Good equivalent of the now pricey Aero Ektar 178 2.5?

[Memento Mori]

Interesting!

[Memento Mori]

Another possible option. Perhaps. Put a +3 diopter on an f/4.5 300mm lens. That would give you (approximately) a 160mm f/2.3. There might be some aberrations, but they might be interesting.

So now my brain is trying to sort out how that works. The diopters I’m used to are teeny things at the eyepiece on a dslr.

Do you have more info on this approach?

[Dan Fromm]

So now my brain is trying to sort out how that works. The diopters I’m used to are teeny things at the eyepiece on a dslr.

Do you have more info on this approach?

PMFJI. The diopters mentioned are sold as "close-up lenses" or as "close-up adapters." They screw into lenses' filter threads. Cheap ones have single elements, are to be avoided. There are also multi-element diopters that are more expensive but work better.

[Mark J]

1 Diopter means a focal length of 1m ( 1000mm )
2 diopters is a focal length of 500mm.

I don't know if you can get diopter lenses big enough to cover a 300mm f/4.5 - can you ? I have never checked.
There will be aberrations .... if not colour, then some other things....

[Dugan]

I have "+3" and "+4" single-element close-up filters in 72mm size...so yes, they're out there...maybe check with www.filterfind.net for other sizes.

[Memento Mori]

I snagged a clean (couple of minor blemishes off to the side on the front element) Buhl 9” f/2.5 for about $250 today. Came with RAF alloy lensboard. Happy enough for now