Thanks jt!
More notes about using this lens reversed:
With no extension (other than the reversing ring itself), magnification is approximately 1.7:1. Adding 26.5mm extension (the Helicoid Extension Tube K at minimum) gets approx. 2.7:1. 46.5mm (Helicoid Extension Tube K at full) is approx. 3.5:1.
Pupillary magnification of this lens (i.e., ratio of exit pupil to entrance pupil) is roughly 1.5 with the lens mounted normally, hence 0.67 with the lens reversed.
Effective f-number increases with magnification. This stuff is rather beyond me, but the usual quick-and-dirty formula is effective aperture = nominal aperture * (magnification +1). A slightly more involved formula takes into account the pupillary magnification. Either way, with the lens's aperture ring set to 5.6, a magnification of 1.7:1 (no extension) is already taking you beyond f/11 and well into diffraction territory on APS-C. I say f/5.6 because that is where this lens (version 1, anyway) starts to get sharp, and subjectively I have found it the best aperture to use at 3.5:1 magnification. You might be able to stop it down to f/6.7 or even f/8 on no extension and gain some sharpness, but I haven't tested this yet.
In short, this is a decent lens for reversing at f/5.6, getting you to high magnifications without much extension. But, ideally, find a lens that is sharper at wider apertures.
US 50-cent piece
Nominal aperture f/5.6, 3.5:1 magnification, K-5, uncropped