Zeiss Distagon 28mm c/y on NEX 5N
28/2.8 C/Y Distagon is one of the very best lenses in this class,doesn't matter APS-C or FF sensor.
I currently have two 28/2.8 MMJ Distagons and two NEX5N bodies.Used to have one more 28/2.8 AEG Distagon ,two NEX3 and one NEX5 bodies,so some real life experience.With the adapter it is slightly smaller than 18-55 kit,very solid and compact.
On a NEX it is sharp and contrasty with excellent color rendition - typical Zeiss.
With NEX5N it's performance is a bit better compared to "classic" NEX5/3 especially in the corners due to improved sensor's microlenses and thinner low pass filter which gives overall impression of increased sharpness.
As for Biogon 28/2.8 - this is a very good lens as well,smaller than 28/2.8 Dist but with three gotchas important for me
1.Unlike 28/2.8 Dist you can not focus the lens without the aid of adapter (no focusing ring on the lens)
2.Biogon is not retrofocal lens so
corners are slightly softer compared to Distagon,esp wide opened
3.You can use Biogon (i.e. in full ,reaching infinity included) only with native Contax G bodies or mirrorless cameras (NEX,Pen,Panny ect) but not with DSLRs like Canon or Nikon ("leitaxed" lenses only).
Adapters - I buy them in Hong Kong and/or China.They are cheap and usually good quality,but you can also choose from some more respectable (and more expensive) brands.