My suggestion would be to think in terms of 1) focal length vs. amount of extension, 2) potential problems due to proximity of the lens to the subject, and 3) finally any exposure compensation that might be needed.

At infinity, assuming a conventional lens design, the focusing mechanism of the camera will place the typical lens at approximately its focal length away from the camera. For 1:1 reproduction, that distance doubles. So, for example, with the 80mm lens, you'd need a total extension of 160mm (the combined distande of the extension tube and the focusing mechanism within the lens). The lens to subject distance is similarly 2x the focal length for 1:1 magnification.

Thus, the shorter extension tubes (e.g. 8mm, 16mm, etc.) are really intended to allow somewhat closer focusing of standard lenses for non-macro shots. For these purposes, the Proxar close-up lenses are another alternative. I use a 24mm extension tube, for example, with the 180mm lens in order to get tight headshots that would otherwise fall inside the lens's closest focusing range.

For "true" macro work, you may find the Hassy bellows will provide far more flexibility. But, extension tubes are much less expensive.

Within your set of lenses, the 80mm is probably the most suited, even though it is not optimized for close work. With the 50mm lens, you'll be right on top of the subject, making it more difficult to light the subject well. The 180mm would provide far greater distance, but require too much extension to be practical for work approaching 1:1.

For an in-depth discussion of the compensation needed because of the additional extension, take a look at http://www.largeformatphotography.in...ws-factor.html

There's also a PDF on the Hasselbald site at http://www.hasselbladusa.com/Archive...on/Closeup.pdf that includes helpful information on the various extension tubes, along with exposure compensation figures.