Peter Gilbert , Aug 08, 2012; 09:03 a.m.
IMO the best macro lenses are the old manual focus lenses made of real metal and with proper focus scales and focusing helicoids. None of this modern fly by wire Wheel of Fortune focusing for me.
I'm using a Tamron 90mm f2.8 Adaptall 2. I have this with a Nikon Adaptall adapter (plus Nikon to NEX E adapter), plus the Nikon PK13 and PM5 extension rings; and I also use a Fotodiox Adaptall 2 to NEX E adapter directly. I'm sure any older MF Canon, Pentax or Nikon macro lens would be just fine for you.
With the excellent touch screen focus (on my 5N; I understand your 7 does not have that, too bad) and MF assist with magnification I'm a real happy camper.
If you decide to stay with a native E mount lens, I see that Kenko now makes auto extension tubes for the NEX. You need to stick with a prime, and the longest focal length you can get so you currently have a choice of one..the 50mm you mentioned. Rumor is that a "mid tele" lens is coming, maybe around 70 to 90mm.
I bolded the word macro since I assume you really do mean macro (1:1 or larger) and not "close up" (1:3 or so)
Just for giggles, here is a shot made with the 5N and Tamron