[pure-silver] Re: looking for a 240-300mm enlarging lens
- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
- To: <pure-silver@xxxxxxxxxxxxx>
- Date: Sat, 19 Feb 2005 01:44:16 -0800
----- Original Message -----
From: "Shannon Stoney" <sstoney@xxxxxxx>
To: <pure-silver@xxxxxxxxxxxxx>
Sent: Friday, February 18, 2005 6:28 PM
Subject: [pure-silver] Re: looking for a 240-300mm enlarging
lens
>
>
> What is a process lens, as opposed to an enlarging lens?
>
> On ebay there was one 240mm enlarging lens for $200.
>
> --shannon
>
The are close. Process lenses were designed for making
photomechanical plates either line or half tone. Their most
important quality was freedom from geometrical distortion
and good sharpness. They were originally used with rather
high contrast materials so flare was not a problem.
Most process work was done at magnifications of from
about 5 times reduction to about 5 times magnification so
most process lenses are optmized for unity magnification.
This is where the object and film are at the same distance
from the lens.
I should say here that one of the fundamental properties
of lenses of all types is that they can be corrected so that
certain aberrations are simuntaneously minimum at one one
distance. At other distances there is some loss of quality.
This distance is chosen based on the expected application of
the lens. For general purpose camera lenses is it infinity,
for enlarging lenses it is whatever distance corresponds to
the most often used magnification. For a lens intended for
35mm work this is probably around 10 to 15 times. For 8x10
it is more nearly 2 to 4 times.
So, for 8x10 a process lens will be pretty near its
optimum distance or magnification ratio.
Most process lenses fall into two catagories: older
standard process lenses and more recent wide angle process
lenses. The wide angle types were intended for use on
cameras that made large photo-offset plates. These were
usualy found in smaller printing plants where the camera had
to be as compact as possible. Most of these lenses are of
the Plasmat type, very similar to common high quality
enlarging lenses and a great many LF camera lenses. (Symmar,
Componon, Sironar, etc.). The older type are most often
four-element air-spaced types sometimes called Dialytes or
Celor (after the first lens of this type). The properties of
these is that they are usually pretty slow, typically f/8 to
f/12 depending on focal length; they have relatively narrow
coverage, typically 45 to 50 degrees at infinity focus; the
size of the image circle does not increase significantly
when stopping down; and they tend to maintain their
corrections very well for change in distance. For this
reason these old process lenses work very well as general
purpose lenses.
Some standard process lenses are apochromatically
corrected. This means that three colors of light are brought
to a common focus rather than two as in most lenses. The
reason for this difficult correction is that these lenses
were intended for making color separation plates for
printing. Their symmetry also gives them very good
correction for lateral color, i.e., the size of the image
remains constant with color.
The later, wide angle, type lenses have nearly double the
coverage of the older type and are faster. They are more
like enlarging lenses. They are seldom apochromatically
corrected although they usually possess very good color
correction.
Most, if not all, of the wide angle types are coated.
Coating was not used on the older process lenses until
rather late. For instance, the Goerz Red Dot Artar is coated
but few of the older Artars are. The reason is simple: flare
could be compensated for by a slight adjustment in the
exposure of the very high contrast plates or films these
lenses were used with.
Goerz began making the Red-Dot version of the
Apochromatic Artar in the late 1950s or early 1960s (the
date is uncertain) for a larger range of application than
the earlier version. These lenses were widely used for large
format color enlarging and for making original color
illustrations, especially table top work. Goerz began
coating them to improve the contrast and also offering them
in a choice of optimum magnifications so that they could be
chosen for their original purpose of making half-tone plates
or for use on a camera or enlarger. These lenses are very
slow but of outstanding quality. Because the entire process
they were originally used for has been supplanted by
electronic imaging they became plentiful on the used market
at very attractive prices. The supply is not quite as
plentiful as it was about two years ago but such lenses are
still available at very good prices and are hard to beat for
performance. Even the very old ones are excellent where a
little flare can be tollerated.
The coverage of a lens increases as the magnification
approaches 1:1. At unity magnification its coverage is
double the diameter it is at infinity. So, if a lens is to
be used on, say, an 8x10 enlarger for relatively small
prints a shorter lens will provide adequate coverage. For
instance, a 10 inch Artar will cover an 8x10 at 2X
magnification and maybe more, although it would not cover
8x10 if used in a camera at infinity focus.
I have a couple of Artars of varying age. They are all
exceptionally sharp lenses.
---
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx
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