The specifications are given by the name of the lens: The lens projects an image circle of 180° field of view, and has a maximum aperture of f/4. The focal length is 7.3mm. This is pretty much all you need to know about a fisheye lens. I'll get back to the significance of the focal length later.
To understand just how wide this lens is, I compare it with the 16-50mm f/3.5-5.6 kit zoom lens at 16mm, which is already quite wide. The images are taken on a tripod at exactly the same spot, using the Sony NEX-3N:
|Sony E 16-50mm f3.5-5.6 OSS @ 16mm f/8||Yasuhara Madoka 180 @ f/8|
The lens appears fairly solidly made, with a metal construction in a matte black finish. The supplied lens cover is plastic, and fits over the lens by means of friction, which works just fine: