Oh. Correction for pupillary magnification. Well-known, been around forever, but often ignored.

When I bought my PB-4 in 1970, a sheet giving magnification vs. exposure factor curves for many, many Nikkors was packed with it. The curves for w/a lenses, all retrofocus, are terrifying.

IIRC, there's a full discussion, with formulae for lens facing normally and lens reversed in Lefkowitz.