Preventing Gray Snow in the Digital Age
Camera meters assume you want whatever you point them at to come out medium tone. That can be a problem this time of year since snow's not at all supposed to look medium tone. Thankfully, digital cameras make this easier than ever to prevent.
The traditional remedy for this problem has been the exposure compensation control that biases your camera's meter by an amount of your choosing. As a point of clarification, this only ever worked if you were using fully automatic metering, or either shutter or aperture priority. Photographers who took full control by using manual metering likely discovered that exposure compensation didn't work for them. In this mode, both aperture and shutter speed settings stayed wherever the photographer set them. Changing the exposure compensation affected neither.
But in most metering modes, you could brighten up snow by understanding that snow was supposed to be brighter than medium and accordingly adding a stop or so on the exposure compensation dial. If you set exposure compensation to +1, the camera would aim for one stop over medium instead of its usually preferred medium.
But was +1 compensation the right choice? A typical camera has a five-stop exposure range so +1 is only halfway between the medium of zero and the brightest that can be recorded at +2. And many digital cameras can reliably preserve detail somewhat above +2 from medium. Arbitrarily setting exposure compensation seems at first glance to be better than accepting medium gray snow, but on further consideration, is dull, still somewhat dingy, +1 snow that much better?
The standard explanation as to why the rule of thumb for snow was +1 compensation was that it was hard to safely pick the brightest point in the snow to meter off of. If you risked it and went to +1.5 or even +2 you might just find out after the fact that there was snow elsewhere brighter still. You knew this after the fact when you saw the burned out, featureless results in that area. Bummer. Picking +1 for exposure compensation essentially served to make sure you didn't up with either medium gray or burned out white snow. It was a compromise.
But why compromise when a digital camera allows you not to have to?
With a digital camera you can see the results while you are still in the field. No more having to wait for the film to get developed to find out if you nailed it or if you blew it horribly. While you're still on location you can check your results and adjust accordingly. Almost every digital camera other than the smallest and cheapest models provides at least one way to judge exposure via the camera's LCD back display.
A histogram displays the distribution of brightness within an image from pure black to pure white. It doesn't tell you where in the image any given brightness is, but it does tell you how much at each brightness exists, somewhere. Some people will tell you that the perfect histogram looks like a bell curve — high in the middle and sloping down towards zero on both ends. This is exactly what the histogram of a snowy scene would look like if you shot it with a camera on fully automatic with no exposure compensation, so clearly not every image should end up with a bell curve histogram. The mid-point on the left to right axis of a histogram is medium gray and this is exactly what we are trying to avoid here. You need to expose above the midpoint of medium gray.
If you add too much exposure compensation you'll push the histogram so far to the right that it gets bunched up against the side wall. This is what clipping looks like in a histogram. You don't want that either. What you want is an exposure with highlights as close to white as possible without noticeably hitting the right hand wall. Shoot your best attempt, check the histogram, and if you don't like what you see, revise the exposure compensation accordingly and shoot again. It's that simple.
Another method many cameras provide is commonly known as "blinkies." Your camera manual though probably refers to it less colorfully as a "highlight warning" display or something similar. What it does tell the camera to add a blinking overlay to burned-out areas of the image review display. So rather than looking at the histogram to check your exposure, you can see if there are any problem areas on the image display itself. This works well for snow scenes since the critical determining factor is whether you have any burned out highlights or not. If you see no blinking spots at all, you probably can go a bit higher on the exposure compensation dial. If you see a lot of blinking areas, you've clearly gone too far. A few random blinking pixels should generally not be cause for worry, so this gives you something to shoot for. If you really want to nail the exposure, bump up the compensation until you first start to see blinking highlights, then back the setting off a tad and go with that as your final setting.
You can utilize one, the other, or both of these tools to optimize your exposure setting. The histogram tells you how much of problem you have while the blinking highlight display shows you where the problem areas are. Even with only one method, you have far more control over your winter snow scene exposures than used to be possible with film cameras. In his age of digital photography, we have some amazing tools available. If you're still just setting exposure compensation at +1 and calling it good, learn to use what your camera offers to finally make your snow look like snow. There's no reason to accept medium gray snow anymore.