 BetterPhoto Member |
ASA/ISO why are there different asa/iso ratings for films
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- Gregory LaGrange Contact Gregory LaGrange Gregory LaGrange's Gallery |
Films are made with different sensitivities to light so that you can vary the film type to different light levels. This allows you to expand your subject choice when light levels change.
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Pete H |
I don't believe ASA (AMERICAN standards association) is used any longer. Now it is (International standards organization. "It takes a village to raise an f/stop."
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Alan N. Marcus |
Hi Jacqueline, A good question! Films are coated with light sensitive chemicals. The bases of modern films came from a discovery by J.H. Schulze in 1725. Mr. Schulze experimented with using paper stencils which he placed on sheets covered with a mixture of silver nitrate and chalk. He obtained images but was unable to render them permanent so they faded away. The first permanent image came in 1816 made by Nicephore J. Niepce. This image was made using sliver chloride. As it turned out three salts of silver will do nicely. Each has a different sensitivity to light. The slowest is silver iodine, next is silver chloride with the fastest made using silver bromide. Thus different sensitivities are produced by blending these three salts of silver. All three are sensitive only to blue light. H. Vogel discovered he could dye the silver crystals (normally their color is much like table salt). The dyed crystals were sensitive to blue and green light. Thus orthochromatic films came along in 1873. In 1902 A. Traube, using Ethyl Red dye achieved sensitivity to red light and panchromatic film was born. As time passed, it was discovered that one could dope silver salts. Doping introduces impurities that greatly enhance sensitivity to light. These are gold, cadmium and the like. It was also discovered that if the gelatin binder contains impurities, the speed of the material is altered. Sulfur is a good example but the prize goes to mustard seed. It seems mustard added to the mix increases film speed. Measurement of film speed started in earnest in 1888 when F. Hurter and V. Driffield patented the first exposure calculator. With the invention of the electric light meter by Western Electric (Weston) in 1932, followed by General Electric, each established a different method to rate film speed. Consistency was achieved. In the US it was ASA (American Standards Association. In Great Britain it was BSA, in Germany DIN Deutsche Industries Norm, in Russia GOST translation – Russian standards association. All used a different method now standardized by ISO (International Standards Organization.). Alan Marcus (Marginal Technical gobbledygook)
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