Archived posting to the Leica Users Group, 2000/10/09
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]Ohhh, it is fairly simple. Long long time ago, lenses were not coated. So they scattered light. Well, so we will use less elements, said constructors. But wait, Carl! You need many elements to correct optical errors! With three lenses you will have low sharpness in corners, not so flat field and little astigmatism! Ernst, what to do? Well, let's cement these elements together! So then Carl (Zeiss) started production of Jupiter 3, later named Sonnar... Jokes aside: every air-glass surface reflects some light. Despite of numerous efforts, even multi-extra-supercoated one. But you, as a constructor, want to correct aberrations. To do this, you have to have some lens surfaces to tweak their parameters. Also, with 2 lenses you simply cannot have flat field, nor good image. You need at least three elements, like in famous Cooke triplet. There you have six air-glass surfaces, because this lens is made from two plano-convex and one double-concave lens (I may be wrong, but then I will be flamed to death :-) Contemporary zooms have lot of these air-glass surfaces, so zooms have huge flare, low contrast and so. Jupiter/Sonnar have 7 lenses in three groups, what results in very good contrast and low flare. So is Leitz optics, How many air-glass surfaces famous Summicron have? Compare this to typical Sigma zoom :-) St. (Stanislaw B.A. Stawowy)