As I understand this thing, it's removable. Is it there as a protective thing? Does anyone ever put a protective UV or whatever on this lens?
I have an upcoming swim meet and I want to use this lens; but the crowds around the pool are terrible; I need to be "mobile" and would like to shoot w/o the hood.
You can remove the front element quite easily. I do this but have never done a side-by-side test to see if it makes any improvement, I have just heard it's best to do. As far as I can tell it's just there for protection as it looks like a huge UV filter.
Be careful with the hood though; that is one spendy item. It's like $300.
According to Thom Hogan's review of the 200-400, the removable front element is just for protection. Thom claims that removing it actually improves far-distance shooting a little.
The "big guns" of Nikon all have this front element, although usually not removable. It's purpose is to protect "true" front element (first piece in the optical formula). If you look at the lens diagrams all the big lenses use ED glass as the first element. ED glass is more fragile than regular optical glass so this protective element is necessary. If you look closely most of these protective front elements are actually slightly curved into meniscuses (menisci?) to help reduce reflections, unlike a standard screw-on filter. Nikon probably has to design them into the lens themselves since they are far larger than most screw-on filters and thus are not commercially available.
The 200-400's removable front element seems to be unique since the frontmost element (aside from the protective piece) is the only one of the long lenses not made of ED glass, thus they may have decided to make it removable since the element that would then be exposed is not the more fragile ED glass but instead standard optical glass. That's one theory at least.
