How to fix colour casts in film scans
Most colour casts in film scans start at capture: auto white balance changes from frame to frame, or the light source has a poor spectrum. The next most common causes are in the conversion: the orange mask is not fully removed, or the black and white points are set per channel on a frame with one strong colour. Mixed or expired film and colour crossover cause the rest, and crossover needs a different fix in the shadows and in the highlights.
Find the cause first
Before you touch a slider, find something in the frame that should be neutral: grey concrete, a white shirt, a cloud, black asphalt. Read its RGB values with the eyedropper in your editor. Then check one neutral area in the shadows and one in the highlights. If both lean the same way, the cast is even. If they lean different ways, it is crossover.
| What you see | Likely cause | Fix it |
|---|---|---|
| The cast changes from frame to frame on a roll shot in similar light | Auto white balance in the camera | Once per roll |
| The same cast on every roll; greys go neutral but skin or foliage still look wrong | Light source with a poor spectrum | Once, in your setup |
| A cyan or blue tint, or an orange one, strongest in the blacks | Orange mask not fully removed | Once per roll |
| A sunset turns grey, a forest turns magenta | Per-channel end points | Per frame |
| One roll looks different from the next; old film looks muddy | Mixed or expired film | Per roll, then per frame |
| Shadows lean one way, highlights another | Colour crossover | Per frame, with curves |
Auto white balance in the camera
Auto white balance guesses the colour of the light for each frame. Through an orange negative, the guess depends on what is in the picture, so it changes from frame to frame. Each RAW file then carries a different white balance, and each conversion starts from a different place.
How to tell
Open two frames from the same roll in your RAW editor and compare their “as shot” white balance values. If they differ, auto white balance is on.
The fix
- Next time: set a fixed white balance and keep it for every frame and every roll. A daylight preset works, or a custom white balance measured off the bare light source with no film in the holder.
- This roll: RAW files keep the white balance as a setting, not as part of the image. Select all the frames of the roll in your RAW editor and give them the same white balance before you convert.
The guide to film scanning exposure covers white balance and the other camera settings.
A light with a poor spectrum
Colour negative film stores the picture in three dyes, and the camera sees them through its own red, green and blue filters. Cheap white LEDs have gaps and spikes in their spectrum. Colours that fall into a gap come out wrong, and no white balance setting can put back what the light never showed the camera.
The colour rendering index (CRI) is a rough score of how fully a white light covers the spectrum. Makers of scanning lights quote it: CineStill rates its CS-LITE at a CRI of 95+. Another approach is narrow-band light, which lights each dye layer with its own colour. CineStill’s CS-LITE+ works that way, and CineStill says generic high-CRI lights produce colour cross-talk. Both kinds of light are used for good scans. Pick one that its maker sells for film scanning and that states its spectrum or CRI.
How to tell
Make a neutral grey in the frame truly neutral. If skin tones, reds or greens still look off, the light is the likely cause. To confirm, scan the same frame with a different, known-good light and compare.
The fix
Use a high-CRI or narrow-band light made for scanning. Turn off room lights too: a coloured ceiling lamp that reflects off the film adds its own cast.
How I do it. I scan on an LED backlight. I have not yet given in to a more professional scanning light, like the ones from Intrepid Camera Co. or Negative Supply.
The orange mask not fully removed
Colour negative film has an orange tint, called the mask, built into its layers to correct its dyes. You see it in the clear film base. A plain inversion turns that orange into a strong blue-cyan. A good conversion balances the three channels so that the bare film base comes out as a neutral black. If the software measured the wrong area, such as the holder or the light around the film, part of the mask stays in the picture.
How to tell
Look at the deepest shadows and at the gap between frames after conversion. They should be neutral black. A cyan, blue or orange-red tint there points to the mask.
The fix
Give the conversion a clean reading of the film base: the clear rebate along the edge or the gap between two frames. Film from the same roll, developed together, has one base colour, so one reading per roll is enough. The guide to inverting colour negatives in Photoshop shows the manual method.
Black and white points set per channel
To remove the mask and use the full tonal range, converters often set black and white points for red, green and blue separately. That works well when the frame has something close to a neutral black and a neutral white. When one colour fills the frame, such as a sunset, a forest or a room lit by tungsten bulbs, the converter takes that colour for a cast and pulls it towards grey.
How to tell
The cast is the opposite of the scene’s main colour, and it shows only on frames like that. The rest of the roll is fine.
The fix
Fix these frames one by one. Copy the settings from a normal frame on the same roll, shot in similar light, and paste them onto the difficult one. Or warm or cool it back by eye, using a neutral area as your guide.
Mixed or expired film
Each film stock has its own base colour and dyes, and two rolls developed on different days can differ too. Expired film adds to this. Kodak Alaris says that speed, contrast, colour balance and fog level change gradually after manufacture, and that colour films suffer more than black and white because poor storage affects their layers to different degrees.
How to tell
The cast changes when the film stock or the roll changes. Expired film also looks foggy, with lower contrast and weak blacks.
The fix
- Treat each roll as its own batch. Take a new film base reading for each roll, and never paste settings from one stock onto another.
- With expired film, expect to correct frame by frame, and accept a little shift. If a frame cannot be saved in colour, convert it to black and white.
Colour crossover
The three dye layers should respond in step. When they do not, a grey can be neutral in the midtones but lean magenta in the highlights and green in the shadows, for example. That is crossover. Common causes are strong underexposure or overexposure, expired film, a home C-41 process that ran off its time or temperature, and cross-processing.
How to tell
Do the two-point neutral check from the top of this page. If the shadows and the highlights need opposite corrections, it is crossover.
The fix
One white balance cannot fix crossover, because it moves all tones the same way. You need separate curves for the red, green and blue channels: correct the shadow end of each curve, then the highlight end. Split toning can help with what remains. If a whole roll shows the same crossover, from the same process, fix one frame and paste the curves to the rest. If it comes from exposure, every frame differs. The guide to thin and dense negatives explains why exposure errors cause crossover.
Per roll or per frame
Work from the setup down, so you never fix the same cast twice:
- Once, for your setup: a good light source, room lights off, a fixed white balance.
- Once per roll: a film base reading, and one reference frame with neutral things in it. Correct that frame, then paste its settings to the whole roll.
- Per frame: only the frames that still look wrong. These are usually frames with one strong colour, frames with crossover from bad exposure, and scenes lit by mixed light.
If the scan also looks grey and hazy, fix that first: see why film scans look flat. Flare and wrong end points shift colour as well as contrast.