> For a given number of elements, a varifocal lens can be of better optical
quality than a true zoom,
> because it doesn't have to hold focus as you change focal length.
Perhaps I'm not thinking ;) but it is not directly clear to me why this is the
case. I suppose that the zooming mechanism for a non-varifocal (what is the
proper term for this?) lens must be more complicated than for a varifocal one,
but how does this compromise the optical quality of the lens? Is this due to
less than optimal placement/control of the elements, because of the more
complicated zooming mechanism? Hmmm, that would in a way make sense, so perhaps
I _am_ thinking after all. ;)
I wish I had a proper answer to your question, but I don't. I admit -- somewhat
shamefacedly -- that I'm going by what I've read in magazines. I'm blindly
believing self-proclaimed experts.
The IS-1x lens is not the first varifocal lens, by any means. I remember at
least one other -- a Series 1 -- from Vivitar. The varifocal design made it
smaller, or sharper, or both. It didn't sell very well, because users don't
like refocusing when they change focal length.
"Common sense" suggests that no one would design a varifocal lens unless it
were smaller, or simpler, or needed fewer elements for high optical quality
than a "true" zoom. But I can't tell you the optical reasons why this is so.
Anyone out there who can?
As far as I know, none of the zooms that come with $300 wonderbricks has an
aspheric element; only the IS-1x lens does. Assuming varifocal lenses are
inherently less-expensive for a given quality level, Olympus presumably used
these savings to incorporate aspheric surfaces -- a nice example of a "virtuous
circle." There is no question this is a very good lens.
Popular Photography's recent round-up of SLRs explicitly described this lens as
"superb." I've been meaning to write Jason and ask him by what standards it is
considered superb. I don't believe they've ever published an SQF chart.
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