thanks Stuart, but as i mentioned conversion factors can be solved by apps or (for 30 euros less) by free websites. maybe i should rephrase: you see a particular shot you want to reproduce, now how do you gauge if the lens used was 27 or 35, or 35 vs. 50 mm?. FWIW i concede a tele shot should be easily differentiated from a wide angle shot.
regards, Ray