Mamiya Sekor-C 37mm f/4.5 Fisheye for RB - what a fun lens...
I've been using this lens for almost a year. from an easthetic and build quality point of view, it has the most gorgeous glass I have ever seen, the front element is absolutely huge, and it weighs a ton.
I am, however, having a lot more fun with this lens than what I first expected. It's very reawrding to try and compose images which do not have the "fisheye look" to them. These were two recent prints I made (both from Ilford HP5+):
As with most good fish-eyes, resolving power and contrast are extraordinary (all elements of the design can be tailored to this, since no distortion correction is necessary) but what I find particularly enjoyable is that this lens cannot be made to flare or produce ghost images, even with the sun right in the frame, or (as per my second shot) a very bright light shining into the lens.
This is an extremely multi-character lens that I only really learnt to value after a couple of months of use. It has a permanent place in my RB kit now, that's for sure (to the great disdain of my acheing body that has to carry the massive weight!).
Any other users here?