I received a PM a couple of days ago asking for some input as to what equipment I thought would be appropriate for photographing dogs performing in the field. From the question, I assumed that the person asking wants to be able to get good, close photos of their dog running retrieves either at trials, tests, or on training days. He had seen some of the images I have posted, and probably some of Tony Zappia's and a few others who post here. I know from Tony's posts that those amazing photos he has of dogs pouncing in the water were NOT taken at tests or trials, but were (as were the close shots of my dog pouncing) taken by setting up conditions conducive for photography. I have shot two hunt tests, and NO trials. The distances involved even with junior dogs at Tests are actually pretty extreme.
The following four images were taken at those tests and are full frame, un-cropped files. All were shot using a 600mm lens (that's a huge lens). All were taken from pretty close to the line (probably no more than 10 or 15 feet back.) The other salient points here are: 1. the dogs already have birds (so they are obviously on their way back), and 2. they are relatively small in the frame. If you want photos of your dog picking up the bird, you probably just aren't going to get it at a Test, and no way at a trial.
This next photo was taken during a photo session using a 300mm lens, and as you can see, fills the frame much more, but I could decide what distance I was going to be from the dog and didn't have to worry about interfering with him.
So this leads me to three points. If you want to take shots like this one, or like the ones Tony Zappia has posted, you don't really need a gigantic lens, probably a 200mm would be sufficient if you are willing to squat in the pond to get the shot. If you want photos of your dog running in with a bird in its mouth, you still don't need a huge lens, just wait until the dog gets close. If you want to take photos of your dog while it is way out in the field, you need a really extreme lens, or....... you can get one of the new higher end point and shoot cameras with an extreme zoom. Nikon, Canon, and Sony all make them with lenses that are at least equivalent to an 800mm - 1000mm on a DSLR (still probably not long enough to get that close up of the dog pouncing on the bird at a test). When using one of the point and shoots zoomed way in (400mm +) you really will have to be on a tripod even if they have image stabilization. They are just so small and light that there is no inertia to help you hold them steady. Hope this helps.