Again, what is your expectation concerning image quality, amount of noise acceptable, etc? As an amateur bird photographer, I don't NEED the ultimate image quality, and might be much happier boosting the ISO and hiking with my 1.25 kg Canon 400mm f/5.6L if I am also carrying another 25 to 30 pounds of camera, tripod/head, camping gear, food, water on a longer distance multi day hike. If I hoped to sell images, I guess that I would just suck it up and take a 4 kg faster longer lens instead.