I have some lenses from 1840s and 1850s, and have some questions. One of the lenses, a Derogy Petzval, came with washer stops. Another, an 1865 Voigtlander, has a slot for stops. An early anc. Darlot I have has a slot and a set of Waterhouse slots. These have all raised some questions in my mind. How did they decide on what diameter to make the holes? I'm assuming there was no real system in place at the time because there was no ISO rating, no shutter speeds, no uniformity between makers. I'm going to guess that they simply made the stops so one let in half (or twice) as much light as the next size? Or, was it pretty arbitrary?
Second question is when did an organized system of exposure come about? I've read there was a convention in Paris in 1900 that dealt with it, but I know there were at least three systems still going on at that time. I have equipment of that vintage such as a Kodak with stops numbered 1-2-3-4, a 1930s Elmar lens with stops such as f9, f18 (Continental System?), and the British (now standard?) system based on "1" and the square root of two. (e.g f2.8, f4, f5.6.....) on my 1906 Heliar in Compound. OTOH, early Weston meters had several scales in the 1930s and 1940s. How and when did our current system come about? Was it the invention of accurate light meters in the 1930s that really drove the standardization?
George B.
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