I photographed this variegated squirrel in Costa Rica under extremely dark conditions. He was in deep shade late in the day, and to fill the frame for visual impact I opted for a 500mm lens plus a 1.4 x teleconverter, giving me 700mm of focal length. I was shooting wide open, which accounts for the extremely shallow depth of field, and the ISO I chose was 2000. That made the shutter speed 1/50th of a second -- something I hate to do. The general rule is that for sharp pictures while hand holding a telephoto, the shutter should be the reciprocal of the focal length or faster. In this case, that would have been 1/700th of a second.
I was using a tripod, though, but still . . . with the long lens, there was little play and it just wasn't rock solid. So, I doubted I could get a sharp picture, but I tried. I took several images, and this was the only one that was sharp. A great argument for getting a camera capable of extremely high ISO settings is for shooting in dark conditions like this one. 25,000 and higher ISO settings are now possible, and that translates into faster shutter speeds and/or more depth of field.