General Information on Newton Rings and history of solving the problem in photographic uses.

Newton rings are created when is trapped between two smooth surfaces like glass and film. As light from your scanner or enlarger passes through this air pocket it’s refracted.  This refracted light creates an interference pattern first noticed by Sir Isaac Newton.  They look somewhat like the circular rainbow effect of gas on water.

In the photographic process there are different methods of getting rid of Newton Rings.

Glassless Film Holders

The most obvious is to use a glassless film holder.  Since the film is suspended over an opening there is no possibility of creating Newton Rings.  While this is effective, glassless holders have their own set of problems.  Since there is no overall support for the entire film area the center may be in a different focal plane than the edges.  This causes soft focus at the edges of your scan.  Also, film can expand due to internal heat of the scanner or enlarger.  Since the film is typically rigidly held in place by clamps, as it expands the only place for it to go is up or down.  So the center is ultimately thrown out of focus.

Glass Film Holders

Glass has been used to flatten film during darkroom exposure virtually since gelatin base substrates were invented.  Almost immediately photographers began noticing Newton rings on their enlargements.  It didn’t take long to figure out where they were coming from.  You only had to look at the film holder in the right light and see the rings. It was also noticed that Newton rings almost exclusively occurred between the smooth base of the film and glass.  The emulsion, it was determined, was “rough” enough to not cause trapped air.

Here is where the experimentation began.   All sorts of solutions were tried. 

Powders

Corn Starch, baby powder, or other fine powders- These were placed in an atomizer and a small amount sprayed into the air, creating a thin cloud of dust. Film was waved within the cloud and some particles settled on the film. It was then sandwiched between two sheets of glass.  The dust, in theory, allowed air to escape from between glass and film.  Since photographers are usually clean freaks in the darkroom this method didn’t have much of a following.

Fluid Mounting

This is an excellent method for eliminating Newton rings.  To use this method, photographers build or purchase a fluid mount film holder frame.  It’s designed to hold two sheets of glass and contain mounting fluid within the frame.  Film is sandwiched between these two sheets of glass with a slight film of fluid between film and glass.  The fluid prevents air pockets from forming and eliminates Newton rings. 

One drawback is that before the digital revolution negatives were hand retouched with dyes. The wet mount fluid removed any retouching dyes, so the technique fell out of favor.  Fluid also can attract airborne dust to the negative.

Since most retouching is now performed digitally, fluid mounting has made a small come back.

Optically coated glass

Some success has been achieved by sandwiching film between optically coated glass. The glass coating is much the same as used on camera lenses and reduces reflection and refraction of light passing through it.  In theory the coating causes light to travel through any existing air pockets with less or no refraction.  The high cost of this glass has prohibited wide spread experimentation or use so the jury is still out on its effectiveness.

Anti-Newton Ring Glass

AN glass first came into wide spread use in the late 1960’s.  Enlarger manufacturers were desperate to find a solution of Newton rings for their professional clients.  Photo labs were creating wall size prints and needed to sandwich film between glass to ensure overall print sharpness. Unfortunately Newton Rings were causing high paper wastage.  The Newton rings were being enlarged along with the negative and ruining images.  

Labs couldn’t use fluid mounting due to heavy use of negative retouching, and glassless carriers allowed a small amount of film curl at the edges causing soft focus in those areas of the print. 

There are no records on file crediting any specific person or company with the invention of AN glass for photographic purposes.  But, with some experimentation, photo labs found it effective and a relatively inexpensive method of solving the Newton Ring problem. 

AN glass can have its problems depending on how it’s used.

The most vexing problem with using any glass sandwich and film is cleanliness.  Dirty environments, poor film cleaning and glass cleaning techniques cause dust problems.  In any glass/film sandwich you have to clean 6 surfaces.  But, benefits far out way the problems.  Dust spots can be retouched from prints and files, Newton rings cannot.

Using two sheets AN glass together can soften an image when enlarging or scanning.

The extra sharp light created by condenser type enlargers can cause the AN glasses texture to show up in broad even areas of a print.  Background paper and cloudless or overcast skies are typical problem areas.  The advent of diffused light enlargers has eliminated this problem in the darkroom.

Images scanned at high resolution (typically over 4,000 dpi) can cause the glass texture to show in digital images. 

Some of the newer films have very thin emulsions resulting in the occasional Newton Ring. Using two sheets of AN glass can solve the problem but at medium resolution and above you may see image degradation.

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