just turn up the asa dial, shoot right through that cap!
+1!! I am curious, though, how high should one set the ISO in order to shoot through the lens cap, and obtain a good contrast black and white negative? I have always preferred taking the easy path and just removing (and leaving in my camera bag) the lens cap. However, if one can, in fact, shoot through, the cap, then I (and countless other rangefinder shooters) wouldn't have to be concerned about losing lens caps (and those damned Leica caps are often north of $30 per!).
Some Nikons (F6, F5, F3P, F3HP, F2ASx4, F2A, F2 and a D800), with Ai/Ais Nikkors ranging from 15mm to 600mm; Leicas (M6X2), with Leitz glass from 28 to 90mm and a pair of Hasselblads (500c/ms), with Zeiss glass from 50 to 250mm. A bit of stuff for a no-longer practicing professional, but justifiable for a now-converted hobbyist who absolutely loves taking/making pictures.