Hi folks, First post here and I'm asking what guess is going to be the near impossible question for people to answer of which lens should I go for. My current camera is a 5D Mk II and I'm looking to replace the 75-300mm f4-5.6 Mk III lens that I inherited from my dad. I'm a generalist with my photography, I like to shoot wideangle, a bit of maco, and so on and so forth. At the moment I'm mainly using the 75-300 on trips to the zoo or safari park but have also used it recently on holiday to photograph birds at a feeder (during a visit to some gardens that had a hide set up for that purpose) and attempted to photograph the moon. I've used it at falconry displays and I'm hoping to get to the odd air show now and then now my eldest daughter is old enough that she shouldn't be upset by the noise. The main reason I'm looking to replace the 75-300 is, as you may have guessed from the list of things I'm using it for, is that I'm looking for something that goes longer. Often times at safari parks I find 300mm is a quite lacking and I need to crop in quite a bit to get a usable image. Needless to say, the lack of IS means I have to try and remember to boost the ISO rating a bit more than I'd otherwise like to when the sun inevitably decides to hide... As I hope you may have guessed from the lenses mentioned in the subject line, I'm on a bit of a budget right now and for the foreseeable future and so I'm considering the Sigma 150-600 Contemporary or a 2nd hand 100-400 Mk I as the prices aren't too dissimilar. While the 100-400 obviously doesn't go as long, I do have the Mk I Canon 1.4 TC (bought 2nd hand years ago when I was planning to get the 300 f4 but then forgotten about as my main interest shifted to other things) which would get me to 560mm. While this would put the effective aperture down to f8 (if my calculations are correct) meaning I'd have to focus manually, I'm not sure how much of a disadvantage I'd find that versus the 150-600 as I'd have to manual focus manually anyway at 600mm. While it's going to be a few months yet until I make a purchase, I'm hoping to inform myself as much as possible beforehand. I'm also keen to get a better idea of which I'm likely to go for as I'm hoping to get a new photo backpack for my birthday in a couple of months time and I'd like to get one that the lens I get will fit in. So, any thoughts and advice gratefully received.