Santa must know that I’m both obsessive-compulsive and a photographer, having brought me an ExpoDisc.

It can almost be thought of as a filter that you hold over your lens. You use it to take a sample of incident light, which you then use to set your camera’s custom white balance. Most modern digital SLRs (at least) include the option to set white balance to any photo you’ve taken, which should ideally be a gray cast.

Most of the time, cameras do a decent job of automatically guessing white balance. And most non-photographers (or perhaps, most non-obsessed photographers) aren’t even sure what white balance is. In a nutshell, it correct color casts. The photo at right was taken (of the ExpoDisc and its carrying pouch, in front of my Christmas tree) after I calibrated my camera to the light in the room. If I hadn’t used the ExpoDisc, it would have looked similar, probably, since this falls into the 95% of the time when the camera can do a pretty good job. But there’s a difference between getting it pretty good and getting it perfect, and there’s also something to be said for the times when the camera just can’t get it right. (Locals: try taking a picture in Margarita’s in which everything doesn’t look red.)

But to demonstrate precisely what white balance “does,” I took a few photos with it horribly misadjusted:

This isn’t particular objectionable, but it’s wrong. Everything looks a little yellow-orange-reddish. White balance was set to shade (7000K), which was too extreme and caused the red color cast. This can be said to be a “warm” shot, referring to the red cast. I happen to like photos just a little bit warm, but it’s better to get the exposure right and then apply a little warming in Photoshop than to try to shoot with a misadjusted camera.

This one should look more objectionable. I set my camera’s white balance to Tungsten (3200K). Since I was actually in a room with white walls and lit by brilliant sunlight, there’s a horrible blue cast to the image. The opposite of a “warm” photo, this is cold. (Incidentally, cheap digital cameras seem prone to having this way-too-blue effect.)

Using the ExpoDisc is quite easy. It seems to come in a variety of sizes, but mine’s a generous 77mm, which is big enough to sit on the front of any of my lenses. Rather than screwing in, you simply hold it in front. On my Canon XTi, you simply take a shot through it, which results in a grayish-white photo. Or, if your white balance was way off, an orange or blue photo. (Hint: switch to manual focus, or the camera will try in vain to “focus” on light that’s impossible to focus on.) A quick option in the menu lets you set “Custom WB” to the photo. Kind of like a gray card in the days of old, but much easier and quicker.

I’ve probably been overdoing it a bit, changing settings every time I move. But the results are going to be worth it.