Long lenses and DOF

Depth of field is a constant consideration. I photographed these Siberian tigers in China on one of my photo tours with a 500mm f/4 telephoto, and as you can see one of the cats was slightly behind the other one. That meant that in order to get both sharp, I had to use a fairly small aperture because long lenses have such shallow depth of field. I chose f/11, but it wasn't enough. The tiger in the lower part of the composition is just a little bit soft and that's not acceptable. The reason I couldn't see how much of the image was sharp was because it was so cold -- minus 35 degrees Celsius (at minus 40 both Fahrenheit and Celsius are the same) -- and the LCD on my Canon 5D Mark III was completely covered in ice. My warm breath froze on the glass and I was essentially shooting blind. In retrospect, I should have raised the ISO and used f/22. My settings for this were 1/1000, f/11, and 640 ISO. I like the photo a lot, but it's not perfect.