How to flatten curly film for scanning
To flatten curly film, cut it into strips, put it in sleeves and leave it under a stack of heavy books overnight or longer. Then scan it in a holder that grips the film or holds it taut. If it still bows, add glass, with anti-Newton glass against the film, and stop the lens down a little to cover what is left.
Why film curls
Film is two layers glued together: a gelatin emulsion and a plastic base. Gelatin swells and shrinks with humidity much more than the base does. When the air is dry, the emulsion shrinks and pulls the film into a curl, most often across its width. Kodak’s bulletin on processed film says that at very low humidity, film curl may become a problem. Many films have a backing layer to balance the pull, but it cannot cancel it in every condition.
Film also remembers its shape. A roll stored on a spool or in a cassette keeps a curve along its length.
- 35mm usually cups across its width and curves a little along its length. The strip is narrow, so the bow is small.
- 120 is wound tightly on its spool with backing paper and has no rigid cassette. It often curls strongly along its length, and because the strip is wide, the cup across it is bigger too.
- Sheet film such as 4x5 has a thicker base and is usually flatter. But a sheet is large, so even a slight bow can put its centre or its edges out of focus at close range.
How you dry the film matters too. Film dried fast, in warm and very dry air, tends to curl more than film dried slowly at room temperature.
Is it curl, or is the camera tilted?
Soft edges are not always curl. Zoom to 100% in live view, or on a test shot, and check the centre and all four corners.
- Centre sharp, both long edges soft: the film is cupped across its width.
- Sharpness changes along the strip: the film curls along its length.
- One side or one corner soft: the camera is not parallel to the film. Level the camera and the holder first. The guide to scanning film with a camera shows how.
What you need
- Archival film sleeves
- Heavy books, or a flat board and a weight
- Clean sheets of paper
- A holder that grips the film or holds it taut
- Optional: anti-Newton-ring glass that fits your holder
- Thin cotton or nitrile gloves
Dry it flatter next time
- Hang the film with a clip at the bottom, to add a little weight along its length.
- Let it dry at room temperature, in still air. Avoid heaters and hair dryers.
- Wait until it is fully dry before you cut and sleeve it.
The guide to drying film covers the rest, including how to keep dust and water spots off.
Flatten it in sleeves under weight
- Cut the film into strips that fit your sleeves and your holder.
- Sleeve the strips.
- Put the sleeve page between two sheets of clean paper, inside a large heavy book or under a stack of books.
- Leave it overnight. Stubborn rolls, especially 120, can take several days.
- Keep the room at a moderate humidity. For processed colour film, Kodak recommends 21 °C and 20 to 50% relative humidity for storage up to six months. Kodak Alaris warns that humidity below 25% can make negatives brittle.
Do not use heat, and do not bend film hard against its curl. You can crack or scratch the emulsion, and neither repair works as well as time under weight.
Use a holder that holds the film flat
Holders keep film flat in three ways.
- Edge grips. Magnetic or clamshell holders press the film flat along its edges. They work well for 35mm. Wide 120 strips can still bow in the middle.
- Curved tracks and tension. Some holders bend the film gently along its length, which stops it from cupping across its width. CineStill describes the VALOI 360 120 holder as having a double S-curved track that mechanically flattens the negative. It is made for full, uncut rolls, and its maker warns that very short strips may not lie as flat. With a holder like that, keep your 120 strips long.
- Glass. A glass carrier flattens anything, at a cost. See the next section.
Whatever the holder, put the film in with care: the edges straight in the gate, and no buckle where the holder grips it.
Add glass if it still bows
Glass on top of the film, or on both sides, holds it flat. It also brings three new problems:
- Newton rings. Smooth glass on smooth film makes rainbow ripples. Use anti-Newton-ring glass, with its textured side against the film. The Newton rings guide shows how to find that side.
- Dust. Each sheet of glass adds two more surfaces to keep clean.
- Reflections and flare. Mask the light to the picture area, so the glass does not bounce stray light into the lens.
For 4x5, a sheet of anti-Newton glass on the film is often the simplest way to get the whole sheet sharp.
Focus with care, and stop down a little
Some curl will remain. Depth of field covers the rest.
- Focus between the centre and the edge. With cupped film, focus on a point partway out from the centre, so the zone of sharpness can reach both. Then check both.
- Stop down in steps. Shoot the same frame at each aperture, from wide open to well stopped down. Compare the centre and the corners at 100%.
- Stop where the corners turn sharp. Past that point, diffraction softens the whole frame. At close focus, the effective aperture of most lenses is smaller than the marked one, so diffraction shows up sooner than in everyday photography.
Higher magnification means less depth of field. If you scan a large negative in several tiles, each tile is shot closer and is less forgiving of curl than a single shot of the whole frame. The guide to scanning 120 film covers this for medium format.
How I do it. For the 6x7 negatives from my RB67 and for my 4x5 sheets, I use two plates of glass or a film holder.
Common problems
- Rainbow ripples after you add glass. See Newton rings.
- More dust spots since you added glass. See dust on film scans.
- The film is flat but one side is still soft. The camera is not parallel to the film. Level both.
- The film curls again after a few days. It is stored in air that is too dry. Store it flat, in sleeves, under light weight, at a moderate humidity.