Snow Confuses Auto Exposure (It Sure is a Gray Day Today, isn't It?)
Here in the Pacific Northwest, days are short and often overcast. But just because it may be a gray day outside, doesn't mean photographs of snow need to look gray too. Now could someone convince my auto-exposure of that?
If you judge only by the snow by the side of the road on the freeway, you might conclude that gray snow is normal. But to do so would be unfair to the snow that hasn't fallen victim to the snow plow, the stuff you perhaps may be on the way to ski down as you drive that freeway. Or the stuff on those mountain peeks all around you that you stop to photograph because they look so pristine, so fresh, so white. Funny thing though is that photographs of that nice, clean, white snow can easily come out as dingy gray as would the snow mixed with mud by the side of the road. Something must be wrong with the camera meter, because clearly the auto-exposure should be able to see the difference, right?
If only it were that simple. More to the point though, how do you know the difference yourself? Think about it for a minute. One obviously appears brighter white than the other, but only because you have the context and experience to make such a determination.
If you've never tried this before, I urge you to perform a simple experiment. Find or buy three sheets of foam core, poster board, or other similar card stock, one white, one medium gray, and one black. You want the sheets to be big enough that you can easily point your camera at one and fill the frame completely. Set up all three side by side leaning against a wall somewhere under even lighting. Now photograph each card in turn with various exposures. It should be obvious that with a short enough exposure, insufficient light will reach the camera sensor to create a visible exposure regardless of the tonality of the target, and that with a long enough exposure, you should be able to overexpose sufficiently to get pure white with any. And with a carefully selected exposure for each, any one of them could be rendered as medium gray. The tonality of the result depends as much on how you photograph it as it does on what you photograph. Fun and games with exposure, right?
Now look at your results and see if you can tell which image was created from photographing which target. If any of the three could come out looking black, white or gray depending on the exposure, who's to say which is which? Now here's the real dilemma: how is the meter in your camera to know which it is looking at? All it can see is what makes it through the lens in a sort of "tunnel vision" effect, and as you've seen, you can make black or white come out white or black at will. It all depends on exposure. Which one is the "real" answer?
Now try it a different way: keep the exposure the same but vary the light level. In pitch blackness, all three target cards will come out photographed as black. At the opposite extreme, overwhelm the scene with bright enough light and black, white and gray will all come out as burned out white. If yours don't, just increase the exposure time a bit more and eventually they all will. Pick just the right exposure and all three can be made to come out a perfect 18-percent reflectance medium gray. Don't believe me? Give it a try and see. Doing so will teach you more about how your camera meter works than reading any book, or dare I say, any online article. Well, other than this article of course.
Now, some cameras do have incident light meters somewhere on the top to attempt to factor in the general brightness level in determining exposure, but even this won't tell the camera what something is supposed to look like. It could be a sunny day with the camera pointed into a dark tunnel, or you could be photographing on a gloomy, overcast day with camera and a really bright flashlight. Really, there's no way your camera could know what something is supposed to look like in terms of brightness.
Camera makers instead decided long ago to standardize their meters to always render things a consistent brightness as determined by something of "average" reflectance. Every camera meter in the world does its level best to render what you point it at as being medium toned, a brightness level technically known as 18-percent reflectance, a level halfway between darkness and light. You might think that halfway would be the 50 percent mark rather than 18, but remember, the human eye sees doubling and halving of absolute brightness as being equal steps, the "stops" of photographic exposure. So right around 18 percent is the point where the consecutive doubling has made half the jumps possible between what we see as solid black and solid white and is thus seen as medium toned.
For many subjects, an assumption of being medium toned works out fairly well. Average out the exposure for everything like paint mixed in a bucket and the net result will come out close to medium in many cases. Some things in the scene will be brighter than average, and some darker. That's what makes "average" average, after all. The problem with snow is that it is (or generally should appear) considerably brigher than medium. And if it constitutes a significant portion of the frame, the scene is no longer average even if your camera doesn't realize that. If enough of the frame is be filled with snow it will confuse the camera meter as it bravely attempts to achieve 18-percent reflectance. You and I know that pure snow should be white, but the camera doesn't even know its snow, let alone how bright it should be rendered. It's just not possible. And so we end up with gray.
But by giving us a consistent reference point, the camera meter does provide us with a useful basis from which to adjust exposure to achieve the look we want. Want your snow to look white rather than gray, easy. Just tell your camera to do so. You can do this either with the exposure compensation dial when shooting in automatic or semi-automated metering modes (aperture priority and shutter priority), or by adjusting the meter reading to read as you want when shooting in manual mode.
When in doubt, check the histogram. But be careful not to overdo your exposure adjustment. You want the snow to be brighter than gray but don't make it so bright that you lose all detail. Most cameras today have a blinking highlight option that can help to alert you if you go too far, like an alarm blinking at you, demanding your attention. Your camera may not be able to decide the best exposure for a snowy scene, but it can tell you all you need to know to decide for yourself.
See, your camera meter isn't so dumb after all. You just need to understand what it can and can't do on its own, and what its trying to tell you so you can take matters into your own hands and get the exposure you want.