If you’ve ever taken a photo where most of the background was white snow or bright white sand on a sunny beach, you’ve probably been disappointed by the results. That crisp, white snow comes out looking grey and murky, while your subjects come out looking very dark and under-exposed. There is a very good reason for this, and fortunately it is very easy to correct.
The problem lies with your camera’s light meter. It is designed to measure the light and set the exposure for an average scene, and under average conditions it will normally do a very good job. This is because an average scene has an unusual property; it reflects approximately 18 percent of the light falling on it, equivalent to a mid-tone grey. It doesn’t matter if it’s a landscape of rolling fields, a city street or your own living room, the average amount of light reflected is always around that magic 18 percent figure.
Your camera’s light meter is calibrated to take account of this fact, which is why it will produce the correct exposure in any normal situation. In fact professional photographers will often use a special 18-percent grey card to help make very accurate exposures. They will have their model hold the card, or place it in the scene to be photographed, and take a spot meter reading from the card.
However good your light meter is though, it will start to have problems whenever it tries to measure a scene where the light reflection is different from the average, and snow/beach scenes are a prime example. The bright surface reflects much more than the usual 18 percent of available light, but your camera meter doesn’t know this. The light meter measures the light reflected by the scene, and assumes that it is supposed to be the usual 18 percent. As a result, what was supposed to be white comes out as a mid-tone grey, and as a result any non-white elements in the scene will be very under-exposed.
We can correct this to some extent by the using exposure compensation feature found on all digital cameras. By adjusting it to +2 or +3 we increase the exposure and the brightness of the image, restoring the white background to white, and ensuring that our subjects are now correctly exposed. Increasing the exposure does have two potential drawbacks however.
First, it usually means using a much longer shutter speed, which increases the risk of camera shake. If your camera has image stabilisation make sure it’s switched on, and if you have a tripod or monopod, use it. The second problem is that of over-exposure, which could result in the snow or sand being completely burned out, leaving a plain featureless area of white in your picture. This is always a risk, so if your camera has exposure bracketing it is a good idea to make use of it.
Another good way to avoid this problem is to use spot metering, if your camera has this feature. It is a little more complicated to use than the normal multi-zone metering, but the results will often be much better. For most cameras, the way to use spot metering is to aim the centre-spot of the frame at the subject’s face, half-press the shutter button to take a light reading, and then keeping button half pressed, compose your picture and take the shot.
But what if you’ve already taken your photo, and it looks grey and dull as described above? Fortunately it is quite easy to fix this using image editing software, especially a good package such as Adobe Photoshop, Photoshop Elements or Corel Paint Shop Pro. The tool that can correct this problem is Levels, which is used to make fine adjustments to the brightness and contrast of an image. In Photoshop it can be found by using the keyboard shortcut CTRL + L, or in the Image > Adjustments menu.
The levels dialogue includes three buttons for setting various tone points in an image, one of which is used to set the white point. Simply click on this button, and then click on any part of the image that is supposed to be white. The program will then adjust the brightness histogram to make that point white, and in the process brighten up the whole image.
If you are feeling a bit more creative you can achieve much more accurate results, with usually less loss of detail, by manually adjusting the levels sliders. Move the left (black) slider to the right, and the right (white) slider to the left to equalise the histogram, then carefully adjust the mid-point slider to lighten or darken the image until the snow or sand is mostly white but with some visible details.