I had a friend tell me that I can reverse my 50mm prime lens for macro usage. While searching the web for an adapter that allows this, I found this post by David Chin on stacking lenses for macro use. He didn't use any special adapters, he just held his 50mm (in reverse) up against another lens. So I thought I'd give it a try.
I used my Nikkor 55-200mm zoom, attached to my camera body, with the 50mm reversed and hand held on top of it. I also used an off camera flash to get enough light to permit me to use a fairly short exposure time.
Here's a picture of a Panamanian coin, poorly focused.
To get a sense of scale, the eagle's wingspan is 14mm. I had the zoom set to 66mm here.
This is a moon snail shell. With the zoom at 66mm again.
The hole in the centre of the shell is about 0.5mm wide.
And here's part of a fax I received. The zoom is set at 200mm.
The 'e' is about 1.5mm wide.
Except for the fax, I cropped a bit out of each of these photos. As David Chin's article noted, vignetting is a problem. I noticed this especially with the shorter focal lengths. It wasn't so bad at 200mm. I used f/16 for the coin and shell, and f/25 for the text, in order to maximize depth of field. Unfortunately I can't control the aperture on the 50mm lens. It's set at f/1.8 and is stopped down by the camera right before the shutter opens.