Q. What are good close up lenses compatible for a Nikon D3100?
A. A short macro lens that is not good for insects but can be used for portraits, the Micro Nikkor 60mm/2.8G (not D!) gets the vote. It costs some $550, but currently it's more expensive due to the disaster in Japan. Micro-Nikkor AF-S 60mm f/2.8 G. It will autofocus with your D3100. (Don't buy the premise that you won't miss autofocus.) It gets high marks for sharpness. The focal length is useful for portraits, too.
It is less expensive than the Micro-Nikkor AF-S 105mm f/2.8G IF-ED VR. However the 60mm does lack VR, and the working distance is short for photographing live insects.
