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Auto­-Exposure Bracketing

by Steph Doran (follow)
Blog (393)      Articles (166)     

Have you ever been in a situation where you want to photograph a scene, but you’re faced with some really shadowy areas and other incredibly bright areas, like in the photo above? Are you not quite sure which exposure settings to choose?

Or maybe you’ve seen some stunning landscape images, with bold skies and rich colours in the foreground, but when you’ve tried to take a similar photograph, it just looks washed out, and just nowhere near as good? If so, then exposure bracketing is about to become your new best friend.

‘Exposure bracketing’ (or simply ‘bracketing’) is a term that describes the technique of taking three or more pictures of exactly the same scene, but with different exposures for each picture.

Auto-­Exposure Bracketing (AEB) is very similar to bracketing, except that it’s automatic (i.e. the camera does it for you.) When you are bracketing manually, you set your initial exposure, take the shot, and then take two more shots of the same scene ­one slightly lighter and one slightly darker than the original. AEB uses the same principle, except that the camera will change the settings for the lighter and darker shots, which means you don’t have to touch the dials. This is useful because it saves you time, and you are less likely to miss ‘the moment’ by fiddling around with settings.

When using AEB, we can choose the amount of light by which the camera will ‘under’ or ‘over’ expose the bracketed images. These amounts are measured in stops of light. Usually, the camera will bracket in increments of ⅓ or full stops, but you can manually choose these increments in the setup. If your scene’s light is not incredibly contrasting, you may want to set the exposure value (EV) to ⅓ stop. This means that the camera will take one ‘normal’ image (0 EV), one ‘darker’ image (­⅓ EV), and one ‘lighter’ image ( ⅓ EV.)

However, if you have a highly contrasting scene, similar to the photo below, you might need to have a greater difference between your shots, perhaps even 2 full stops! In this case, you would get one ‘normal’ image (0 EV), one ‘darker’ image (­2 EV), and one ‘lighter’ image ( 2 EV.)

Without bracketing

Lighter

Darker

Exposure bracketing was more commonly used in the days of film, as photographers could not simply look on their digital viewfinders, or read their histograms to make sure their image was correctly exposed. Of course, they would also have to wait until the film was developed to know if their shots were too light, too dark or just right. So exposure bracketing was often used as a ‘back up’, to make sure that at least one of the exposures was useable.

Now, we have the ability to review our images as we shoot, and even adjust the exposure later using editing programs, so exposure bracketing is not used quite as often for everyday shots.

So, in this digital age, why would you still want, or need, to use AEB? While we are less likely to use bracketing to capture a ‘back up’ frame, it still has many relevant uses. Here are a few reasons why you may choose to experiment with AEB:

1) To determine the ‘best’ exposure for your scene, without having to manually change settings.

2) To ensure you have a correctly exposed shot when shooting quickly.

3) To capture a scene that has tricky lighting.

4) To blend multiple images, creating a HDR (high dynamic range) image, using editing software. The term ‘dynamic range’ refers to the ratio between the lightest and darkest ‘capturable’ parts of the image.

Exposure bracketing can be a useful tool for beginner photographers, as it can help you to recognise common situations where you may need to over or underexpose a subject. As the camera takes three consecutive pictures, you are essentially given three versions of each scene you capture. This makes for a fantastic comparison. Say, for example, you were photographing a bouquet of whiteroses in a living room. You may take your time setting up the shot, examining your camera’s light meter, and carefully choosing your settings. However, even if you match your light meter up exactly with the camera’s ‘zero,’ you may find that your roses look light grey, rather than the crisp white that you were expecting. Using AEB will allow you to see a lighter and a darker variation of the same image.

Lightest to darkest, from left to right

Using exposure bracketing is very convenient when you are faced with a tricky lighting situation and aren’t quite sure which exposure to use. For example, if you are photographing a black dog in a park on a very sunny day, you could get all kinds of results depending on which camera settings you are using. Letting the camera choose an exposure will most likely result in correctly exposed midtones, but little or no detail in the shadows or highlights. This is not good for our black dog, because his dark coat is at risk of becoming a silhouette. However, using AEB will give us three exposures for this scene, allowing us to choose the best one. For this particular scene, we may find that the slightly overexposed shot is best for our subject, because it will provide us with more detail in the darker tones of the image.

A similar situation occurs when shooting subjects with the sun behind them (i.e. the subject is between the light source and the camera and is “backlit”). The camera’s meter will expose for the overall scene, which could result in a bit of detail in the background, and a bit of detail in our subject, but not an optimum exposure for either area. But using AEB to take the same shot will give you two additional choices. The over­exposed image will blow out the background, but will give you detail in the subject, whereas the under­exposed image will give you detail in the background, and the subject will fall into silhouette. Neither result is better than the other, it just depends on what kind of image you are aiming for.

Now, back to our original problem – the fantastic landscape photograph that we just cannot seem to replicate. How do those photographers make their images look so professional? The answer is exposure bracketing. In most cases, landscape scenes will have a dynamic range that’s greater than the dynamic range of the camera. If we have very bright and very dark areas, the camera may not be able to capture both extremes in detail, within a single shot. For example, if you have a darker foreground (perhaps black stones, or shaded bushes), capturing a shot that includes this shadow detail will require sacrificing detail in the brighter areas of the image (e.g. the sky). This is because the camera’s dynamic range is not large enough to include detail in the very dark and very light parts of the image.

We can overcome this problem by using the bracketing technique, and blending the photographs later in post-production. By using AEB, we can capture one exposure for the highlights, one for the midtones and one for the shadows. Between the three shots, we will have some detail in all areas of the scene. Then, using editing software, we can combine these images (either automatically, or by manually brushing them in with layers) to create an image with a ‘high dynamic range’. These types of images are commonly referred to as ‘HDR’ images, because the dynamic range has been increased. HDR is a commonly used technique in landscape, architectural and product photography.

AEB is a powerful tool, and one which many new photographers are afraid to try. However, experimenting with this technique will not only improve your photography skills, but it will aid your understanding of exposure. Try it next time you are in a tricky lighting situation, it can be interesting to see how much of a difference a stop or two can make to your final exposure!

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