After three days in Yellowstone Park this week, I was suffering from the flu. It is a stunningly raw and beautiful park in the winter but it can also be extremely cold. The steam from the thermal activity freezes on whatever it can - including the big bisons that gravitate there to keep warm. I have waited for a very cold day like yesterday in Yellowstone for a long time, but looking at me this morning I have certainly paid the price.This kind of photography may test one's appetite to work in extreme conditions, but it also tests our maths. I have never taken a photograph of anything on an aperture of F18 before, but I did yesterday as I wanted as much depth of focus as possible. My inclination was to be greedy and to try and have the bison’s nose in focus and also her eyes. So I sacrificed shutter speed and resolution and hoped for the best. It was impossible to check too much in the field, but when we returned home, I found one gem. I hope she is feeling better than me today. She will be - they are as tough as hell.