new Metabones adapter gives nex crop factor higher-quality full frame look.

You're a slightly off (see http://en.wikipedia.org/wiki/F-number#Notation):

f-number is focal length / aperture diameter, but the f-number is expressed as f/X, where X is the previously calculated ratio.

So in your example, with focal length 10mm, and diameter 20mm, the f-number is 1/2, which would be expressed as f/0.5, not f/2.

Decreasing the focal length to 5mm then gives you f/0.25, not f/4.

HTH