The difference between ISO 100 and 125 (1/3 stop) on black and white film would only be noticible for deep shadow details if you were underexposing (and using a spot meter, had done all of the zone- system testing, and were holding your development to +/- 1 degree, ....). Since you are over-exposing (and from the sounds of it, guesstimating exposure), it will disappear into the noise.(posted 9760 days ago)From experience with several old Zeiss folders, your shutter speed is likely to be a bit slow (altho I bought a beater 1934 Super Ikonta C which was within 1/3 stop at all speeds).