The Nikkor 200-400/4 is large and heavy because it is a constant f/4 throughout its focal range. Since f-number = focal length / entrance pupil diameter, it follows that
front element diameter > focal length / f-number.
So, for a 400/4 lens, the front element must be at least 100mm diameter (and is often larger due to other optical and mechanical considerations). This is why, in theory, one could make a 200-400/2.8-5.6 with a 77mm filter diameter, but it is unlikely because those "other" considerations usually mean that it is not possible to sustain a near-maximal theoretical aperture throughout the entire focal range, and some compromise is necessary. After all, you need space for the cams, gears, and electronics to operate the diaphragm and focus/zoom the elements. A more realistic design would be something like 200-400/3.5-5.6. I would not care much for a 200-400/4-5.6 unless such a lens had superb imaging quality over a more aggressive design.
Another possibility, though perhaps less appealing, is to design for an 82mm filter diameter and try something like a 200-400/2.8-5.0. But now you've made the lens heavier and bulkier, and 82mm is not a very popular filter size.
On a somewhat different note, I don't know why, but I personally find non-Canon lenses to be rather...ugly, Nikkors especially. Nikon bodies seem ugly to me, too. There's something about their shape and proportion that makes them look "wrong" somehow and I can't put my finger on it. Conversely, Canon lenses are very pleasing to my eye. I mean, just look at the 85/1.2L II. It's just...beautiful. Like a fine crystal paperweight. About as heavy and solid as one, too. If I had one in my camera bag, it could double as a blunt weapon in case I ever got mugged. (Just kidding, I could never bring myself to throw a $1700 lens.) But back to my point, you don't see Nikon making that kind of gear, do you? []
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Originally Posted by Daniel Browning
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