How to scan colour negatives with a camera.

What you need

A camera on a copy stand shooting raw, a lamp, and a holder that shows the frame and the clear film around it. That clear border is not spare margin: it is unexposed film base, and it is where the roll's own base density gets read.

The lamp is either a narrowband RGB lamp that can put out three narrow bands one at a time, or a white one. Narrowband RGB or white, and what to look for in either.

The camera and the stand

Shoot raw. Fix the camera and the film in place and do not move either between exposures — Trichroma Lab corrects a small residual drift, but no software stands in for a tripod. Turn in-body stabilisation off as well: on a stand it has nothing to steady, and it can move the sensor between exposures.

Shoot single frames rather than a burst: some cameras write fewer bits per photosite in continuous drive. On a Nikon, choose lossless compressed NEF. Trichroma Lab reads NEF, NRW, CR2, CR3, ARW, DNG, RAF, ORF, RW2 and PEF.

The holder

Let it show the rebate — the clear strip around the picture — and keep the black of the holder out of what is read as base. A sliver of holder read as film makes the base read too dense; the whole roll then grades wrong while every frame still looks plausible on its own.

Exposure: set it once, on the empty gate

Put the camera in manual, take the film out and point it at the empty gate. Under a narrowband lamp, set each band so the sensor fills to between 75 and 90 % of its range, with nothing at the ceiling, and write the three settings down. Under a white lamp, one exposure, set so the brightest colour fills between 60 and 92 % of the range.

Then leave the shutter, the ISO, the aperture and the lamp where they are, and shoot every frame of the roll on them. A frame shot that way looks dark on the camera's screen and the meter will call it underexposed. That is right: nothing in the holder passes more light than the empty gate, so once the gate sits under the ceiling, no frame on its settings can reach it. Expose for the film instead and the empty gate goes past the top, where nothing can be measured against it.

If the empty gate reads too bright, take light away — dim the lamp, or shorten the shutter or lower the ISO for every shot of the set — and shoot the roll on that new setting. Frames taken at other settings than their empty gate carry the difference into every density on the roll.

Flats: the empty gate, with every roll

Shoot the empty gate with every roll — three frames under a narrowband lamp, one under white — and keep them with the roll. It costs seconds. A density is the film's light divided by the lamp's, so the shot of the lamp alone is as much a part of the scan as the frames; and a calibration of the rig is right only until the lamp, the lens or the aperture changes.

Shooting the roll

Under a narrowband lamp, three exposures per frame, one per band, without moving the camera or the film between them — or, since 1.2.0, one exposure under all three diodes at once, which Trichroma Lab separates into the three bands. Under white light, one exposure per frame. Manual exposure throughout, on the settings of the empty gate.

Name nothing. In Trichroma Lab a card straight out of the camera is a roll the app can read: which three exposures belong to one frame is worked out from file names, capture time and shot order, and from the pictures themselves.

Then develop it

The quick start takes the files from here to a finished scan. How trichromatic (narrowband RGB) scanning works explains why the empty gate and the rebate matter; why camera scans look flat is what happens when they are skipped.