>
> You know, I understand what lens elements are, but what is the optical
> advantage of distinct lens element groups?
>
Well, a group is just two or more elements glued together. By
gluing the surfaces, the optical designer loses two degrees of
freedom to work with in the design. Two separate elements have 9:
the curvature of the 4 surfaces, the element separation, the 2
element thicknesses, and the 2 glass types. By gluing you lose one
curvature and the separation, since the surfaces are in contact.
You also lose the ability to move the elements independantly for
zooming, internal focusing, or changing aberration correction (like
in the Zuiko Close Focus Correction lenses.) gain less expensive
mounting, better alignment and robustness, and less air-glass
reflecting surfaces. Unwanted reflections are greatest where the
materials have large differences in light speed, as with air-glass.
So two air-glass surfaces are traded for one less reflecting
glass-glass (the two glasses will be slightly different, otherwise
there's no point in having a group rather than a single element.)
Paul Farrar
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