ChrisR wrote:They do all tend not to have deeply recessed front elements which helps to get the lens physically close, though being retrofocus makes them more likely to vignette..
I'm still confused. Would we not be using these short lenses on the front of the combo, reversed, with their normal rear elements facing the subject? In that case, and with the other lens focused at infinity, I'm thinking the working distance should be equal to the flange focal distance minus however much the lens normally protrudes into the mirror box, regardless of the focal length of the lens. Lenses that don't protrude much would have a weak advantage, but no lens can protrude very far because of the mirror.
What am I missing here?
I don't see much confusion! Yes I know they'd be used reversed in this application.
As I wrote, the fact that the front lens elements tend not to be recessed much makes it possible to locate them very close to the "tube" lens.
Add that to
1) they can be quite wide apertures - one I have is a f/1.9
2) They're Prime lenses with FLs in a useful range, around 24-45mm
3) retrofocus means the WD is at least reasonably good -
A SK 40mm enlarger lens reversed has 27.8mm WD, not as good as a reversed Canon 40mm STM at 37mm WD.
A SK 28mm f/4 WD (rear) is about 21mm
The 24mm STM reversed gives about 34mm WD.
Robert's Xenon 28mm gives 20mm WD.
(Even a Canon MP35 (front) is about 27mm)
4) Some of them eg the Canons are good, lightweight, optically modern designs and not comparatively expensive.
5) several of us probably have one or two.
I reckon that's enough to suggest "consider it if you have one"?