[rollei_list] Re: Anti Newton Ring Glass Effect on Enlarging
----- Original Message ----- From: "Don Williams" <dwilli10@xxxxxxx> 
To: <rollei_list@xxxxxxxxxxxxx>
Sent: Thursday, May 28, 2009 6:49 PM
Subject: [rollei_list] Re: Anti Newton Ring Glass Effect on Enlarging 



At 08:00 PM 5/28/2009, Elias wrote:

Hi All,
I think this question is best for all the optics aficionados here: My enlarger uses AN glass above the neg, and clear glass below. With my b&w Rollei negs this works very well, but with color negs not well at all. Even though the emulsion side is in contact with the clear glass, I still get rings. I'm thinking of getting another piece of AN glass for below the neg. (emulsion side). Will that have any effect on the projection, and the resulting print? 

Thanks,

Elias
It all depends on the texture of the color neg. My ANR experience is based on glass mounts for 6X6 projectors, and of course I have tried all combinations, however color positives are not as smooth as color negs. I got the best results with ANR glass on both sides, particularly in theater-size high power projection systems that would eventually cook the slide.
Why not just get another piece of ANR glass for the bottom? Alternatively, experiment by exchanging the glasses and observe the result. You could also just examine what happens when you put the neg in emulsion up.
I think it will depend on the depth of focus of your enlarger as to whether the ANR glass distorts the projected image.
Can you see the rings on the paper holder or only on the print? 

What happens when you use B&W paper with color negs?
In the olden days a little dusting of corn starch, or something similar did the trick. 

Good luck.
Newton's Rings are caused by destructive interference between two reflecting surfaces where the spacing is a fraction of a wavelength. It occurs where glass is used in a negative holder beause of the smoothness of the glass and the difference in index of refraction between the surfaces. With a difference in index there would be no reflection. There are two ways of preventing Newton's rings: one is to use a slightly roughened surface on one side in order to break up any coherent reflection; the other is to use an anti-reflective coating on the surface of the glass. The first is acceptable and works well when it is in contact with the support side of a negative. Since the emulsion surface of some color materials is as smooth as the support side there can be Newton's rings formed on the emulsion side as well as the support side and generally speaking one wishes to avoid any diffusing surface in the image path. There are glass sheets made for covering pictures available with excellent anti-reflection coating, much the same as the anti-flare coating on lenses. This glass has very little visible reflection and should suppress reflections enough to eliminate Newton's rings without using diffused surface glass. 

--
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx


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