[karmakvisser]

It seems that the Kenko rings work a bit better then the Nikon steel versions with the exception of the flexing. It seems that the flexing is mostly controlible if not disconcerting. However, the ability to meter seems to give the Kenko's an advantage. I saw some very impressive macro photography using this technique.

Here is a post I found on Nikonians.

The Kenko rings do allow autofocus, but this is not so useful in macro photography.

The Kenko rings allow metering with a D50 and CPU lenses, which to many users is more important than AF. The Nikon rings don't. You can still use the Nikon rings with lenses that have an aperture ring, but you must expose manually by trial-and-error and by checking the histogram in test shots. Metering with the Nikon rings (with some limitations) is possible only with the D200, D2 series and probably some older models like the D1 series.

If you have the older Micro Nikkor 105mm without VR and with an aperture ring, you can use the Nikon PK-11, PK-12, PK-13 and PN-11 extension rings (with manual exposure on a D50).

If you have the Micro Nikkor 105mm VR, you cannot use the Nikon extension rings and must use the Kenko ones, regardless of metering and AF, and of which camera body you have.

The bodies of the Kenko rings are made of plastic (like their teleconverters), which makes them flex a little unter stress (especially when you use all three rings stacked together). With heavy lenses, flexing may be enough to cause intermittent electrical contact between the body and lens.

The Nikon rings are all-metal and built to closer tolerances, so they are much stiffer. They have neither electrical contacts nor mechanic AF transmission, but they do have the AI aperture ring couplings (except for very old pre-AI rings like the M series) and the lever to control the diaphragm during exposure.

The Nikon PN-11 has a built-in collar with tripod socket, so it can carry heavy lenses and balance the weight well.