Hi, thank you for contacting JustAnswer.com. My name is Russell. I will do my best to provide the right answer to your question.
If the new lens lacks sharpness, there are two questions:
1. is it fully compatible with the camera body? please mention the maker and model # so that I can tell you about this point.
2. if it is really not focusing sharply, is that with Automatic focusing, or is that with Manual focusing also?
- if only lacking sharpness in Auto Mode, then it may need warranty repair (for free, to answer your question) by Nikon.
- if it lacks sharpness in Auto Mode and in Manual Mode both, then it is a defective lens (optically, most likely) then it should be returned for an exchange as such, and exchange for another of the same capability or equivalent... provided it is still recently bought enough for this to be a valid approach. (Return/exchange provisions usually expire very soon after purchase, some as short as 7 days and depending upon the store.)
Let me know, in case I need to provide more helpful information for this problem. Thanks!