With winter just around the corner, and the thrilling prospect of snow, ice and winter landscapes, now might be a good time to look at how to nail the exposure in these tricky conditions.
Why do snowy conditions present exposure problems?
Generally we can rely on our camera’s internal light meter to do a good job of measuring the light in a scene. If we are in manual mode then we can read the exposure from the scale in our view finder and adjust the aperture and shutter speed accordingly. If we are in a semi-automatic mode then our camera will select an appropriate aperture or shutter speed for us, again based on the reading from the internal light meter. This works reasonably well in an ‘average’ scene with a mix of tones from dark to light because this is how the camera is calibrated. However, if the scene is mostly white your camera has no way of knowing this and will assume it has a broad range of tones. This means that your beautiful white snow will actually come out grey as your camera has not let enough light through to the the sensor to show it as white; it is under exposed.
What can we do about it?
We need to ‘over expose’. Actually this is a bit of a misnomer, as what we really need to do is expose correctly for the scene. In the semi-automatic modes we use the exposure compensation setting on our camera, and in manual we adjust the aperture, shutter speed or ISO until our meter reading shows over exposure. A reasonable over exposure to aim for is one to two stops. You’ll have to use a little bit of trial and error to make sure that you have it right and it’s a good idea to have your camera’s highlights warning on so you can review the image and make sure that you have not gone too far and lost all the detail in the highlights.
Checking the histogram
It’s also a good idea to take a quick look at the histogram in your camera. What you need to see is a cluster of pixels at the right-hand side of the graph which shows that we have very light areas of your image. Make sure that the graph does not spill off the edge which means that you have over exposed too much.