What this page is. Eighteen words this site uses, each defined in one or two plain sentences here and again at the place it is used. None of it is needed to convert a roll of film: it is here for the moment a technical page uses a word you would rather not guess at.

The software these words describe is Open Negative Initiative, a free macOS application for converting and editing scanned film: colour negatives, positives and black & white, from a camera scan or a scanner. It runs on its own, with no host application to buy. macOS 13 or later. Apple Silicon and Intel.

C-41
The standard development process for colour negative film. Nearly every colour negative you can buy is a C-41 film, which is why one conversion approach covers so many stocks.
Transmittance
The fraction of light a piece of film lets through. A transmittance of 0.1 passes a tenth of what falls on it. A raw scan is proportional to transmittance, which is why it needs converting before it behaves like a photograph.
Density
How much light a piece of film blocks, written as the base-ten logarithm of the inverse of transmittance. Density 1.0 passes a tenth of the light, 2.0 a hundredth, 3.0 a thousandth. Film has been measured this way for as long as film has been measured at all.
Stop
A doubling or a halving of light. On a density axis a stop is always the same distance, about 0.301, wherever you are on the scale. On a linear scale it is a tiny step in the shadows and an enormous one in the highlights.
Linear, or scene-linear
Numbers proportional to the light that reached the sensor, with no curve applied. A camera RAW is linear. A JPEG or a finished scanner TIFF is not: a curve has already been baked into it.
Gamma
The exponent of the curve applied to an image. A colour negative is exposed at a gamma near 0.6, and RA-4 colour paper works around 2.5 to 3; multiplied, the chain prints at about 1.6. Scanning skips the paper, so that contrast has to be put back.
Characteristic curve
The graph that says, for one layer of the film, how much density you get for a given amount of light. Each colour layer has its own, and they do not climb at the same rate, which is the whole difficulty of a colour negative, and why one global correction cannot undo the mask.
Colour head
The part of a darkroom enlarger that doses the red, the green and the blue of its lamp separately. It is the physical equivalent of three gains, one per channel, applied before everything else. Also called a dichroic head.
Quantile
The value below which a given share of the pixels sit. The 0.9 quantile is the value 90 % of the picture is darker than. The automatic balances read quantiles, which is a measurement and not a guess about the scene.
Median
The 0.5 quantile: the value half the pixels are below. Aligning the medians of red, green and blue assumes the scene averages to grey, which is true of many photographs and false of a sunset.
Luma and chroma
Two halves of a colour: how bright it is, and what colour it is. Film grain lives almost entirely in the brightness half, which is why colour noise can be removed without touching the grain.
Demosaicing
The step that turns the single-colour measurement under each sensor photosite into a full colour image. It happens at the very beginning, while the signal is still clean.
Rebate
The unexposed border of a film strip, between and around the frames. On colour negative it is orange, the same colour as the rest of the film base.
Flat-field correction
Dividing a frame by a photograph of the empty light source, taken with the same setup, to remove the falloff of the lens and the unevenness of the lightbox. It only works if the reference is decoded exactly like the frames.
Pedestal
A small constant added to every value before the logarithm is taken, because the logarithm has no answer at zero and a scan does contain zeros. Adding to everything keeps the pixels distinct; clamping the small ones would make them identical for ever.
Numerical guard
The floor under the pedestal, sitting an eighth of it lower, that catches whatever still arrives below zero. Values piled on it are the ones no setting can separate.
Graduation window
The stretch of the density axis the levels are drawn on, from −0.35 to 3.0 here. It starts below zero because a scan can transmit more light than the reference, and those pixels have to be reachable.
Sidecar
A small text file written beside each original, holding every setting for that photograph. The original is never modified, and the settings travel with the folder.

Where these words do their work

None of this is needed to start

Download for macOS. Free, no account, and nothing is applied to a photograph until you click it. If you would rather start by using the thing, the six-step guide gets you to an exported picture without one word of this page.