[pure-silver] Re: Durst L1200 (condensor) & uneven illumination
- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
- To: <pure-silver@xxxxxxxxxxxxx>
- Date: Tue, 24 Jul 2007 07:25:19 -0700
----- Original Message -----
From: "Georges Giralt" <georges.giralt@xxxxxxx>
To: <pure-silver@xxxxxxxxxxxxx>
Sent: Tuesday, July 24, 2007 3:07 AM
Subject: [pure-silver] Re: Durst L1200 (condensor) & uneven
illumination
Hmmmm !
This may me think about my Laborator 1000.
As I use Nikon EL lenses, I think the light fall off will
be of the same order
than the Schneider's one.
Do you think it would be wise to test for eveness without
any lens ?
I would be _really_ glad to be able to remove the frosted
glass and drawing
paper from the filter drawer, to recover the high light
output I used to have !
Cheers
If you have a diffusion enlarger of the kind that has a
ground glass or opal glass diffuser you could use a light
meter with a small probe, or maybe a spot meter to measure
the center to corner illumination level. The light reaching
the lens will depend on the angle of the light rays from the
diffuser to the lens so simply placing the meter against the
glass will not give an accurate measure.
For diffusion enlargers using a light box or a
condenser enlarger about the only way to measure uniformity
is through an enlarging lens. This is what counts anyway.
The size of the light box is important and should be the
right size for the focal length of lens used. The same for
the condensers, they should be large enough for the format
and of the right focal length to focus the light source at
the lens.
You can find the focal plane of the condensers by
removing the lens and holding a card under the lens board to
see where the beam focuses. You will probably have to move
the bellows up. Ideally, the condensers should focus at the
entrance pupil of the lens. For the type of lenses most
commonly used for enlarging now (Plasmats) the entrance
pupil will be somewhere between the outside element of the
lens and the iris. If you don't know at all assume it to be
at the iris, you won't be far off.
Some enlargers allow constant focus of the condensers,
some only focus ranges, either by moving the condensers in
steps or by using different condensers for each format.
These enlargers, like the Omega D-2 or D-2V, have large,
diffuse, lamps so exact focus is not too important.
Nonetheless, when using my D-2V I find that the condenser
setting sometimes makes a difference. In my case I use a
135mm lens for 4x5 in order to get reasonably large prints
with the limited pole length of the enlarger. When making
smaller prints, i.e., say 8x10, I find the corner brightness
better if I use the setting for a 160mm lens. This is
because the lens is further away from the condenser than is
assumed for the 135mm setting. I still have to burn in the
corners but not as much.
If you have an enlarger with continuously variable
condenser focus you can adjust the focus for each change in
print magnification to obtain the best corner brightness.
Sometimes the best way to observe this is to remove the
negative after getting the magnification and focus set and
adjusting the condenser visually. A printing exposure meter
may help. I would check focus at both maximum aperature and
at the working aperture, or at least about two stops down
from maximum.
Unless the enlarger has some form of compensation built
into it there will be some fall off no matter what you do.
I wrote earlier about light fall off in lenses. While
there is some variation among designs the fall off for
conventional lenses will all be about the same. Some wide
angle enlarging lenses may employ the tilting entrance pupil
principal, if they do they will have significantly less fall
off than a conventional lens of the same focal length. There
will still be some fall off. In a camera this is corrected
by a "center filter", a neutral density filter which is
tapered in density from center to edge at the right rate to
correct a particular lens. These will have a considerable
overall light loss and must be used a relatively small
apertures.
Some enlargers have some equalization in the light path.
At one time Elwood supplied a diffuser with tapered
diffusion, heavily sand blased in the center and tapering to
nearly clear at the edges. The old Elwood used an eliptical
mirror as a sort of condenser to they were really a
combination of condenser and diffusion enlarger. I don't
think any of the light box diffusion enlargers have any sort
of equalizer built into them but one could make a sort of
center filter to go above the negative.
---
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx
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