A Nikon N75 lens will be fully compatible with your D80.
It will certainly mount on the D40 and it will meter. However, unless it is an AF-S type lens with the built-in motor, a Sigma HSM lens with the built-in motor or a Nikon AF-I lens, you'll have to manually focus.

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Please Help to setup my Nikon d80 camera for taking jewelry picture. how i set my camera to take jewelry picture. please give all setup. i'm using closeup lense with lighten box.
You have asked this question already and I have sent you a link to the best advice I could find at -
http://www.tabletopstudio.com/documents/jewelry_photography.htm
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What are 2 lenses I should start out with for my camera? I would like something for everyday use and something for zooming in long distance. Under $500 would be nice.
What is difference between 70-300mm and 75-300mm?
Is a VR a must have?
Hi Cincy. Great Questions!! There is no real need to [...]