I do a lot of lens modding.
I suppose lens mount conversion is not meant?
I converted some Canon FD lenses (20/2.8, 24/1.4L, 85/1.2L, 100/4, 300/2.8L, 400/2.8L, 500/4.5L, 800/5.6), some Minolta Rokkor lenses (28/2.0, 35/1.8, 50/1.4, 58/1.2, 100/4, 135/2.8) to Canon EF mount. Furthermore I build some lens adapters that up to know didn't exist as a product, like Minolta/Sony lens to Canon EOS adapter, and Minolta MD/MC/SR lens to Canon EOS camera adapter.
More Frankenlens like are my experiment to fit old lenses with automated EF iris - great for macro work, I did this with three lenses up to now.
Furthermore I made a Sony 135 STF (smooth transfer function) copy, to get creamy bokeh.
Another way to get smoother bokehwas to find and modify a lens to work as a copy of the Nikon DC (defocus control) lenses, with controllable spherical aberration.
For macro stacking I build a 35mm lens with long focus travel and steady entrance pupil to avoid perspective change.
Furthermore I build a telecentric adapter to get telecentricity when needed.
I use several projection lenses, and for theses I sometimes adapt those into a cheap focusing helicoid.
To use wideangle lenses for macros I made a 8mm thin EOS extension tube, and now a even shorter 6mm EF extension tube (automatic).
DIY Tilt-Lensesfor crop DSLR cameras where my first lens modification. I didn´t like the idea to have the lens attached with some flexible hose, so I found the way to use the tiltable housing of a spotlight. With this I could make tilted images at night and such.
Someone used this concept and made a very detailed instruction, this seems to be the most common DIY tilt lens here in Germany today.
I made and work on some more lens modification projects, for example I want to optimize the apodization filter for the Sony STF copy.
And I made a lot of mistakes, I better not mention
I try to get cheap lenses, like the Helios 44 for some experiments - sometimes for 3 Euros or such prices.