Your whole roll, balanced at once

Why measure the whole roll

A frame alone cannot say where its white would have been. A thin negative and a flat scene hand a converter the same data, so a frame balanced on its own pulls both towards the same picture, and the difference that made one of them thin is lost.

A roll can say it, because a roll is one film, one development and one digitisation. Read together, its frames show what is the film and what is the scene: the window shared by all of them is the film, and what one frame does differently is your photograph.

Two rolls of the same film never come out of the tank identical. The emulsion batch, how old the film was, the developer's temperature and time, a push or a pull: each shifts the colour and the contrast of the whole roll at once. The light of your scanner or your camera setup adds its own shift on top. Expired film drifts further still: as it ages it loses contrast and leans toward one colour, more so after a summer in a warm drawer.

All of it is shared by every frame of the roll, so it is measured once, for the roll, and the Roll tab holds the result. Start a new roll, and Prepare the Roll reads it afresh.

Even out your light first

A light panel is rarely even, and a macro lens darkens its own corners: the edges of every scan come out darker, sometimes tinted. On a negative, darker means denser, and the corners of your photo print lighter than the centre.

Shoot the empty light panel once, with the setup you scanned the roll with, and click it at the second step, Set Up the Flat-Field. That shot becomes the flat-field: it evens out the light whatever you crop to, and every frame is measured through it. Skip it when your light is even, as a scanner's usually is.

Pick the bare film

Every colour negative carries its orange mask on clear film, between the frames and along the rebate. That bare film is the scene's zero: nothing of the picture is in it.

At the BASE step, choose a scan and click the border or the gap between two frames, under the loupe; Picker Size sets how wide an area it reads. Click well clear of the picture, because a click on the image shifts the blacks and colours of the whole roll. Skip the step, and the black is read across the roll's frames.

Light leaks and extreme frames

A light leak is light that reached the film outside the exposure: a worn door seal, a back opened too early, the first frame fogged while loading. Once converted, it shows as an orange or red flare. An extreme frame is a real photograph that strays far from the rest, a night shot or a frame into the sun.

Flag them at the fourth step, Flag the Frames Left Out. They are still converted, and they open on the roll's window, without counting in its measurement: left unflagged, one of them can wash out or tint the whole roll.

An average follows its strangest member: one frame fogged orange pulls the whole roll toward orange. A median is the middle value once the frames are sorted, so it follows the roll, and a stray frame only moves it by one place.

That is why the roll's gains are each channel's median across its frames, and why the window's ends are read with the extremes trimmed. Flagging still matters for a roll with several strays, or a short one, where each frame weighs more.

Keep your setup still for the whole roll

The roll's measurement trusts that every frame was digitised the same way. Scan the whole roll in one sitting: the same light panel at the same brightness, the same lens and distance, the same holder, the same exposure, with the camera's own white balance fixed rather than automatic. Let an LED panel warm up first, and keep room light off the film.

Then the only differences left between your scans are the ones on the film, which is exactly what Prepare the Roll reads, and its measurement takes the roll's own drift off before you edit a single frame. The flat-field shot belongs to that same sitting, so take it before you move anything.

A camera on aperture priority or auto ISO picks a new exposure for every frame, and a dense negative gets more light than a thin one: the scans no longer say how dense each frame really was. Prepare the Roll reads each scan's shutter speed, aperture and ISO, marks the frames digitised with other settings than the rest, and evens their exposure out to the roll's.

A different aperture also changes the vignetting the flat-field measured, so the cleanest roll comes from one manual exposure, kept from the first frame to the last.

Where the measurement reads

Inside the measuring frame you draw at the last step, clear of the holder and the film's edges; the Frame slider runs every scan under it so you can check it on each one. An edge caught inside skews the whole roll.

The frame is cut into small cells, each read at its median: a speck of dust under half a cell never counts, and an extreme has to cover an area to count. With a film base picked, the bare film is every channel's black. A channel clipped at the scanner's black level is beyond any setting: that frame wants scanning again, a little brighter.

Each frame, read in two steps

A scanned negative leaves its three channels in different places and over different spans. Step one brings red's and blue's medians onto green's with a gain, the job a darkroom enlarger's colour head does with its lamp, never past the window's ends.

Step two then places each channel's black and white at those gains: the three channels land on one another, ready for your edit.

The roll's gains are each channel's median across its frames. The colour of each end comes from a median with the most extreme frames left out, a tenth at each end; its depth from the frames sorted darkest first, a fifth of the way in for the black and four fifths for the white. A flagged frame takes the window and stays out of the count.

So a few frames' deepest shadows and brightest lights are clipped for the roll's contrast, and the Black point and White point of the Color tab take them back on the frames that need it.

What the roll shares, and what each frame keeps

The Roll tab holds what the roll shares: the measured roll, the film type, the flat-field, the source encoding, the source balance, the density ceiling, the per-channel levels, the dye separation and Balance Strength. Moved on one photo, a setting there reaches every photo measured with the roll, each keeping its own look.

Your own look lives in the other tabs, frame by frame, and sits on top of what the roll shares: a frame edited before a change in the Roll tab keeps its edit after it.

A photo of the folder that was left out of the measurement says so in the Roll tab, where its settings act on it alone. JOIN THE ROLL gives it the roll's measurement and settings, its own look kept; ROLL OF THE, followed by a count, makes the photos measured together a roll of their own.

PREPARE THE ROLL…, in the same tab, opens the measurement, and a Prepare the Roll you leave halfway picks up at the step where you stopped.

Auto White Balance, on top of the roll

The roll holds the film's colour; what changes from frame to frame is the light the scene was shot in. Auto White Balance reads it by Shades of Gray: each channel's sixth-power mean of the rendered frame, its bright areas weighing more than a large warm subject, brought onto green's by Temperature and Tint. Those two move the colour alone, never how light your photo is.

Balance Strength, in the Roll tab, sets the share of that correction Temperature and Tint take. It starts at 67 %, which keeps a warm evening warm; each photo keeps its own correction, and the share is the roll's.