I wouls say keep away from those things, I have one of the fisheye adapters I bought for my old pns andthe quality is very bad and the added weight at the end of your lens is very akward. You have a d40x right? So you should have an 18-55mm and that should do fine unless you really need a wider angle.
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"Thirty-six satisfactory exposures on a roll means a
photographer is not trying anything new." - Freeman Patterson