I need expert advice! Which Nikon should I buy?

Weddings are usually in dark churches so a fast lens (3.0 minimum) and good mid/high iso performance are a must (image stabilization doesn't hurt either) . Both the Nikon D3 and D700 are good choices in this regard, especially when mounted with a good lens that unleashes what they can do (such as a 70-200 f/2.8, which covers some of the most common focal lengths in even photography). Because they're full frame, you get less noise at higher ISO's along with a ton of resolution. I personally liked the D3 more while handling it, simply due to its portrait grip, but the d700 is a good alternative as well.

edit:

forgot to mention this: I've found that 35mm or APS film is often a better medium. It offers much greater dynamic range and when scanned with a good film scanner (something almost every pro photo lab has) you can get some absolutely amazing results. My old APS Canon IX takes better pictures than my olympus 510 after the film's been scanned, and it was about 1/10th the price (lens included). Digital's biggest advantage is that you don't have to keep buying film to keep taking pictures. Thus, you may want to consider an old F5 or F6, so long as you can afford the film in the long run.

You do not need a D3, but you do need a good camera. And its usually a good idea to have two bodes. You could get a D90, D200, D300, D700 and of course the D3. A D300 with a D90 as a backup would make a great team. You do however need an f/2.8 telephoto zoom like the 70-200. You will also appreciate a mid range zoom like the 17-55 f/2.8 or 24-70 f/2.8. You might be able to get away with a mid range prime lens and a little longer prime like a 28mm or 35mm and a 50mm. Then you will probably want a wider option like a 12-24. Everything should have f/2.8 or faster maybe with the exception of the ultra wide angle. You will also want at least on dedicated flash.

you do not choose a D3, yet you do choose a stable digicam. And its in lots of circumstances a stable concept to have 2 bodes. you are able to desire to get a D90, D200, D300, D700 and of direction the D3. A D300 with a D90 as a backup might make a large team. You do however choose an f/2.8 telephoto zoom like the 70-200. you will actually rejoice with a mid selection zoom like the 17-fifty 5 f/2.8 or 24-70 f/2.8. you would be waiting to flee with a mid selection top lens and somewhat longer top like a 28mm or 35mm and a 50mm. then you will probable desire a much broader decision like a 12-24. each and every thing could desire to have f/2.8 or quicker perhaps different than for the extremely huge perspective. you will additionally choose a minimum of on committed flash.

For weddings I would recommend a well-regarded F/2.8 zoom mounted to a Nikon F5 or Nikon D700. (Keep a 50mm f/1.8 handy for those "just in case" shots.)

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