Thank you, Lars and Hans.
Lars, I was at full extension, so 560mm with the t/c.
The 400/4.5 is a lovely lens, with great image quality. It is better than the 100-400 at 400, but much worse at 100-300
The 100-400 is my most used lens for wildlife and sport because of its versatility. It has excellent close focussing, and can operate as a sort of faux macro. On this trip I was weight limited, so it was the ideal choice. I am very happy with the image quality, and while I would use the 400 where I knew most of my shots would be at that range, I knew that there would be many situations where the puffins were close up, and where 400 would be much too long. So just the 100-400 it was. Seeing some colleagues suffering from three days of hiking with too many lenses justified my decision.
So I am a big fan of the 100-400, but it is always worth renting a lens to try it out before buying.
