How Auto Exposure Bracketing Can Improve Your Photography
Modern day cameras are technological masterpieces of engineering, with electrical, mechanical and optical systems all working together. However, like most devices, they do have some limitations, one being their dynamic range and the other being an auto exposure system which doesn’t always get it right.
If you take a photograph on a bright sunny day, with no filters, you will often find it difficult to keep detail in both the highlights and the shadows. If you expose for the brightest part of the scene, such as the sky, the shadows will appear too dark with virtually no visible detail. If you try and modify your settings to lighten the dark areas, you then find the brightest areas are just white patches. This is due to the dynamic range of the scene being larger than that of the camera.
What is dynamic range?
In non technical terms, it is the difference between the lightest and darkest areas of light, or the ratio between the brightest whites and the darkest blacks. It is often used when describing the attributes of a camera, but it could equally be used for scanners, monitors, printers or even your eyes, as well as the actual scene being photographed.
The dynamic range of a digital camera is normally quoted in stops, which like f-stops refer to the doubling or halving of light. A stop is not an actual unit of brightness or luminosity, but uses the power of two to indicate ranges or ratios. For example, a camera with a dynamic range of 12 stops could also be quoted as having a contrast ratio of 4,096:1 (2 to the power of 12 = 4,096).
The dynamic range of modern full frame cameras is around 10–15 stops, but older cameras, or those with smaller sensors can only manage 5–10. However, a typical landscape scene on a bright sunny day may be around 20 stops, so there is often a need to make compromises.
Use your histogram
Many photographers will have heard of the term histogram, but tend not to use them or do not fully understand them. However, they can prove very useful when reviewing images, and deciding what settings to change to improve the exposure. They are also handy on sunny days, when it is hard to determine the image brightness on the rear screen. Some cameras will show a histogram before you take the shot, while others will only display one after an image had been taken.
A histogram is basically a graph which represents the tonal values of an image. The scale along the bottom displays brightness, from 0% on the left (black) to 100% on the right (white). The vertical scale indicates the amount of each level of brightness. As a general guide, a histogram for an image with no extremes should have the majority of the values in the middle of the graph, with lower values on either side. However, this approach to setting your exposure can’t be used if the scene or subject is primarily black or white.
If you take a photograph and the histogram has no values on the left, with the bulk of the graph on the right hand side, then the image is likely to be over exposed. If on the other hand the graph is pushed up against the left, then it is probably under exposed. In these situations you can adjust your settings to move the values so they are more central, and this should give you a better exposed photo. However, if the values are spread right across the graph and are clipped on both sides, this means the dynamic range of the scene is greater than the camera’s.
One option is to expose for the highlights and not worry about the dark shadows; most people agree that darker areas with limited detail are preferable to large white patches where the sky should be. Many landscape photographers manage the issue of high contrast scenes by using neutral density graduated filters. These have a dark top half which covers the sky and helps equal out the light levels, allowing the camera to achieve a good exposure. Other photographers prefer to edit their images in Photoshop, often by using layers and masks to selectively edit different parts of the photo. But if you don’t own a selection of ND filters and you aren’t very proficient with editing, what else can you do? One option is called auto exposure bracketing.
Auto Exposure Bracketing
A lot of modern DSLR, mirrorless, bridge and some compact cameras come with a feature called Auto Exposure Bracketing (AEB). Different camera manufacturers use slightly different menus and terminology, but they all operate in roughly the same way. It normally only functions in aperture priority, shutter priority and program, but not in full auto or manual modes. First you have to select the function, which may be called AEB or Brkt, and then you have two or three options.
First you have to decide how many shots you want to take, as some cameras offer the choice of between two and seven, although three is normally adequate. Then there is the amount of compensation, which can be any where from one third of a stop to three whole stops. Finally you may have to choose the order in which the photos are taken, although this doesn’t really matter.
To take the images you can either take three (5 or 7) individual shots, or hold down the button in high speed / burst mode until the camera stops. If you have selected three exposures you should see one under exposed, one correctly exposed and one over exposed. If your camera doesn’t have an AEB mode, you could carry out the procedure manually. Because of the time required to take three or more shots, AEB is not suitable for moving subjects.
After loading the files onto your computer, you can then choose the best image and delete the others. If you have selected raw files you will then be able to tweak the highlights and shadows as necessary. This does not have to be done in Photoshop, as most manufacturers supply image editing software with their cameras, which often includes a raw editor.
However, if the scene had a very large dynamic range, as described earlier, you may find that none of the images are very good, but all three have some useful data. In this case you need to combine all three exposures using your editor, to give an image with a high dynamic range (HDR). This should then show shadows, mid tones and highlights, all with a lot of detail.
HDR photos can often be produced by cameras automatically, but they are often too processed to look realistic and they are usually only available as Jpeg files. A whole article could be written on HDR photography, but that is for another time.