Hi Tim
Thanks for your information The answers to some of the questions are:
I would be shooting in all types of weather dull and sunny. I could also be shooting in low light situations and be shooting in all different ranges .
I would like to capture dogs running and jumping both head shots and whole body.
Any advice would be most appreciated
Thanks
Barbury
OK, so let's assume for a headshot you want a field of view of 1' and for a running dog with a bit of space around you want a field of view of 10'. Now, since you haven't specified shooting distance I'll assume a range between 20' and 150'.
At a distance of 20', in order to achieve an FOV of 1' you'd need a focal length of 450mm.
At a distance of 20', in order to achieve an FOV of 10' you'd need a focal length of 45mm.
At a distance of 150', in order to achieve an FOV of 1' you'd need a focal length of 3370mm. (not easy to find or afford, and you probably won't be expecting headshots at 150', fortunately)
At a distance of 150', in order to achieve an FOV of 10' you'd need a focal length of 337mm.
So it looks like a focal length range of around 50-450mm would serve you well. Probably the closest is the Sigma 50-500, but that is a slow lens, with an aperture of only f/6.3 at the long end. By the sounds of things you'd be much better off with an f/2.8 lens, with a constant max aperture as well. But you won't get such a lens to cover anywhere near the zoom range my calculations suggest you might need. Probably the best solution would be a 70-200/2.8 plus a 1.4X teleconverter for the longer shots, or a Sigma 120-300/2.8, which would be better for the longer shots, but you won't be able to get a wide FOV close up.
I also have no idea of the focusing performance of the Sigma lens, and whether it is fast and accurate, since you will have quick and demanding subjects to track, so you'll have to turn elsewhere for the answers to that. I would think on balance that a longer lens will serve you better. You'll get better subject isolation, from the narrower FOV, and probably end up needing to crop less, and can probably avoid using a teleconverter completely. So much really depends on your distance from the dogs. Shooting at greater distance will actually make your job easier, and simplify things for the AF too.
If you were to get the Sigma 120-300 then at 20' you would have an FOV of 3'9 x 2'6, which will give you a fairly tight frame on a dog running towards you, but not so tight as to be a problem. It would be very snug for a dog that was running across your FOV and which you were panning. Maybe too snug. Of course, if you are further back than 20' then the problem reduces. It really is important to know your shooting distances in order to pick the optimum lens solution. You've got a decent budget, so it's definitely worth putting in the effort to pick the right lens. The Canon 100-400 might be ideal in focal length, but that f/5.6 aperture could be a worry in poor light, with focusing being less assured and needing a good old dose of ISO to keep the shutter speed up.
If you can narrow down the range of distance you might even find that a fast prime would suit better, like a 135/2L. But really, since I don't know your shooting distances, I don't know what would be best for you. I wish you luck.
