Film negative conversion

Consistent scans, measured rather than estimated

Metrakon converts camera-scanned colour negatives into finished positives. Instead of analysing each image and estimating a correction, it measures how your film stock behaves on your scanning rig, once, and converts every frame through that measurement.

The result is what minilab scanners did well: a whole roll that lands at the same density with the same colour, without an operator correcting frame by frame.

The problem with per-image correction

Most negative conversion works from the image: it reads each frame's statistics — white point, black point, average colour — and estimates a correction. That approach fails in exactly the situations that fill real rolls.

Scenes without neutral content drift. A red room, a forest, a sunset — average-to-grey logic reads the scene itself as a colour cast and corrects it away.
Mis-exposed frames get corrected toward the wrong target. Over-exposed frames come out bright and desaturated; night shots get lifted to grey.
Frame-by-frame analysis drifts frame by frame. Two photos taken seconds apart can render differently because something in the scene changed the histogram.

A conversion can only be as consistent as the thing it anchors to, and scene statistics are not stable. The film's physical behaviour is.

What Metrakon measures instead

A short calibration session — a colour chart photographed on one roll of the stock, bracketed in one-stop steps — captures how that film actually responds on your light and your camera.

A density fingerprint Dozens of measured points along each colour channel's response curve, across ±3 stops. Neutral reads neutral at every density level, not just at one — which removes the shadow and highlight casts that a single white balance move can never fix.
A fitted colour matrix The chart's colour patches fit a cross-channel correction for the overlap between the film's dye layers and the camera's colour filters. It passes statistical quality gates or it is not used.
The film's own base Each roll's unexposed film base is measured and subtracted in density space, so the orange mask and the scan light's colour cancel exactly — per roll, not estimated per frame.
Exposure as arithmetic On a negative, an exposure error is a uniform density offset. It is removed with a gain that preserves colour ratios, placing every frame at the same print density.

Consistency, compared

The test below is one exposure bracket — the same scene shot seven times, from several stops under to several stops over — scanned once, then converted by four tools from the same RAW files. The only manual adjustment in any tool was density, brought toward a similar level where the tool offers a density control; nothing else was touched. A converter that anchors to the film renders these frames nearly alike; a converter that estimates from each image drifts.

Each row is one tool across the bracket. Read along a row: the differences within a row are what that tool would do across an ordinary roll.

5 / 4

Metrakon colour balance drift across the whole bracket (mean R−G / B−G, 8-bit levels) — including the deep unders

up to 19 / 13

The same drift for the other tools — visible as the warm-to-cool swing along their rows

16 vs 42–56

Median brightness spread (8-bit levels) over the normal and over-exposed frames — even with density matched by hand where the tool allows it. Metrakon's levelling is automatic.

Inspect a frame

Pick a frame, then switch tools — the image swaps in place, so differences show as a flicker.

Method: identical RAW camera scans (white-light source) converted in each tool. The only manual adjustment anywhere was density, brought toward a similar level where the tool provides a density control; Negative Lab Pro offers no simple density control and was left at its defaults. No colour, contrast or other settings were changed in any tool. Deep under-exposed frames are left dark by Metrakon by design rather than lifted. Full-resolution exports were downsized identically for the web. Product names belong to their respective owners.

Scanning light, measured at the sensor

Digital cameras apply substantial white balance gains to raw sensor data before you see anything — on a typical body, around ×2.6 on red and ×1.6 on blue. A light mix that looks well balanced on the camera's histogram can leave the sensor badly starved of red, and colour negative film blocks most red to begin with.

Tuning the light against raw, gain-free sensor values instead of the camera's processed preview produced a substantial improvement in red-channel quality for the same scan.

34% → 89%

Raw red channel level at clear film base, before and after tuning

~2×

Cleaner red content for the same exposure and film

4.58 → 4.74

Measured white density — the red response now resolves to the top of its range

Sensor-balanced scanning light — Drag to compare. The same negative, converted identically; only the scanning light mix differs — daylight-balanced on the left, balanced for the sensor on the right.

Daylight-balanced scanning light — Drag to compare. The same negative, converted identically; only the scanning light mix differs — daylight-balanced on the left, balanced for the sensor on the right.

White light works too

None of this makes an RGB source a requirement. Metrakon works with white light — an LED panel, a light table — because calibration is measured for whichever light you actually use. The chart session is identical either way, and a white-light profile is as consistent across a roll as an RGB one.

The difference is how much colour information reaches the sensor. Measured on the same negatives scanned both ways, narrowband RGB resolves about 20% more separation between colours, because narrow channels read closer to the film's actual dye densities. White light gave no noise advantage in exchange. So RGB extracts more from the same negative, but white light is a perfectly workable starting point.

One rule holds regardless: a profile belongs to a light source as well as a film stock. The overlap between channels changes with the light, so a profile calibrated under one cannot be converted to another — scanning the same stock under a different light means one more five-minute chart session.

Who it is for

Metrakon is built for volume: film labs digitising customer rolls with a digital camera, and photographers scanning their own work who care more about consistency than about per-frame control.

It is not a general-purpose editor, and it deliberately will not convert without a calibration. That constraint is what the consistency is built on. Calibrating a stock takes about five minutes and is done once per film stock and light source.