[pure-silver] Re: Durst L1200 (condensor) & uneven illumination
- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
- To: <pure-silver@xxxxxxxxxxxxx>
- Date: Fri, 3 Aug 2007 13:13:40 -0700
----- Original Message -----
From: "Georges Giralt" <georges.giralt@xxxxxxx>
To: <pure-silver@xxxxxxxxxxxxx>
Sent: Friday, August 03, 2007 10:48 AM
Subject: [pure-silver] Re: Durst L1200 (condensor) & uneven
illumination
Hi !
So, as Cor asked, I've made some more experiments.
I tried to measure light evenness on my Laborator 1000
baseplate without lens but, even at the highest column
position, the Analyser Pro keep
Measurements without the lens are meaningless. The
condenser is supposed to concentrate the light at the
entrance pupil of the enlarging lens. Even when the light
source is partly diffuse, as it is in most condenser
enlargers, the focus of the condensers must approximate this
for good uniformity.
The condensers and light source should be such as to
completely fill the entrance pupil. That is, the condensers
should not vignette the source.
Virtually all lenses have some vignetting due to the
lens mount. This vignetting is usually gone when the lens is
stopped down beyond about two stops. One can seen it
visually by looking back through the lens from the focal
plane. The reason that some lenses have oversize outer
elements is to avoid this mechanical vignetting as much as
possible.
The light fall off of lenses comes from several sources.
One is the vignetting of the aperture. This can be seen from
either side of the lens when looking at it from an angle.
The iris becomes cat's eye shaped. Another reason is the
inverse square law, simply the distance from the exit pupil
to the focal plane becomes longer as the point of interest
moves from the optical axis of the lens. Another reason has
to do with the magnification necessary to produce an
rectilinear image. In a standard lens the fall off is
proportional to cos^4 theta where theta is the half-angle,
or the angular distance from the optical axis to the point
of interest.
By modifying the lens to introduce coma into the pupils
its possible to eliminate one term and reduce the fall off
to cos^3 theta. This is what is done in the Roosinov lens
AKA tilting entrance pupil lenses. You can see this effect
by looking at the iris of such a lens while tilting it. The
iris appears to follow you and actually becomes larger as
you tilt the lens.
A lens which is not required to be rectilinear can have
even less fall-off. For instance fish eye lenses, which have
substantial pincushion distortion, can have nearly uniform
illumination. In fact, its possible to design a lens where
the illumination actually increases from center to edge.
By tapering the illumination provided by a condenser or
diffusion system the fall off can be partially or completely
corrected. I have not seen a condenser system in an enlarger
which provides for this but the use of tapered diffusers in
diffusion enlargers is old art. An example is the old Elwood
enlarger, many of which were equipped with an equalizing
diffuser consisting of a glass plate which was heavily
sand-blasted in the center with the amount of blasting
falling off to a nearly clear circle at the margins and
corners.
Of course, one could also introduce a center filter
arrangement into the condenser system, but again, I've never
seen one applied to a practical enlarger. I believe, but am
not certain, that it may be possible to obtain this kind of
effect by using non-spherical condenser lenses. Such lenses
are common in slide and motion picture projectors.
---
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx
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