The AIS 180/2.8 ED has unit focusing as Erik says. The lens is optimised for far-medium distances, at close range the performance starts to decline since the optics don't adjust for close range (no floating elements), which may be why the close focus is limited to a rather long 1.8m. Performance still remains very good provided you stop down a little. When using extension tubes, the performance at close range continues to decline as the amount of extension is increased. Even with the PN-11 (52.5mm extension, plus built-in extension of the lens focused close), quality results can be obtained at medium apertures. This is one of the combinations John Shaw used for long distance closeups.
The AF 180/2.8 has IF focusing - the inner focusing optics move with respect to the fixed front and rear elements. This allows the lens to be better corrected over a wider range of focus distances, so the focus limit was reduced to 1.5m. When using this lens on extension tubes, if the lens is "focused" at infinity the optics will be corrected for far distances, even though the whole setup now focuses relatively close, so the results will be less good (similar to the unit focusing AIS 180/2.8 ED). However if the lens is focused close, the correction will improve and you should get better results. In general, when using any lens with floating elements (CRC, IF, RF) on extension tubes, you will get best results if the lens is also focused close and not at "infinity". But don't believe me, try it for yourself and see what happens.
If you intend to do a lot of closeups with this focal length, it may be best to investigate the 180mm and 200mm macro lenses from Nikon, Sigma and Tamron. The AFS 300/4 (VR and non-VR) also focus very close for non-macro lenses (to about 1:4 magnification) so are very useful for closeups.
