Alex Lofquist , Jan 09, 2009; 05:09 p.m.
Are you talking about an extention tube or an auxilliary close-up lens (diopter)? Nikon made several high quality but inexpensive attachment achromat lenses to fit on the front of the camera lens which should serve you well if you can find one. Depending o the lens diameter and magnification desired they are listed as 3T, 4T, 5T, and 6T. They are no longer manufactured but often found in camera stores or on ebay. What is the front diameter of the lens that you expect to use? What is the focal length?