Choosing Canon 400mm prime or 100-400mm zoom lens? Help!

[MeadsMan]

I went for the prime,because for me quality is at the top of the list,the only down side of the prime,is if things get to close,it has happened to me now and again,but not very often.I can imagine it being more probmatic where things are more tame,such as parks and gardens,but I have used it in my garden,wich is not huge by any standards,only large birds like Wood Pigeons posed any over frame filling problems.Is there also not an issue with the Zoom putting dert on the sensor ?

[Sam_swfc123]

Thank you very much for all your replys, been extremely helpful! One last question though. What do you reckon to the Sigma 150-500mm OS HSM lens?because I wouldn't be able to get a Canon lens till christmas, but I would be able to get a sigma lens next month? Please comment. Thank you
regards,
Sam

[glsammy]

The Same comments apply. The image quality will not be as good as with a Canon 400mm F5.6. Don't get me wrong, it's a fine lens but it simply will not be able to produce shots as good as the prime lens.
It does of course has OS, only you can decide if this is important to you.

[nigel_b]

As I found with my 500mm prime, its usually easier to take a step back if you have too much lens than step forward if you have too little. The only issue I found was it can sometimes be fearsomely difficult to line up a shot with such a narrow field of view and here the zoom can certainly help you.