[pure-silver] Re: looking for a 240-300mm enlarging lens
- From: Shannon Stoney <sstoney@xxxxxxx>
- To: pure-silver@xxxxxxxxxxxxx
- Date: Sat, 19 Feb 2005 09:15:03 -0600
Thanks, this was really helpful.
--shannon
> >
>> What is a process lens, as opposed to an enlarging lens?
>>
>> On ebay there was one 240mm enlarging lens for $200.
>>
>> --shannon
>>
Richard wrote:
>
> 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
>
>=============================================================================================================
>To unsubscribe from this list, go to www.freelists.org and logon to
>your account (the same e-mail address and password you set-up when
>you subscribed,) and unsubscribe from there.
=============================================================================================================
To unsubscribe from this list, go to www.freelists.org and logon to your
account (the same e-mail address and password you set-up when you subscribed,)
and unsubscribe from there.
Other related posts:
