correctly exposed. The images to the left and right are under-exposed and over-exposed, by one stop.

Getting exposure right is critical to taking a good photo. Thankfully all modern cameras, and even most older cameras, have a built-in light meter. This measures the brightness of the image and guesses the right exposure for you. Today's digital cameras employ ever more clever ways of doing this and, frankly, they do a damn good job. But in tricky lighting situations it can go horribly wrong.

The way a camera's so-called "auto-exposure" works is to adjust the exposure so that the photo is recorded, on average, as a mid-tone. This works fine in most situations, since most scenes are a mixture of light, dark and mid-tones. But this is not always the case, and that's when things tend to go wrong. A classic example is a scene with snow, such as the one below. The camera sees all that white and thinks it is overexposed, so it tries to darken the image to a mid-grey. The result: nastily underexposed photos. The more white in the scene, the more it will tend to underexpose.

Exposing for snow

Photographing snow scenes can trick your camera into under-exposing (top). Increase the exposure (bottom) to compensate.

To compensate for this we need to override the camera and increase the exposure manually. Conversely, when photographing something dark, the camera will tend to over-expose, to create washed-out photos that lack the dark tones we are after. Then we need to decrease the exposure to compensate.

Tricky lighting situation

You can't always rely on your camera to handle a tricky lighting situation such as this intense flame from a hot air balloon.

When things get really tricky, then you have no choice but to take over completely. The photo above was taken during a hot air balloon flight over Buckinghamshire. My first priority was to get some aerial photos over Thames Valley and the picturesque village of Hambledon. Shortly after take-off from Henley, at first light it was still too dark for aerial photography from a moving balloon. So until the sun came up I turned my attention to the pilot, world record holder Mark Shemilt. This gave rise to an excellent example of when a camera really struggles to judge the exposure correctly. The overall lighting was very low, with the sun still below a very misty horizon. By contrast, the balloon's burner was blindingly bright - and smack-bang in the middle of the frame. To make matters worse, as I took this shot, Mark was adjusting the burner, so the flame danced and flickered on and off, with the light intensity varying accordingly. This threw the camera off completely, and I had no choice but to switch to full manual control. I pinned down the right exposure only after some trial and error.

Using the histogram to check exposure

Exposure is a difficult thing to judge in practise, especially when you're viewing the photo on a 3-inch screen and the midday sun is beating down on you. (Or, indeed, if you're 300 feet above the ground with only an inch of wicker basket between you and certain death!) Thankfully, most digital cameras will display a histogram. This shows you the amount of dark and light areas in your photo. Black is on the far left of the histogram, and white is on the far right. Have a look at the histogram below: this is from a correctly exposed photo. There is a good balance of light and dark tones and, importantly, none of the pixels are completely black or completely white. This tells you that the camera has been able to capture the full tonal range in the photo.

Histogram

When in doubt, use the histogram to judge correct exposure.

If the histogram leans to the left or right, especially if it appears chopped off at one end, then it is likely that the exposure is wrong. Note, however, what I said earlier about bright scenes such as snow having lots of light tones. In those situations you would expect the histogram to be weightier on the right hand side. Likewise, if you want to capture a photo with lots of dark tones, then the histogram is meant to lean to the left. Just watch out that the histogram isn't skewed so far one way that it gets chopped off. If that happens then you'll either have highlights that are burnt out, i.e. completely white, or shadows that are solid black and lack any tonal variation or detail.

Using exposure compensation

Great, so now we've cracked exposure. There's just one problem: you cannot set "exposure" on a camera directly. You'll never find an "exposure" dial on a camera, not even on an old manual camera. Exposure is actually set indirectly, by three out of the four fundamental controls that I'll get onto very soon. The nearest thing to an exposure dial is something called "exposure compensation." What this actually does is change those fundamental controls for you, in order to increase or decrease the exposure, i.e. lighten or darken the image. All but the most basic cameras have this function, and it's a useful first step away from using the camera in full automatic mode. So if you're not familiar with exposure compensation then I highly recommend digging out your camera's instructions and having a play. Get used to changing the exposure compensation and see how it affects the brightness of a photo, without worrying about what the camera is actually doing. That comes in the next article.

Expert tip: Many cameras have a feature called Exposure bracketing. With this feature switched on, the camera will take three shots at different exposure settings, so that if the exposure is slightly off on the first then hopefully one of the other two will be right. This feature came about in the days of slide film, which allows very little room for error. With digital cameras you can check the shot using playback, so today exposure bracketing is less useful. But if you need to get a shot in a hurry and the lighting is tricky, then this feature might just save your bacon.

Don't stop now

One final note on exposure: it is measured in stops - a reference to the stops on a dial. Stops actually refer to changes to the camera's exposure settings. Imagine turning a dial to increase or decrease the exposure. The amount you turn the dial is measured in stops. Regardless of the current exposure setting, changing it by +1 stop always results in a doubling of the exposure, whilst -1 stop means halving the exposure. So +2 stops equals 4x the exposure, +3 stops equals 8x, and so on. Most cameras actually allow settings to be changed in thirds of a stop, for greater precision.

Next article: Controlling shutter speed.

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