With digital photography and temperatures well below 0⁰ F, not only do you have to keep yourself warm; but you have to protect the integrity of your camera from condensation inside the lens. I have found that leaving the camera outside of my jacket does that and then I let it warm up very slowly when done. It seems to work because my camera is on image 40,986. On this day the raw temperature was -29⁰ F when I took this timed photo of myself bundle against the cold.