How to invert colour negatives in Photoshop

Photoshop’s Invert command (Cmd+I) turns a colour negative into a flat, blue picture, because it does not remove the orange mask and it treats the three colour channels the same. To invert a colour negative properly, first make the film base neutral, then invert, then set a black point and a white point for each channel with Levels. Finish with Curves for contrast and for casts that remain.

Why Invert fails on colour negatives

Invert maps every value to its opposite: 0 becomes 255, 255 becomes 0, in each channel. This works for a black and white negative. A colour negative has two more problems.

• The orange mask. Colour negative film has an orange base. It is densest in blue, then in green, then in red. After Invert, the orange turns into a strong blue-cyan cast over the whole picture.
• A different range in each channel. A negative uses only part of the tonal range, and each channel uses a different part. Invert does not stretch the channels, so the picture is flat. It also does not line the channels up, so the cast in the shadows is not the same as the cast in the highlights.

That second problem is why a single white balance or Auto Color cannot fix a plain invert. You need to correct each channel at both ends. The theory behind each step is in how to convert film negatives into photos.

What you need

• Photoshop, with Camera Raw for RAW files.
• A RAW file or a 16-bit TIFF scan of the negative. Include a strip of clear film (the film base) in the frame.
• The Info panel open (Window > Info), so that you can read the values of your samples.

Step 1: Open the scan flat

RAW file: Photoshop opens it in Camera Raw.

1. Choose a flat profile, such as Adobe Neutral, and leave the Basic sliders at 0.
2. Select the White Balance tool and click the clear film base. The orange turns grey. This removes the mask before you invert, on the raw data. If you do this, skip step 2.
3. In the workflow options, set 16 bits/channel and a wide colour space such as Adobe RGB or ProPhoto RGB.
4. Click Open.

TIFF from a scanner: open it. If it is 8-bit, choose Image > Mode > 16 Bits/Channel. This does not add data, but it stops further loss when you stretch the tones.

Step 2: Remove the orange mask with Divide

Use this step for TIFFs, or if the base still shows colour after Camera Raw.

1. Select the Eyedropper tool. In the options bar, set Sample Size to 11 by 11 Average, so that grain does not change the reading.
2. Click a clean part of the clear film base, away from the edge of the holder and any light leaks. The foreground colour is now the colour of the base.
3. Choose Layer > New Fill Layer > Solid Color, and accept the foreground colour.
4. In the Layers panel, set the blend mode of this layer to Divide.

The film base turns white or near white, and the orange is gone from the picture. Divide scales each channel by the base colour, so it removes the mask at all densities, not only in the base.

Step 3: Invert

Choose Layer > New Adjustment Layer > Invert. The picture is now a positive. It is still flat and has a colour cast, and the film base is now black.

Step 4: Crop to the picture

Crop away the film base, the sprocket holes and the holder. They are no longer useful, and their very dark and very bright values would confuse the histogram in the next step.

Step 5: Set the black and white points per channel

Choose Layer > New Adjustment Layer > Levels. In the Properties panel, do this for each channel:

1. Choose Red from the channel menu.
2. Hold Option and drag the black input slider to the right. The picture turns white, and the first pixels that appear will clip to black. Stop when the first detail appears, then move back a little.
3. Hold Option and drag the white input slider to the left. The picture turns black. Stop when the first pixels appear, then move back a little.
4. Repeat for Green and Blue.

Now each channel uses the full range, and the channels line up at both ends. Move back a little at each end, because a specular highlight or a dark corner should not decide the points for the whole picture.

Then set the midtones. Find something that should be neutral grey: concrete, a grey card, a white shirt in shade. Click it with the gray point eyedropper in the Levels Properties panel. If there is nothing neutral, move the middle slider of each channel until the midtones look right.

Step 6: Add contrast and fix the remaining casts with Curves

1. Choose Layer > New Adjustment Layer > Curves.
2. On the RGB curve, make a gentle S: lower the shadows a little, raise the highlights a little. This gives the print-like contrast that the negative does not have.
3. Check neutral areas in the shadows, the midtones and the highlights. Put a Color Sampler on each (the tool is under the Eyedropper) and read the Info panel. On a neutral object, the R, G and B values should be about equal.
4. If one end has a cast, fix it in that channel’s curve. For example, if the shadows are blue and the highlights are neutral, choose Blue, put a point in the middle to hold the midtones, and pull the lower part of the curve down a little.

Step 7: Reuse the settings for the whole roll

All frames on a roll share one film base. If you scanned them with the same exposure, the same Divide colour fits every frame.

1. Select the four layers and choose Layer > Group Layers.
2. Drag the group onto the next frame.
3. Fine-tune Levels and Curves for that frame. Change as little as you can, so that the roll keeps one look.

You can also record the layers as an Action and run it on the roll with File > Automate > Batch. Save each result as a 16-bit TIFF or PSD for your edits.

What still goes wrong

• Crossover: a cast that changes with brightness. For example, green shadows and magenta highlights. Levels fix the end points but not the shape between them. Fix it with the per-channel curves in step 6. More fixes are in colour casts and tints in film scans.
• Noisy blue shadows. The mask absorbs blue light, so the blue channel of the scan gets the least light. When you stretch it, its noise shows. Expose the scan as bright as you can without clipping, as in exposure and white balance for film scans.
• Frames that do not match. If you set Levels again from zero on each frame, each frame gets its own balance, and the roll drifts. Start every frame from the same group.
• A wrong film base sample. A light leak, a fogged edge or the holder’s shadow gives the wrong Divide colour. Sample again in another place.
• Lost shadows or highlights. You moved the Levels sliders too far. Move them back until the Option clipping view shows only a few pixels.
• Flat colour compared with a lab scan. Levels and Curves do not model how film responds to light. A dedicated converter does this better. See negative conversion software compared.

The same method in Affinity and GIMP

The method uses only tools that most editors have, so it works the same way elsewhere.

• Affinity (free for individuals): use a Fill layer in the Divide blend mode, an Invert adjustment, and a Levels adjustment with its channel menu.
• GIMP (free): set Image > Precision to 16-bit, use a layer filled with the base colour in the Divide mode, then Colors > Invert and Colors > Levels, which has a channel menu and black, grey and white pickers.

Sources

• GIMP manual: Levels
• Affinity
• Kodak Professional Portra 400 data sheet (E-4050), Kodak Alaris: the characteristic curves show the different base density of each layer.