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I used to own canons 100-400 f/4.5 - 5.6 IS L lens (canons most popular zoom) and I loved it, ok it was affectionately known as the dust pump, but I started to notice that friends were achieving much sharper images straight from camera than I was, the reason they had primes (fixed focal) i had a zoom.
Well as you can imagine I couldn't have that so I looked into what prime I could get, surprisingly at a little under £1000 the brilliant canon 400mm f/5.6 lens was in my price range! Now this isn't the fastest of canons L series lenses, but as most of those can cost more than a small family car!! But sped aside it is a very sharp lens, and as long as the light is good, or I can up the ISO levels it is great.
Right the technical bits, at weighing 1250 grams it's quite light for a lens of its size, 90mm x 256.5mm with a filter size of 77mm, it is a good weight and length for hand holding. Made with 7 elements in 6 groups.
Right that done onto my thoughts, the lens is sharp, very sharp, even at f/5.6 it is still sharp, stepped up it gets better! Ideal for wildlife it is light enough to be carried around all day without totally killing your shoulder and can even be hand held while still achieving great shots (so long as you have a good panning technique).
There are better lenses out there but not for the price, and the best things about canons L glass is it holds its value, with second hand lenses (if you can find them)selling for not much less than new.