[pure-silver] Re: History... Clarification
- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
- To: <pure-silver@xxxxxxxxxxxxx>
- Date: Mon, 19 Oct 2009 22:56:48 -0700
----- Original Message -----
From: "Jean-David Beyer" <jeandavid8@xxxxxxxxxxx>
To: <pure-silver@xxxxxxxxxxxxx>
Sent: Monday, October 19, 2009 5:12 PM
Subject: [pure-silver] Re: History... Clarification
Ray Rogers wrote:
Were any of these non-circular-ish apertures found on
common (pictorial) lenses?
No. They were Waterhouse stops found in process lenses.
Interesting...
Are there any other unique features or properties to
process lenses?
They tend to be dialites, 4 elements 4 groups. They tend
to have longer
focal lengths and not have super big apertures; i.e., f/9
is kind-of usual.
They are usually optimized for 1:1 reproduction ratio
--
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Perhaps just a little more about process lenses. As
above, but it should be noted that lenses with fixed
elements can be corrected for ceratain aberrations at only
one distance. For ordinary camera lenses this us usually
infinity but for lenses which will usually be used at closer
distances the correction is optimized for that distance.
Most process lenses are designed to have their best
correction at equal object and image distance or a
magnification of unity. Usually the correction stays good
over a range of 1:5 to 5:1. Enlarger lenses and macro or
micro lenses are other examples of lenses corrected for
specific ranges of distance usually for some range of
magnification or reduction.
Most of the process lenses over the years were Dialyts
as stated above but some other types were used. Most were
symmetrical lenses since the the equal distance for object
and image results in a completely symmetrical optical path
which corrects geometrical distortion, coma, and lateral
color automatically. Early process lenses were generally
achromats, i.e. corrected for two colors with good
correction in the blue region since that was what the
conventional collodion process plate was senstive to. When
three and four color printing began apochromatic process
lenses began to be produced. An apochromat is a lens that
has common focus for three colors and is corrected for
spherical aberration for two colors. By making the lens
symmetrical, or nearly so, the lateral color is also
corrected so that the iamges at three colors are at a common
focus and also of the same size. Probably the most often
used lens of this type was the Goerz Apochromatic Artar, a
four element air spaced lens of the "Dialyte" type. As
Jean-David pointed out these are slow lenses with rather
narrow coverage. Because the stop used for photographing
with the half-tone screen is critical to the quality of the
dots produced fast lenses would be of no use so the fastest
process lenses are usually around f/8 and longer focal
length ones as slow as f/12 or even less.
Keep in mind that the coverage of a lens is doubled at
1:1 so wide angle coverage is not necessary. Nor is it
desirable since using a longer lens results in less
aberration generally and more even illumination.
Since the plate is of extremely high contrast a very
small difference in brightness of the image projected by the
lens will result in a transition from white to black so that
uniform illumination is important. This is also the reason
that dots of different brightness are converted to dots of
different size on the processed plate. Simply the threshold
between light and dark is very sharp.
A great many process lenses came on the market a few
years ago because the cameras they were used on became
obsolete. Some of these, like the Apochromatic Artar are
excellent generatl purpose lenses despite the specialized
correction. This type of lens has rather stable correction
with distance plus they are so slow and narrow that whatever
aberrations occur at infinity focus are small. However, the
lens must be longer than in focal length than the diagonal
of the film or plate to get good coverage at infinity focus.
So, for an 8x10 plate with diagonal of about 12" a 14" inch
lens is about right. Note that the coverage of Dialyte types
does not increase much as they are stopped down.
Most process lenses are equipped with an iris
diaphragm, often stopping down to very small stops, f/90
being common and the barrel mounted ones, which were
intended for use on process cameras usually are fitted to
take Waterhouse stops. When a Waterhouse stop is used the
iris is set wide open. The iris diaphragms in these lenses
usually have a lot of leaves, 20 or more, so that the iris
is very close to being exactly round at all stops.
Goerz made some late Apochromatic Artars which were
intended for graphic arts or table top photography which
were mounted in shutters and were corrected for greater
magnification. I think all of these were the late "Red Dot"
series. Some were even coated although Goerz was one of the
last lens makers to coat. Flare is of no consequence in
half-toning since its effect can be corrected by a slight
adjustment in exposure.
This is more than I intended to write so I hope its of
some interest. I have been accused of telling people how to
make a watch when they just wanted to know what time it is
and I ratehr think there is some truth to it.
--
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx
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