According to DB <db002a3121@blueyonder.co.uk>:

    Alone, they will give you a single reproduction ratio for a
given fixed focal-length lens.

    They probably won't work well with most zoom lenses reversed,
Although I have not tried that.

    To get a *range* of magnifications, you will need to add either
extension tubes or a bellows.

    Your metering will most likely not work with a reversed lens,
nor will auto aperture -- it will stop down to whatever the aperture
ring is set to.  (And if it is one of the new lenses for the Nikon
digitals, you won't even *have* an aperture ring, so it will stop all
the way down.

    What the reversing of the lens does for you (mostly) is to allow
you to use the lens in the direction for which it is designed when you
want the image to be larger than the original object.  

    Aside from that, it typically does give you quite a bit larger
image with no other accessories than you could get without it or
extension tubes.

    You don't mention which camera it is for, so I can't offer any
more details.  I have the Nikon reversing ring (for 52mm filter thread
lenses), but I did a lot more of this back in my early film days with a
Miranda F and a bellows and extension tubes -- though I got a few
interesting high-magnification shots with just the 50mm f1.8 on the
reversing ring alone.  Luckily, the metering of that camera did not
depend on electrical connections to a CPU in the lens -- unlike with my
current Nikon D70.

    These days, I use an old Medical Nikkor, with close-up lenses
designed for it, and a built-in ring flash for even illumination.  This
works well with the D70.

    I hope that this is some help,
        DoN.

Is that just a male-male threaded fitting for attaching the lens
backward on the front of another lens or something that fits in the
camera's bayonete mount with threads for the front of the lens?

I've done the first with just step rings & tape, at least you can test
to see if that's suitable for your needs. I needed to either tape the
front lens aperture open part way or reach my finger around & twiddle it
manually, It'll spring to the smallest opening (too dark to see) when
hanging backward like that.

In the second case, I'm not sure that lenses are optimised properly to
work backwards, maybe someone can confirm. It's possible that's better
quality than extension rings. You should be able to get varying
magnifications with distance & focus and zoom if it's a zoom. Just hold
it in front 1/4 inch or so away & look through the viewfinder to get
some idea what focal range and magnification (pulling pack the aperture
lever to open it up with your finger).

Hi Dave,

Have a look at the following website, see under macro photography.
There is an article on reversing rings and how to make your own.
http://www.insecta.co.za
Regards,
Lambert.

These work extremely well and I'm getting 2.85:1 magnification with a 50mm
f/1.4 AI reverse mounted in front of a Micro Nikkor 105mm f/2.8 AF-D.  Here
is what it looks like.

http://www.geocities.com/ritaberk2006/pics/50_105mm.jpg

Rita