[pure-silver] Re: Durst L1200 (condensor) & uneven illumination
----- 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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