Markewitz carries a hand-held light meter, but many cameras have one built in. Either way, snow will throw it off. "The meter adjusts, turning white snow to gray, and everything that's not white gets darker." His trick: Set your camera's meter to the automatic mode. Then point it at the back of your hand ("It needs a medium tone," he says). Remember that reading, switch to manual mode and set your camera to the appropriate shutter speed.