The artist who painted/drew these portraits mimicked the effect photographers get when using little DoF - so to this artist finds there is some 3D effect in at least some photographs. Why otherwise try to get the same in a painting/drawing?
Correct! Those portraits are in fact photos of drawings of photos of real people. There's a thriving industry in the night markets of Chiang Mai where a small group of artists specialise in reproducing as faithfully as possible, with pencil and charcoal brush, any photograph you give them. They'll attempt to reproduce every hair and wrinkle, and the results are often quite amazing.
[attachment=18143:6074.jpg]
In my opinion, some of these drawings give a clue as to the nature of that 'enhanced' 3D effect that some people tend to notice in MF images. Shallow DoF certainly plays a role, but also a sense of greater accutance in the parts of the image that are in focus. This combination of greater accutance and shallow DoF would result from use of a good MF lens at wide apertures which is also sharp at such wide apertures.
Even though a particular MF lens at, say F4, may be no sharper than an equivalent 35mm format lens at F2.8, the larger sensor of MFDB will ensure equally shallow DoF and the wider pixel spacing (or larger sensor) will result in a higher MTF (than 35mm) at the same 'picture' resolution ( ie. same number of line widths per picture height).
In addition, the lack of an AA filter on the MFDB, will extend such resolution to the Nyquist limit and even create a bit of additional, 'false' resolution which may also contribute the the 'enhanced' 3D effect.