Right, but strap one of those 500mm f/4 puppies onto your D3 and you can make up for light lost to slower glass by crankin' the ISO without having to cringe at the results. That way you can fill the frame with tweetie *and* you get to sit put.
Sorry, Piyono, this isn't making any sense to me. Let's translate it step by step and see where it gets us.
Put a 500/4 (standard big, fast birding lens, indeed
the standard birding lens, assuming you can afford one) on a D3 (a camera that, together with the 5D, delivers less pixels on the bird than any other current SLR). OK, so far we have a big fast lens matched with a low pixel-density body. We can still get good shots of medium to large birds, but we have to be quite a bit closer than it is usually possible to be. With small birds, we are completely screwed because we have to be so close that we can't focus on them. (MFD is around 4 metres with a 500mm lens.)
And you can make up for light lost to slower glass. Now you have lost
me. What slower glass? f/4 is the fastest glass there is in 500mm, it delivers more light than any other lens of equal length. A 500mm f/2.8 would cost maybe US$20,000 and weigh enough to break your back. No-one makes them. (Though there are 400/2.8 lenses: Nikon and Canon both have them; they weigh twice as much as a 500/4 and cost 50% more. They are seldom used for birding and are primarily designed for sport - stuff like night football games.)
What you
could do with a D3 that you couldn't do with most other cameras is use a
slower lens: something like the 50-500 Sigma which is f/6.3 at the long end (and only about 465mm, not actually 500mm at all), relying on the excellent high-ISO capabilities of the D3 to deliver good results. The advantage would be that the slower lens is much,
much smaller and lighter than a 500/4. The disadvantage is that you have sacrificed a lot of effective focal length - you could just as easily use an equally small and light 400/4 on a D300 or 40D, or possibly even a 300/2.8. (If I get bored I'll do the sums for you a little later on.)
A better solution if you want excellent high-ISO performance without greatly compromising your reach is to use a ID III, which is almost as good at high ISO as the D3, and puts a significantly greater number of pixels on the bird.
Don't get me wrong here: the D3 (at least on paper, I haven't seen one in the flesh) is a superb camera that has a lot of people very excited. But it is not by any stretch of the imagination the camera you would design for bird photography. Of the current Nikon and Canon ranges, it's quite possibly the last camera you would pick for birding. Sorry, make that second-last, I momentarily forgot the Canon 5D.
In summary, buying a D3 for birding would be like buying the world's best 9 iron - for driving off the tee on long par fives. Great tool, but entirely the wrong job for it.
