作者: after 时间: 2003-9-24 22:52 标题: [推荐]『徕卡设计师谈镜头设计』
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Meniscus lens - An early lens for landscape photography was a single
positive meniscus lens (in fact, used in a camera obscura before
the invention of photography). Maximum aperture about f/15.
Rapid Rectilinear lens - two cemented doublets, symmetrical, about
f/6 to f/8 (fast for its day, hence Rapid). Historically very
important, given the slow emulsions of the day.
Protar - two cemented doublets, not symmetrical, fairly slow, can
cover a wide field (70 deg or more?). Reputedly not as sharp as
modern lenses, but for large film sizes it covers a lot in a small
package. There are some convertible Protars (giving multiple focal
lengths through exchanging lens cells or using single cells - single
cells will have chromatic aberration). Examples: Zeiss, BL Protars;
others?
Dagor - two cemented triplets, symmetrical, f/6.8 or 7.7 usually.
Wide field, supposed to cover 70 deg at f/16 or so, 75-80 deg when
stopped down further. Spherical aberration when wide open, watch for
focus shifts. Late model Dagors are something of a fetish-object and
can go for absurdly high prices, but earlier ones can be found at more
reasonable levels. Do a Dejanews search (www.dejanews.com) in this
newsgroup for tons of posts about Dagors. Examples: Goerz Dagors;
other firms’’ Double Anastigmats.
Angulon - two cemented triplets, symmetrical, like a Dagor but with a
different shape for one of the cemented interfaces, f/6.8. Wide field,
something like 80 deg. Spherical aberration/focus shift again.
Angulon is a trade-name, I don’’t know if anyone besides Schneider
made lenses of this design. Newer wide angles are sharper and have
more coverage but tend to be huge; older wide angles such as Angulons
and Dagors might be friendlier for field use, especially in large
formats (a 165mm Super Angulon is about the size of a 10 pound barbell).
Double Gauss - many variations. In its simplest form it’’s two
air-spaced doublets of two meniscus lenses, and is symmetrical. The
design is varied by different choices for the shapes of the elements,
replacing some of the elements with cemented or airspaced doublets,
large deviations from symmetry, and so on ... The Kodak Wide Field
Ektars are 4-element double Gauss designs, f/6.3, covering about 80 deg
stopped down. Some older wide angle lenses are also 4-element double
Gauss, as are the Topogon and Metrogon wide-angle aerial lenses.
Also, most fast normal lenses for 35mm and roll-film cameras are
extensive modifications of the double-Gauss design, descending from
the Planar design. Examples: Kodak W.F. Ektars; others (Wollensak
W.A. Raptar?)
Cooke triplet - two airspaced elements in front of aperture, one
behind. Early medium-aperture (f/4.5 - f8 or so?) design.
Tessar - derived from Cooke triplet, two airspaced elements in
front of aperture, cemented doublet behind. Can be made fast,
even up to f/2.8. Moderate coverage, generally 55-65 deg.
Classic simple medium-fast normal lens. Most lenses found
on press cameras are Tessar types. Examples: Zeiss Tessar;
Kodak 127/4.7 and 152/4.5 Ektars (note Kodak used Ektar for
all its premium lenses, it doesn’’t specify a lens design);
some Wollensak Raptars and Graflex Optars (again, Raptar
doesn’’t specify a particular lens design); Kodak Commercial
Ektars (f/6.3, 64 deg coverage); Schneider Xenar.
Heliar - also derived from triplet, has three elements in front
of aperture, cemented doubled behind. Also a fast normal
lens, with 55-60 deg coverage.
Dialyte - two airspaced doublets (one elements convex, one concave),
symmetrical. This type of lens is often used for process lenses,
which can be found relatively inexpensively in barrel mounts.
Although process lenses are designed for close-up use, the dialyte
design maintains its corrections fairly well as the magnification is
changed (so Kinglake says) and these can be used at infinity.
Dialytes are relatively slow, commonly f/9-f/11, and have somewhat
limited coverage, 45-50 deg. Examples: Goerz Celor, Dogmar, Artar;
Kodak 203/7.7 Ektar; Rodenstock Apo-Ronar; Wollensak Apo-Raptar; no
doubt many others.
Plasmat - the basic plasmat design is symmetrical, each half being a
cemented doublet with a meniscus element behind it. There are
variations with more elements etc. The plasmat combines medium speed
and wide coverage, usually f/5.6 to f/9 and coverage 64 - 72 deg.
It’’s now the dominant design for modern LF lenses of normal length.
Actually, it’’s an old design, but did not become widely used until
lens coating became common. Plasmats are technically cconvertible by
using the rear element alone, but they won’’t be as sharp that way and
will have chromatic aberration. Examples: Schneider Symmar and
G-Claron, Rodenstock Sironar, Nikkor W. There are also newer variants
from e.g. Rodenstock and Schneider with even more coverage (which
are really huge and expensive).
I haven’’t really addressed telephotos or modern wide-angles. True
telephotos have a negative element in the rear which makes the focal
length longer than the bellows draw, and usually have an obvious name
like Tele-Something. Modern wide-angles such as Grandagons, Super
Angulons, etc. seem to be derived from the Biogon design - generally
not quite symmetrical, with very large meniscus front and rear
elements, and loads of coverage.
Remember the diameter d of the circle of coverage of a lens,
the focal length f, and the coverage angle theta, are related by
d = 2 * f * tan(theta/2).
