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