Nikon 15mm f/3.5

This is an amazing wide angle lens. No other reasonably priced mass produced manual focus wide lens can touch it. It is a rectilinear lens which means it won't bend or distort straight lines. As long as the subject is 10 feet away you will not see any barrel distortion. Not many extreme wide angle lenses I know of can claim to have no barrel distortion.

Mounted on Sony NEX-7 with Nikon to NEX adapter - looks huge

This Nikkor AI-s lens is not for everyone. First of all, it will set you back close to $1000 used. Secondly, on a full frame or FX camera it is quite wide. It may be difficult  to compose a decent photo that includes so much area.

On DX, APS-C or micro4/3 cameras it makes a great wide angle manual focus lens.

There are times when no other lens will work. When shooting indoor architectural photography this lens shines. It will make the rooms look big without adding any distortion.

For HD video this lens can make some wonderful wide shots for indoor sequences. In fact, this is my favorite lens for indoor establishing shots - it's wide enough to include everything you might want to include in a room, with no linear distortion. Your just going to have to move the lights a little farther away from the subject.

15mm in my hand

On a Sony NEX-7 or similar camera this lens looks big, but it's not unruly. FoV on APS-C cameras is around 22mm. Close focus limit is 1 foot, which works wonderful in tight spots.

Below is a comparison of the Nikon 15mm f/3.5 and the Nikon 50mm f/1.2 lens. Both photos were taken at approximately the same distance from the subject. I'm standing close to 5 feet from the back wall, and a little more than 1.5 feet from my son Hatuey.

A little more than 1.5 feet from subject - Nikon 15mm f/3.5

Approximately same distance from subject - Nikon 50mm f/1.2

Nikon 15mm f/3.5 - notice horizon, no distortion