Thanks to all for your asnwers.
I’ve decided to prioritize the M42 mount because I think the adapter tend to be more compact (for instance the thread is inside the adapter) compared to others.
From your suggestions, I selected lenses that are relatively easy to find and not too expensive. Here is my short list:
- Pentax SMC 50mm f/4 Macro
It may be explicitly titled Super-Multi-Coated (commonly abbreviated keeping the dashes [S-M-C]). If you include the full spelling and the other abbreviation your search may yield more results. The SMC-labeled ones were later versions.
- Pentax-A 50mm f/2.8 Macro
The A denotes it will be a Pentax K-Mount lens with optional by-camera aperture control. There is also a Pentax-M version, which is also K-Mount but is fully
Manual. Both are not what you want if you're sticking with M42.
- Vivitar 55mm f/2.8 Auto Macro
It's a pretty sweet lens! It employs a Xenotar-type formula (5 elements / 4 groups) which actually seems to be a pretty common formula to use for macro lenses at these types of speeds (f/2.8, f/3.5, etc). The same base lens is apparently available under other labels:
- Elicar V-HQ
- Panagor PMC
- Soligor
- Soligor C/D ("computer designed"?)
- Rokunar V-HQ
Like the 55, this 90 is available under the same additional brand names. There is also a f/2.5 variant of all of these - again not to be confused with the Tokina-made "Bokina" Vivitar 90/2.5. These use a revised optical scheme (6e/5g; the f/2.8 90s use 6e/4g).
Now that we've established that you're not against 90mm macros, there are some excellent options in that range worth mentioning.
Looking up pricing for the vaunted Bokina (the Vivitar one or the Tokina AT-X one), its pricing has settled down since last I checked. Its bokeh really is among the best and it's plenty sharp. It only goes to 1:2 on its own, and buying it on its own seems to be the best deal. Once you throw in the rear optic for 1:1, the price gets less reasonable and puts it out of the running for a "budget" option, in my opinion. If you want lovely smooth background bokeh for character these must be mentioned now though!
The Tamron SP 90/2.5s are perhaps the best deal right now in the 90mm vintage macro arena. Bokeh-wise they're barely worse than the AT-X, while having slightly tighter (and slightly differently-flavored) LoCA control. Both versions of it (52B, 52BB) can have the "blue spot" issue on digital, but are otherwise very great macros. I love my 52BB, but not necessarily because of its character, as it just performs so nicely that there isn't a whole lot of character to be had - it's a bit clinical.
There are also some old Zen-era Sigma 50/2.8 and/or 90/2.8 macros that don't command high prices. They probably won't dissapoint - prior to getting a Canon EF 50/2.5 my Sigma 50/2.8 treated me really well, but it also had a bit of a "blue spot" problem!
Edit : I just noticed the Pentax are 0.5:1 magnification, and the Vivitar lenses are quite long at full extension.
Yep. Are these issues? We don't fully know what you want, do we? Most of these are simple primes in that they use unit focus by extension - there are no floating elements or focus groups. Extension tubes can quickly overcome the 1:2 issue when you need to get closer.