Newton rings in film scans, and how to stop them

Newton rings are ripples or contour lines, often rainbow-coloured, that appear where film nearly touches smooth glass. Light reflects back and forth across the tiny air gap between the two, and the reflections interfere. To stop them, keep the film off plain glass: use a glassless holder, or anti-Newton-ring (ANR) glass with its textured side against the film, and keep the film flat and in moderately humid air.

What Newton rings look like

• Irregular rings, ovals or wavy contour lines, like the lines on a map.
• Rainbow fringes in colour scans. In black and white they show as light and dark bands.
• They show most in even areas: sky, walls, soft backgrounds.
• They change shape or move when you reposition the film or press on the glass.

What else it could be

• A pattern in the same place on every frame. That comes from your light panel or a mark on the glass, not from the film. Newton rings move with the film.
• Shiny patches on the film itself. Look at the film in reflected light. Kodak calls these ferrotyping: smooth, shiny blotches on the emulsion caused by heat or moisture with pressure. Kodak Alaris notes that the glossy surface of some plastic sleeves can cause it. Ferrotyping stays the same however you mount the film.
• Drying marks. Water spots are on the film too, and also stay put. See how to dry film without water spots.

Why film on glass makes rings

Kodak’s technical bulletin on processed film defines Newton rings as bands of coloured light around areas where two transparent surfaces are not quite in contact. They come from interference, and they occur when the gap between the surfaces is about as wide as a wavelength of light.

Light reflects off the glass and off the film. Where the gap has one width, the two reflections add up. Where it has another, they cancel. Film is never perfectly flat, so the gap changes across the frame, and you get bands.

Three things make it worse:

• Smooth surfaces. The shiny base side of the film is smoother than the emulsion side, so rings form more easily there.
• Curl. Curled film touches the glass in some places and not in others, which gives exactly the changing gap that makes rings.
• Pressure. Pressing film harder onto plain glass does not remove the gap. It only moves the rings around.

Use a glassless holder

The surest fix is to take plain glass out of the light path. A glassless holder grips the film by its edges, or bends it along a track, so nothing touches the picture area. No glass also means two fewer surfaces to collect dust.

The trade-off is flatness. A glassless holder depends on the film lying fairly flat. If yours is curly, flatten it first with the steps in how to flatten curly film.

Use anti-Newton glass the right way round

Anti-Newton-ring glass has one surface with a very fine texture, etched or coated. The texture stops the film from lying in smooth, even contact with the glass, so the even air gap that makes rings cannot form.

1. Find the textured side. Hold the glass so a lamp reflects in it, and look at the reflection from each side. The plain side gives a sharp reflection. The textured side gives a soft, blurred one.
2. Put the textured side against the film. The texture works only where it touches the film. With the plain side towards the film, ANR glass is just plain glass.
3. Put it against the shiny base side. That is the side that causes most rings. The duller emulsion side rarely needs it.
4. Keep it on the light side if you can. With the ANR glass between the light and the film, its texture sits behind the film and out of the lens’s direct view. Between film and lens, the texture is more likely to show in clear areas at high magnification.
5. Test it. Scan a frame with a large even area, such as sky, and check it at 100%. Look for both rings and any grain-like pattern from the texture.

Buy ANR glass made for your holder, so it sits at the right height and the film stays in focus.

Keep the film off plain glass

If you must use plain glass, a small gap is enough to stop the interference.

• A spacer. A thin mask or frame around the picture lifts the film a fraction off the glass. Keep it thin, so the film stays inside your depth of field.
• Holder height. On a flatbed scanner, use the holder’s height setting or feet so the film does not rest on the scanner glass.
• Wet mounting. Some scanners take film mounted in a fluid that fills the gap completely. It also hides fine scratches, but it is messy and slow. Mounting fluids are chemicals: work with ventilation and gloves, and follow the maker’s safety data sheet and local disposal rules.

Control humidity and curl

Humidity works in both directions.

• Dry air makes film curl. Kodak’s bulletin says that at very low humidity, film curl may become a problem, and it gives Newton rings as an example. Kodak Alaris adds that relative humidity below 25% can make negatives brittle.
• Damp air also causes trouble. Many people who scan with glass report more rings on humid days.

So aim for the middle. For processed colour film, Kodak recommends 21 °C and 20 to 50% relative humidity for storage up to six months. Let the film sit in the room where you scan before you mount it. Bring any cold glass or film to room temperature first, so no condensation forms. Clean glass with a dry microfibre cloth; do not breathe on it.

Quick checklist

1. Move the film. If the pattern moves with it, it is Newton rings.
2. Take plain glass out of the light path, or replace it with ANR glass, textured side against the shiny base.
3. Flatten curly film before you mount it.
4. Scan in a room that is neither very dry nor damp, with the film and glass at room temperature.
5. Check one even frame at 100% before you scan the whole roll.

Adding glass also adds surfaces for dust. Blow off the glass and the film before each strip, as the guide to dust on film scans explains.

Sources

• Kodak: Storage and Handling of Processed Film (TIB 5203)
• Kodak Alaris: Storage and Care of Kodak Photographic Materials (CIS-2017-1)