Fish-Eye.
I have an old 3-aperture, fixed focus, 12mm Panomar on an M42 mount.
Its not a great lens. I have to take it off the camera to see what aperture I'm on.
It makes 'round' pictures... only not 'quite'... it chops the top and bottom off
And it frequently doesn't even get the circle centred on the frame.
It has big-bow front element.
Lens flare is a bludger.
You cant use any filters.
The angle of view is so wide you have to be bog off careful to make sure you look at all four corners of the frame... and tripple check them to make sure you haven;t got a finger, hanging strap, or the person stood behind you learing into the shot.
Meanwhile the magnification is so low, that anything more than about four feet away, is so tiny 'detail' is non existant, while anything less than four feet bows.
THEN there is the small matter of metering. Yeah... forget it. TTL ,metering through the thing is a wast of time. Such a wide field of view spreads the metering centre over such a huge expanse of scene as to be almost irrelevent; especially if you have, in such a wide view, a LOT of sky, or a LOT of fore-ground, or a LOT of shadow.
BUT I LOVE IT
You have to know it's foibles. Sod Sharp. Sod vignetting. Sod exposure accuracy.
This lens you HAVE to use your head, and a bit of 'craft'.
You have to look at the subject, and ponder how the lens is going to distort it.... look at it through the camera, twist it, move it, see what it does... you HAVE to loo at everything 'differently' and anew every time.
Then.... you have to look at the scene, and weigh it up. Ponder what the meter suggests. Look at the scene again. Look through the lens, ponder how much bright sky you have, or how much shaddow in the corners.... maybe get a hand-held meter out, or pop on another lens and compare EV's! Then look again, and ponder some more.
THEN when you are through pondering..... you have to make your final composition... peer at what is in the center looking very very small.... then check the edges.... and the corners, and the edges again.... and then you can press the shutter!
And what you get?
Some-times!
It's just 'fun'.
I got it, ooohhh... about 1995 and the novelty still hasn't worn off.