Finally made the decision to buy a Canon 7D for my wife. Her photography is continually improving. Flying bird and distance photography with her existing canon EOS 450D and 70 -300 lens. Can someone help me make a decision please?
According to the respective lenstip reviews, the 50-500 is produces somewhat better image quality than the 150-500 counterpart. However, it is bigger, heavier and about $600 more expensive. The extra 100mm of range on the wide end is not likely to be a particular advantage to you, but I guess could possibly be useful.
In short, if weight and price are not concerns, go with the 50-500. Otherwise, you may wish to consider the smaller lens.
If weight is more important than focal range (and I note that you already have a 70-300), you might also want to consider the Canon EF 400 mm f5.6 L USM (1,250 g) with or without the Canon EF 1.4x III Extender (225 g). Even with tht extender (which will take it out to an equivalent of a 560mm f/8) it will weigh less than either of the Sigma zooms, although you will lose autofocus which is problematic.
Having used both, I'd say the 50-500 is the one you want from those two unless weight is an important factor.
It's a bit sharper and offers less distortion at the long end, it's also smaller and therefore easier to handle in tight spaces like a blind. Most important however, the AF is faster.
That said, you might want to consider a Sigma 70-200 f/2.8 EX with a Sigma EX 2x teleconverter, or a Canon 100-400 instead. Both options are optically superior, faster focussing, and generally more versatile than either the 170-500 or the 50-500.
While you're not having the extreme 500mm range with those options, in my experience it's not something you really will miss. If you can't get close enough with 400mm, 500 isn't going to make that much of a difference. At most you may lose out on some detail shots (heads for example).
I can't vouch for the quality of the Canon lens (being a Nikon user), but others I know who have it are happy with it. I do however have the Sigma 70-200 with teleconverter and am quite happy with it.
Cropped shot taken with the 170-500: http://www.usefilm.com/image/1294988.html
With the 70-200+TC (not cropped): http://www.usefilm.com/image/1319700.html
Different situations, the swans were a lot further away, but gives some idea as to the relative optical quality.
I am a Nikon user and also looking for a high focal length lens, hopefully up to 500mm and have been talking to folks on the Nikonians web site about it. One fellow's opinion on the Sigmas is that he likes the 50-500 has thousands of images with it and has blown up prints with it and is very satisfied.
Regarding Canon, I can also tell you that -every member- of our camera club who uses Canon, actually has the 100-400 lens, which has the stabilizer and a wide f stop range of 4-5.6, which is better than both Sigmas. The lens is rated sharp and I have seem blow ups taken by these members and they are quite good.
Unfortunately for me, the Nikon 200-400 is very (too) expensive for consideration, unless I can win the next Megamillions.
Dennis -- I'm also a bird photographer who uses a 7d. I'd recommend the 100-400 over both Sigma lenses, but a few months ago, I sold mine and I now use a 300F4 + a 1.4x teleconverter for my bird photography. That is a cost effective setup, and I find that the 300+1.4 is sharper and a faster focus than the 100-400, and it's a LOT cheaper than the 400 MM lenses. Both of these are good values and will work well in bird photography. If you really feel you want to go to 500mm, the 50-500 is the one the bird photographers I know normally go for but most also say at 500mm it can be soft.
I would recommend looking at the 300mm F4 from Canon, and add the 1.4x teleconverter to it.
@Dennis you may want to check this post, for similar advice that I asked month ago ;)