[davidmanze]

With the small size (DO) they do not have the room, probably, to make a longer focus path, hence the "long" MFD.
1:39X is not a thing, though. The Tamron you refer is said to do 0.26X max.mag.

       ....... not to be confused with the G1!!


https://www.tamron.eu/lenses/sp-150-600m...vc-usd-g2/

1:39X is still not a thing, no matter if you write G1 or G2. Both do 0.26x, by the way.

  What do you mean by a thing exactly  ??

It is either something is either a ratio (1:2 for instance) or a magnification (times something), like 0.5X. So not 1:39X. And 1:39 would be 0.0256x max. magnification, which does not seem right either. 

   I'm now confused too about maths .... but which ever way you roll the dice  a ratio of "1:0.14" is pretty poor on the magnification scale of things ......

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Tamron G2 MFD max mag 1:0.39 ..... magnification equals 0.39 squared = 0.1521X     ...... maybe