The world isn’t gray. That seems like overstating the obvious but with your camera, it’s not quite that simple. The light meter in your camera makes some assumptions about our world that don’t necessarily hold true. The most important one is that, contrary to the title of this article, the world is gray. At first blush that may seem ludicrous but it’s the basis for all of our exposure computations.
Middle gray is a tone that is perceptually half way between black and white on a value scale; in photography, it is typically defined as 18% reflectance in visible light. Middle gray is the universal measurement standard in photographic cameras. The camera assumes that much of the light reflected by the object measured is equivalent to middle gray and uses that to determine how to correctly expose the scene.
All of that is well and good unless the scene that you’re composing isn’t mostly gray, or some other middle tone. The image in this post is an excellent example where the concept breaks down. Roughly half of this image, of Cook’s Meadow in Yosemite National Park, is brilliant white. When your camera calculates the exposure it’s not calculating how to expose the scene as you view it, it’s calculating the exposure necessary to make it middle gray. In this case, if left on full automatic, the snow would be grey, the sky day and Half Dome very underexposed. Because exposures calculation is always based on gray, or middle tones, scenes with a lot of black have a similar but opposite problem. If shooting in full manual mode scares you, and it shouldn’t, there are still ways around the problem.
Step one is making sure that you’re using your camera’s narrowest metering option. If you don’t have a “Spot Meter” option, choose a “Center Weighted” mode.
Now, consider the starting point, on your exposure meter, “0.” Pure white, with detail, is generally 2 stops above 0 or +2. Pure black with detail is roughly 2 stops below 0 or -2. If you’re in manual you simply adjust shutter speed to compensate for it (on Canon, move the meter 2 stops to the right, on Nikon, move 2 stops to the left). If you’re in an automatic mode you’ll need to add or subtract “exposure compensation” or EV depending on whether you’re metering on a white or black object. My personal opinion is that if you’re bothering with exposure compensation you might just as well be shooting in manual; it’s fewer steps.
For the Cook’s Meadow image, I pointed my camera at the surface of the snow. I selected the aperture that I wanted, in this case f/20 because I wanted a very wide depth of field, and then dialed my shutter speed in to bring the light meter to 2 stops above 0 (+2). That setting ended up being 1/40 second @ ISO 200. Once I had those settings dialed in I placed the camera back on the tripod and composed my shot. I ended up at 30mm on my 24-70 f/2.8L using my Canon 30D.
Shooting snow, and having it look like you expect, is easy as long as you just remember that the world really isn’t gray.