If you are looking for a lens that is:

• easy to adapt (M42 mount)
• easy to disassemble and clean
• ART - biotar swirl
• super sharp - from 2.0 apterture
• looks amazing

There it is. I can only say good things about this lens. Oh wait. It's super heavy ;-)

V1 is not as good (build quality) but optically the same. Comparison: https://productz.com/en/carl-zeiss-jena-biotar-75mm-f1-5-version-2-lens-vs-carl-zeiss-jena-biotar-75mm-f1-5-version-1-lens

If you can find it - buy it.

Laydees & Gen'l'men, I give you Philander, the lensman. Philander, I want a lens that can see deep into the primordial past; and has a biotar swirl. Can you recommend one?

I'm looking for an 800mm FX AF VR that's under a foot long and less than 2kg.

(It does not exist. :()

I'd like a 10-800mm f/1.2 lens. That's under £100 if possible

No you wouldn't.

And one that defies the laws of physics so it's the same size as my 70-200.

You do not want a 10-800mm of any form or specification.

I've been messing with teleconverters today. Need to wait for a full moon to test what I want, but I've got a 2x and 3x to bump up some old manual 200mm lenses.

Tripod is absolutely essential. Or some very good inbody stabilisation.

Can only get 1.4x and 2x for F mount (Nikon) - will probably be my next photographic purchase, but I'm resisting getting further hardware until I've at least organised and processed some of the photos I've taken this year.

A good head is more important - I'd rather do handheld at 400 or 600 than use a tripod with an awkward head.

I got a mirrorless Sony A6000 last year. One advantage is near enough any lens can be adapted for the body. Everything is fully manual when using old lenses with dumb adapters. Well, set the manual aperture on the lens and shoot aperture priority, so doesn't actually need to be fully manual. I've picked up a few old lenses for very little money, likely rarely or never use most but they're nice to have available.

Current favourites are
Helios 44-2 58mm f2 - from my dad's old Zenit SLR.
Pentax 70-210mm - courtesy of Caer 10 years ago. Used it on my K200D for many years, now use it AP on my A6000.
Pentacon 135mm f2.8 - lovely shallow depth of field at 135mm, swirly bokeh.

There's a part of me that would like to do that - there's plenty of old F mount lenses - but then I'm like "what's the point?"

I suspect the problem may be that I simply don't do enough casual photography to be able to say "I'm going to try subject X with lens Y this week" and without that then any lenses are just going to be clutter.

Pretty much what I've been thinking today. Got 3 lenses for 99p on ebay. Need some cleaning, but thinking I might only use one at a push. Starting to have a growing collection of lenses I won't use.

I should do what you're saying, pick a lens and subject and stick to it. What I really need is a cheap old really wide lens.

Well I didn't mean stick with one thing, more pick a subject each week/month and capture it with a different lens each day/week type of thing - exploring the characteristics of each lens as much as expanding or refining your interpretation of whatever you're shooting.

Obviously it also needs interesting enough subject matter to not get bored doing that.

That's what I meant. Go out with a single lens each time and have certain subject types/situations in mind. Especially if its fixed focal length so you have to be more creative and imaginative.

Can you use cleaner for glasses on camera lenses?

I use my shirt tails. Swirly bokeh eh?

I usually use my t-shirt but the front element of my 70-200 is a little grubby and it's not working on it. Maybe it's got good swirly bokeh though :)

Regular eyeglass cleaner is probably fine. I rarely clean my front element unless it's got crap splashed on it.

It's definitely important to clean if it's splashed in crap :D

I tried my glasses cleaner and it's looking much better now.

Unless there's a special coating, I'd say anything that cleans glass is fine.

I'm also guilty of just using my t-shirt, accompanied when necessary by moist breath.