Why buy an awkward heavy, slow 600 when, for only a few hundred dollars more, you can get better picture quality even after cropping from the superb-in-all-respects Canon 100-400 II? It is vastly easier to handle, much lighter, has better balance, superior IS (IS Mode 3 is a revelation!), significantly faster (yes, one-third of a stop is a significant difference), and has better image quality. In short, it is a no-brainer.

See for yourself: https://www.the-digital-picture.com/...mp=7&APIComp=2

If you really want to go beyond what a 100-400 can offer you, then you need to look at fair-dinkum big lenses, such as 500/4, 600/4, and 400/2.8. Both Canon and Sigma make lenses in this class ... for a price. However you should be able to pick up something like a second-hand Canon 500/4 IS Mark 1 for around about the price of a new 100-400 (or Sigma 150-600). It takes you into a whole different world.

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PS: I knew that thread was around here somewhere: http://www.ausphotography.net.au/for...aithful-friend Some (I think) useful observations on lenses there.