Can automatic dust removal work on Kodachrome slides?

If you have boxes of family slides from the 1950s to the 90s, there is a good chance some are Kodachrome. It is one of the most beautiful and long-lasting colour films ever made, and it has a reputation for being tricky to clean when scanning. The truth is more nuanced than "it doesn't work".

How automatic dust removal works

Many film scanners make an extra pass with infrared light. Colour film dyes are almost transparent to infrared, but dust, hairs and scratches are not. The infrared pass therefore produces a map of just the dust, and software fills in those spots. On ordinary colour negatives and slides such as Ektachrome and Fujichrome, the results are excellent.

Why Kodachrome is harder

Kodachrome was processed in a unique way, and its dyes, particularly the cyan layer, absorb much more infrared than other films. Parts of the picture itself show up in the infrared channel, so the software can mistake fine detail for dust. The standard infrared cleaning built into many scanners was designed around other films, and on Kodachrome it can soften the image or smudge detail such as eyes, hair and text.

What can work

With the right tools and settings, infrared cleaning on Kodachrome is often very usable:

• Gentler cleaning settings. Some specialist scanning software lets you choose how strongly it cleans. Starting with the lightest setting greatly reduces the risk of softening.
• Adjustable detection. Better software lets you tune how sensitive the dust detection is and preview the result before saving, which helps a great deal with Kodachrome.
• Kodachrome modes. A few professional scanners were built with a special mode designed to handle Kodachrome.

Results still vary from slide to slide, especially in dark areas where Kodachrome is densest. The safest approach is to compare the cleaned and uncleaned scans at full size before deciding.

Black-and-white film

Why it fails on traditional black-and-white film

Traditional black-and-white film forms its picture from tiny grains of metallic silver, left in the film after developing. Infrared light cannot pass through metallic silver, so the scanner's infrared pass sees the silver grains that make up your picture and mistakes them for dust.

The result is that the software tries to "repair" the picture itself. Fine detail is smudged away, edges become soft and blotchy, and in the worst cases the whole image looks smeared or painted. For this reason infrared cleaning is switched off for traditional black-and-white film, and any dust is removed by careful cleaning before scanning and by hand afterwards.

The big exception: chromogenic black-and-white film (C-41)

Some black-and-white films were designed to be developed in standard colour chemistry, known as C-41, so they could be processed at any high-street lab. During processing, all the silver is washed away and replaced by dyes, just as with colour film. With no silver left in the film, infrared light passes straight through, so the infrared pass maps only the real dust and scratches, and automatic cleaning works as well as it does on colour negatives.

These films were popular from the 1980s onwards. They often have "C-41" printed along the edge of the negatives, and the negatives usually have a faint orange, purple or grey tint rather than the clear base of traditional black-and-white film.

How to tell if a slide is Kodachrome

Look at the cardboard mount: "Kodachrome" or "Processed by Kodak" is often printed on it. When held to the light at an angle, Kodachrome often shows a slight relief on the surface of the film.

How we handle it

We clean the film carefully before scanning, use gentle infrared cleaning where it works well, and check each Kodachrome slide at full size. Where infrared cleaning would harm the picture, we leave it off and remove the remaining marks by hand. If you have a large collection of Kodachrome or black-and-white film, get in touch and we'll explain the options.