[pure-silver] Re: Durst L1200 (condensor) & uneven illumination
- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
- To: <pure-silver@xxxxxxxxxxxxx>
- Date: Mon, 23 Jul 2007 12:03:28 -0700
----- Original Message -----
From: "Martin Jangowski" <martin@xxxxxxxxxxxx>
To: <pure-silver@xxxxxxxxxxxxx>
Sent: Monday, July 23, 2007 4:24 AM
Subject: [pure-silver] Re: Durst L1200 (condensor) & uneven
illumination
On Mon, 23 Jul 2007, C.Breukel@xxxxxxx wrote:
Martin,
That's great, I have read about lens fall off, but I never
realised it
was that substantial! Thanks for pointing it out!
FYI I used a 80mm Schneider-Kreuznach Componon-S, at
f11-f16 (not my
working aperture, usualy between f5.6-f11, but I had to
reduce my
lightoutput else the Zonemaster could not measure). A
quick search on
the internet did not bring such graphs as the one in your
link, but I
assume it will be compairable with the Rodagon?
You didn't search very thorough ;-)
http://www.schneideroptics.com/pdfs/photo/datasheets/componon-s/componon-s_40_80_1.pdf
Grüße aus Hohenlohe,
Martin Jangowski
The Schneider and Rodenstock lenses are very similar.
The rule for light fall off of lenses having
distortionless projection and a normal pupil is fall off =
cos^theta where theta is the half-angle, i.e., angular
distance of the point of interest from the optical center. A
distortionless lens is one without barrel or pincushion
distortion. For example, fisheye lenses, which have very
large barrel distortion, hvae less fall off than a
distortionless lens. By normal pupil I mean one that has no
coma as in a Roosinov lens. The Roosinov lens has what is
called a tilting entrance pupil and has something like cos^3
theta fall off, one reason thay are used for wide angle
lenses. There are Roosinov wide angle enlarging lenses. Some
lenses have even more fall off than is predicted by the
cos^4 theta rule, for instance the well known Goerz Hypergon
or the more modern B&L Metrogon. This fall off is
unavoidable unless a tilting entrance pupil is employed.
There is additional fall off in most lenses at full
aperture because of vignetting of the pupil by the edge of
the lens mounting. This fall off is reduced by stopping down
but the amount of stopping down varies with the field angle.
For a "normal" focal length lens at infinity focus the
largest stop without vignetting is about 2 stops down from
maximum aperture. Since the angle becomes smaller as the
magnification is increased somewhat larger stops may be used
when enlarging. However, the _effective_ stop may be about
the same because of the bellows correction.
There is another source of fall off in enlarging, namely
the uniformity of the light source. A perfectly diffuse
source would produce about the theorectical fall off of the
lens. A true point source, where a point is focused on the
entrance pupil of the lens may not follow this rule, in any
case, the iris of the lens becomes useless in this
application. Normal condenser sources are partially
diffusing. The usual arrangement is to have a large lamp
with a diffused surface focused approximately on the
entrance pupil. The diffusion may also come from a diffusing
surface somewhere between the lamp and the condensers. This
system is much less critical of exact focus of the source on
the entrance pupil although uniformity and brightness are
still better when focus is exact. This arrangement also
allows the iris of the lens to control the light
transmission although the variation with stops may not be
exact: it depends on the amount of diffusion in the system.
In condenser illumination any deviation of the
condensers from the optical axis or any tilting of the
condensers will produce an asymmetrical uneveness of
illumination. If the focus is not fairly close there will be
an exagerated amount of fall off. Condenser enlargers
require very good alignment.
It is possible in a diffusion system to correct for
light fall off of the projection lens by introducing a
tapered light attenuator in the light path. This can be a
diffusion plate that is sandblasted in the center, the
amount of diffusion tapering off to the edges. A number of
older large-format enlargers used such a system. The problem
is that the correction is right only for a given focal
length of enlarging lens and only when its centered.
Since fall off is proportional to image angle one can
improve matters by using a longer focal length enlarging
lens. The drawback is, of course, the need for greater
distance between the head and paper for a given amount of
enlargement. Nonetheless, a lens of perhaps 1.5 times normal
will give much improved uniformity. It is common to use
somewhat short lenses for LF work. For instance a 135mm lens
is commonly used for 4x5 to reduce the required hight of the
enlarger column. However, unless the lens has a tilting
entrance pupil or is equipped with a center filter the
prints must be burned at the corners (assuming the negative
is not being seriously cropped) in order to get fairly
uniform density. The "normal" lens for 4x5 is 152mm and even
better illumination is gotten by using a lens of around
180mm.
Usually, the lens fall off can be corrected by burning
in using a circular mask. This is a bother but allows the
use of shorter lenses where space is at a premium.
One could probably make a center filter for the
condenser system or for a diffusion enlarger by making
printing onto film and using the negative. The required
minimum density and correct contrast would have to be
determined experimentally.
Note that the eye is much more tollerant of corners
being darker than it is of corners being lighter.
---
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx
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