Just stumbled over this site again;
http://www.pierretoscani.com/focale.html#focale37Animation of how the Micro Nikkor 105mm AF-S VR looses it's focal length as it focuses,,,
One of those lenses that I returned shortly after testing it out - and newer looking back!
The older AF 105/2.8 micro also looses focal length at close range - even more than the AF-S VR version, but for some reason this is more often mentioned as a disadvantage for the AF-S model. Maybe this can be explained as the AF 105 micro has a gentle loss of focal length across the entire focus range, while the focal length of the AF-S version remains constant at medium-far distances but then drops more rapidly at close range.
One thing not mentioned is the rear 1.4x converter of the AF 105 micro is not a fixed unit. From Nikon 1001 nights article:
https://imaging.nikon.com/history/story/0072/index.htm"Gauss-type structure comprised of six elements in five groups with a three-element teleconverter behind this main structure. Three elements in front of the aperture, three elements behind the aperture,
and the front two elements in the teleconverter structure combine to form a total of three groups that each moves independently to focus from close-up to infinity."
Observing my lens I can see that as the lens is focused closer, the front two elements of the rear converter move rearwards, then stop and move forward as the lens approaches 1:1, finally arriving at more or less the same position as it started. This complex design must give a high degree of correction across the focus range providing very good optical quality. Perhaps over-corrected as the background bokeh at medium-far distances is rough. And lack of ED and other special glasses means it is not colour-corrected as well as more recent lenses, so some CA is observed.
