i sold my Canon 500f4 mk1 yesterday as it is too heavy for my shoulder these days and I was told by a nature photographer at my club that sooner rather than later it would no longer be possible to get parts for it - maybe even as early as March next year. Seems very wrong that this can happen to such expensive items but there you go. As he pointed out, it would eventually become a very heavy doorstop. As my lack of use of this lens has bothered me ever since I bought it ( my shoulders started playing up 3 months afterwards), it was the push I needed to move it on. I had 4 offers and, happily, the best was from an independent retailer only an hour from me. The lowest was from a very popular online seller of used photo gear. I still like to photograph bigger birds and am currently waiting for the Bewicks to return to Slimbridge. I shoot full frame and have the 100 - 400 mk 2 and the 1.4 III extender. Now, I could stick with this and maybe I will but I am interested to hear what you might suggest for that extra reach; on seeing my sad face yesterday, the chap in the store suggested the Sigma zoom - not sure which one. Thanks for reading and also for telling me your thoughts.