Where I live (southern Arizona) if you want to do wildlife photography you need to be prepared to walk to your subjects. I find that weight is a big consideration. Lugging a heavy lens/camera/tripod combination is simply impractical most of the time. I was looking for something that gave me significant magnification, good image quality, and was reasonably light. A friend suggested the 400DO to me. I did some research and discovered that it seemed to be a forgotten item in Canon's inventory. My friend, who is a superb wildlife photographer, assured me that I couldn't go wrong with the lens. I swallowed hard and bought it. I must say that I never cease to be pleased with it. The image quality is superb, and it is light and portable, weighing not all that much more than my 100-400 f4-5.6L. It's become my go-to lens for wildlife photography.