HI
I have used most focal lengths for wildlife photos and they all have their good points and bad. The 500mm is a good choice as long as it is a fast lens. The 50-500mm is an excellent lens but is rather heavy and needs a very steady hand. The canon 100-400mmIS is a popular choice though takes a little getting used to. I have recently used a 170-500mm sigma lens which is great but not fast enough. Now I use the canon 400mm f5.6L. This is a prime lens and is VERY sharp! and very fast though lacks the versitility of a zoom but there are few times when this would be a problem.

Good to see you managed to get the 400 at last annette. :)

I was thinking maybe your kit had another dunking as we not seen much from you lately.

LOL Rob yes I have though I had to wait 5 months to get it. I initially got the 500mm because i got fed up with waiting but the 500mm doesnt measure up to the 400mmL by any stretch of the imagination. Only had it a couple of weeks and the weather has been rubbish so not really tested it fully but I have used this lens before and what i have taken have been very encouraging!

Have you got your set up sorted yet?

No not been around much as havent been well at all but have been out with both lenses.

Personally I wouldn't worry too much about IS, a good tripod will do the job just as well... that said I've never used an IS lens so perhaps I don't know what I'm missing. If you do go for an IS lens have a look at the Sigma 80-400 OS (OS is sigma's version of IS) as well as the Canon 100-400 IS.

If you can afford to go for a prime lens and are happy to carry the extra weight, I am sure you won't regret it. I changed from a Sigma 170-500 zoom to a Sigma 500 f4.5 prime last year and am very pleased that I did. True the zoom was about 1/3 the weight of the prime and could easily be used handheld, but the prime is so much sharper and faster. I haven't missed the flexability of a zoom at all, for birds I used it at 500mm 95% of the time anyway.

But really it all comes down to budget and weight - my budget was limited, but I don't mind carrying the weight so I went for a secondhand Sigma prime. If budget hadn't been an issue I'd have got a Canon 500 f4 IS. If weight had been an issue I'd have stuck with the 170-500 zoom which is a very good lens for the money.