The 7.5mm f5.6 is a hemispheric fisheye lens that provides a full 180 degrees of view in a circular image that is 23mm in diameter on the film. This lens will see everything in front and next to the lens. The effects are dramatic, but of limited utility.
This is not your typical Nikkor however. It is not a retro-focus design, and therefore the rear elements are very close to the film plane of the camera. In order for this lens to even be mounted on the camera, you need to lock the mirror up. The official Nikon line is that this camera is only to be mounted on the F and F2. I will add that if you have a Photomic finder on a Nikon F, you will have to remove it as it will get in the way. It will work with the Photomic finders of the F2 however. I can see no reason that the lens will not mount on the F3 or F4 which have full time, mechanical mirror lock up. Besides having limited compatibility, the camera design also precludes the use of TTL metering.
To use this lens, you need to use an external meter or the Sunny 16 rule. When I use this lens, I use a hand-held meter. The MLU also precludes the use of a view finder, so Nikon thoughtfully included one with the lens. The viewfinder is not exact however, giving a 120 degree angle of view. I find it nearly useless. If it will capture everything in front and on the side of you, you just need to align the camera and level it and try and avoid getting your fingers or feet in the frame. The finder fits over the rewind crank of the F and F2 but some have been modified to fit in the standard hot shoe.
This lens is also of a fixed focus design so all elements are fixed. Fortunately with such a short focal length, even wide open at f5.6, pretty much everything will be in focus. The front element of this lens, and the viewfinder for that matter, is very bulbous and sticks out. There is no ability to filter the lens and no lens shade in made. This makes the lens very susceptible to damage. You need to put the screw-on lens cap on as soon as you are done shooting to avoid damage. I do the same with the viewfinder lens cap. Since there is no ability to filter the front element of this lens, Nikon installed a filter turret inside of the lens that is selected via a wheel on the lens barrel. Sadly for us, this lens was made in the later 1960's so most of the filters are really use useful when using black and white films. The filters include a L1A for normal shooting, Yellow 48, Yellow 52, Orange 57, Red 60, and XO.
Construction of this lens is first rate, as was the norm with Nikkors of that era. The lens barrel is metal with a small rubber ring near the lens cap threads. There are six aperture blades that are not rounded and if you get a point light in the view, which is pretty easy when you see half of the world through it, you get a rather ugly flare. The lens coatings are older so the lens does lose some contrast if it flares. The aperture is selected by a lever on the side of the lens and it can be difficult to adjust when it's on the camera. The aperture range is from 5.6 to f22. I like to shoot it from f8 to f11.
I am not a huge fan of writing about image quality. Still, I will write what I have found, but keep in mind that I don't do scientific tests, or even under controlled conditions. Also note that I have only used this lens on film cameras, so the high density pixel DSLRs might display more flaws than I see when I shoot film. The lens is sharp enough for my film use. While it isn't tack sharp, it will pass at proper viewing distances for film under most enlargements. Contrast is moderate, but it can suffer from flare which reduces contrast. There is some vignetting which appears as a ring around the edge of the image circle. This can be reduced slightly by stopping down, but since the angle of incidence of the light rays is so small, there will be some light loss no matter what. There is little CA that I can observe. There is major barrel distortion but that is normal for fisheyes.
While the lens works fairly well, it is more of a collectors piece than a working lens, especially these days. The going rate for these lenses is about $1,000, and it's very limited utility and shortcomings such as no TTL viewfinding or metering make 3rd party fisheye lenses that have retrofocus designs a more cost effective option. The newer optics are much easier to use, and will probably offer image quality as good, or better than this lens.
I have also included a link to a YouTube video I made for operating the lens, and also some photos I shot of, and with, the lens itself.
7.5mm Nikkor mounting video
