Hi again Ian:
<< This lens [the 28 mm f/2.8 Zuiko] has lettering round it that looks really
old - it's something to do with the style of the font and the way it is
applied, and also that the glass is very near the front, rather than set back
into the filter thread. >>
That is called a retrofocus design. It is actually a modern design
necessitated by the SLR and its mirror. The design modification by Olympus is
kind of novel. To quote from the OM System Lens Handbook:
"In the single lens reflex type camera, in which the mirror flips in back of
the inchangable lens, it is impossible to reduce the distance between the rear
of the lens and the film plane below a predetermined figure. It is impossible
to use lenses in the short focus range without modifying their optical
characteristics to increased their Back Focus distance. With the Retrofocus
Type lens, featuring a negative (concave) lens group in front and a positive
(convex) group in the rearit is possible to create short focal length lenses
that provide sufficient Back Focus to allow free mirror movement. For that
reason virtually all SLR wide angle lenses adopt this type of optical design.
The Retrofocus Type adopted by OM Ssytem lenses is an improved optical
construction developed by Olympus, in which a positive (convex) lens element
in the front permits a much lighter, more compact lens design than with the
rather bulky conventional Retrofocus Type lenses."
As an aside, the Back Focus necessary in the OM System bodies is shorter than
many other makes of 35 mm SLRs, thereby allowing for innovative wide angle
designs, and likely the first marketing of f/2.0 wide angle lenses in the 21
and 24 mm ranges.
The old 28 mm f/3.5 was even more novel in that the front element was
essentially flat. I dumped mine when the f/2.8 came out because of vignetting
problems with thick filters and an unacceptable lack of edge sharpness,
perhaps due to excessive curvature of field.
Gary Reese
Las Vegas, NV
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