The 6T is a 2.9 dioptre lens with a 62 mm thread. Costs approximately $US50 (as of May 2004).
Available from Adorama or B&H PhotoVideo, among other places.
Nikon also has a 4T, which is a 2.9 dioptre lens with a 52 mm thread and a 5T (1.5 dioptre, 62 mm thread).
Howard Owen quotes from the 6T manual...
``Both the No. 5T and the more powerful No. 6T close-up lenses are designed primarily for use with telephoto lenses ranging in focal length from 70mm to 210mm. (Although the accompanying lens to subject distant tables includes lenses of 28mm upto 300mm)...Nos. 5T and 6T are threaded on the front to accept filters, lens hoods, or a second close-up lens.''
There is also a quote relating to using both the 5T and 6T together :
``Nos 5T and 6T may be screwed together and used in combination to further extend the range of obtainable reproduction ratios, although this is not recommended for critical work.''Bee photographs by art(less) -- taken from approximately one foot away from the subject
Ron Hautau's gallery -- more insect photographs
Paul Milholland (Yahoo!, FZ10 group, 23rd February 2005)...
With the 6T on, the maximum distance you can be from a subject is just about a foot. You can be closer than that if you aren't zoomed out very far, but once you reach a certain point you're pretty much locked into the 1-foot range. You can vary your framing by zooming in or out, and at 12X you're zeroed in on an area about an inch wide by three-quarters of an inch high.
It's seems a little hit-and-miss at first, and a tripod really helps for macro shots with a close-up lens. Although the extra legs help with camera shake at slow shutter speeds, they're actually handier for the fact that they lock you into the 1-foot distance range. Since the focusing zone is very shallow with a close-up lens, it's pretty hard not to float in and out of it when you're scrunched down for a handheld flower or bug shot.
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P.S. -- Not trying to get you to spend more money, but if the 6T still seems finicky after you've practiced with it, the 5T allows you some wiggle room in terms of subject distance. You can be anywhere in the 18 to 24 inch range, and still be able to focus. At 12X you can zero in on an area about 1-2/3 inch wide by 1-1/4 high. Not quite as much magnification as the 6T, but that's still a pretty small area, and the wiggle room makes the 5T a little easier lens to get good results with.David de Saint Michel demonstrates stacking Nikon 6T and Sigma macro lenses, complete with example photographs.
tchuanye also demonstrates the stacking of a Nikon 6T and Sigma achromatic macro lens. To quote from his photo-gallery website...
The Nikon 6T is a +2.9 diopters lens giving a magnification of 1.218 at x12 zoom. The Sigma is a +1.6 diopters lens with a magnification of x0.672 at x12 zoom. Together, when stacked, the total diopter is +4.5, and magnification of 1.89 at x12 zoom.