I keep thinking about thinning out my gear. But every lens has a job. I think my 10.5mm fisheye has been out with me about 2-3 times in 7 or 8 years!
Of all the lenses I've had, I actually had never owned a fisheye before about two months ago.
The opportunity presented itself for what was a great deal on probably more of a collector's piece-a Nikon 8mm f/8 Fisheye.
This is the old stye "invasive" fisheye, or in other words can only be used with mirror lock-up. It actually officially only fits a handful of Nikon bodies-basically the single digit Fs(aside from the F6) and the Nikkormat range of bodies. These are the only Nikons with real, true mechanical mirror lock-up.
Even better-or worse perhaps-for use on those bodies it comes with an external finder. This is a Nikon F era lens, and the F did not have an ISO standard hot shoe or accessory shoe, but instead had an accessory rail(with flash contacts) around the rewind crank. The F2 use the same form factor accessory rail, while the F3 used the same general design but just different enough that F/F2 accessories don't securely fit it. That means, practically speaking, this is an F or F2 lens(and really an F2 lens because of how weird MLU is on the F...) although there are adapters that let the finder fit either the F3 or a standard dimension accessory shoe.
Some brave souls will use these invasive lenses on a DSLR in live view, but you have to keep your wits about you.
They can come into their own on mirrorless, though, within reason. I have used the Fisheye on my X-T5, and it actually works decently well. There is no focus on this lens, but it's not really needed on an f/8 maximum aperture fisheye. Of course on a full frame camera an 8mm gives a 180º angle of view as a full circle in the frame with black edges. On APS-C it gives you more of an oval(full height, black horizontal edges). Of course too, no external finder is needed on mirrorless either.
Still, though, as I'm sure you know, fisheye lenses are incredibly different to use well. I've not gotten beyond the "Hey, look at how weird this looks" playing around stage.
BTW, I have another of these invasive lenses-the Nikkor-O 2.1cm. Mine is super early-probably 1959 by the SN. A few folks have used them on Z cameras, but I've not been brave enough to try on my X-T5. The rear element gets REALLY close to the shutter. In fact, it's close enough that there's an extra prong on the lens mount(that I didn't even know was a thing before getting this lens) that blocked it when new from mounting on the Nikkormats, as their Copal Square shutters were considered too "thick" and could potentially hit the rear element. I think the F3 is the newest camera it will properly mount on. The results I've seen from Z cameras, though, have been underwhelming. It's a known issue that digital, with the design of the microlenses, is very sensitive to having the rear element too close and can do crazy things. The 8mm fisheye is fine, although most of these effects show up at the frame edges where the fisheye is dark anyway. The 2.1cm of course is a full frame lens, but also sits closer than the fisheye, so using it on a mirrorless camera really isn't much more than proof of concept.
