I sold my Canon 100-400mm f5.6 zoom some years ago as I was not happy with the results. With hindsight I should have got it serviced as have many friends since. It is one of those lenses that seems to need adjusting from time to time. Owners who get them serviced seem very happy with them. I was considering buying another, but after a lot of thought have gone for the latest Sigma 170-500mm. My first results from it are fine. It is sharp and gives good feather detail so I am happy with it optically. However the autofocus is painfully slow. I was amazed to watch it slowly inch its way to focus on the first occassion I used it. Not what I am used to from Canons own lenses. I will stick with it for now as the extra zoom range will hopefully be useful and I will never use it for flight shots.