This medium-sized species often forms hoary-looking patches on base-poor rocks at low altitudes in hill districts but it is readily confused with R. heterostichum s.str. or R. obtusum. It can be hard to tell apart from these two species in the field and microscopic confirmation is strongly advised.
Good jizz characters which suggest potential R. affine are falcate leaves which appear relatively flat, not deeply keeled, and long hair-points (though with predictable Racomitrium variability, muticous forms also sometimes occur).
Sectioning a leaf is the only reliable way to confirm a provisional identification. At its base, the nerve of R. affine is normally 60-80um wide and at the middle of the leaf, it is 3 cells thick, distinctly rounded on the lowerside with cells appearing homogeneous. For a good, contemporary key and comparison with lookalike species, Tom Ottley’s excellent article in Field Bryology is highly recommended; a pdf can be downloaded from this page.Read the Field Guide account