A dumb question from a newcomer to the joys of large format. I've recently acquired a Schneider Tele-Xenar f 5.5 240 mm for a 4x5 wooden field camera. The Copal No. 1 shutter does not have f markings, though there is a blank tab where these would normally be. The brochures I've scoured suggest Copal shutters never have these markings, and don't have the automatic attachments of Compur shutters. And unlike the Compur, the Copal does not have click divisions on the f lever. Could someone tell me what is the easy way to work out the various f stops? Many thanks. Jonathan Manthorpe, Hong Kong.