Re: [Leica] Lens elements & groups

["Stanislaw B.A. Stawowy" <watteau@krakow.neurosoft.net>]

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)