Both StepWedgeDensitone and AltProcessDensitone measure tone values out of an image file. Nothing else about the print reaches them. So whatever your scanner did to the tones on the way out — an auto-exposure, a contrast curve, a sharpening pass — is not a rendering choice you can undo later. It is now part of the measurement.
This page is the one setup worth getting right before either app can tell you anything useful.
Scan RAW, with a straight curve
The short version: scan RAW, with the tone curve set straight, and every automatic adjustment off.
RAW is the point at which the scanner has not yet decided what your print looks like. A rendered scan has a curve baked into it — usually one designed to make photographs look good, which is exactly the wrong instinct when the photograph’s tonality is the thing being measured. A step wedge scanned through a pleasing curve reports the curve as much as the print.
VueScan and SilverFast can both write raw files; a camera copy-shot is raw by default. If your scanner software cannot emit RAW at all, the next best thing is a 16-bit TIFF with the tone curve set to linear or neutral and every automatic correction disabled — the goal is the same, which is a file whose tone values track the print rather than an interpretation of it.
Both apps open Apple-supported RAW directly — DNG, NEF, CR2/CR3, ARW, ORF, RAF, RW2 — and StepWedgeDensitone decodes them with sharpening off, so what you measure is the sensor’s tonality rather than a rendering of it.
Turn all of this off
Every one of these changes tone, and several of them change it differently from scan to scan, which is worse than changing it at all:
- Auto-exposure — re-decides the black and white points per scan, so two wedges from the same session no longer compare
- Auto-levels / auto-contrast / auto-tone — same problem, applied to the histogram
- Sharpening / unsharp mask — moves values at every tonal edge, which is precisely where a patch boundary is
- Colour restoration, fading correction, backlight correction — tone curves under friendlier names
- Dust and scratch removal — invents pixels; both apps would rather see the dust and let you nudge a measurement circle off it
- Descreening — a blur that varies with what it thinks it found
What you want is the dullest, flattest, least helpful scan the software can produce.
Bit depth and colour
16-bit. An alt-process print’s whole tonal range can be compressed into a fairly narrow band of scanner values, and both apps then normalize that band across the full 0–1 scale. Doing that to 8-bit data spreads a few dozen levels over the whole range and the quantization shows up in the readings.
Grayscale or RGB both work — read the tone, not the tint. RGB is the safer default if you are not sure your scanner’s grayscale mode is a real channel rather than a conversion.
Colour space matters less than consistency. Both apps normalize to the print’s own extremes — the wedge’s lightest and darkest patch in StepWedgeDensitone, the detected paper white and maximum black in AltProcessDensitone — so nothing here is absolute densitometry and none is needed. But the same print scanned into two different colour spaces gives two different sets of numbers, so pick one and never change it.
Resolution
There is no single number, because the two apps care about different things.
StepWedgeDensitone averages a disc of 5–40 px radius at each measurement point, so every patch needs to be comfortably larger than that disc with room to spare for a nudge off a defect. A few hundred pixels across the short side of a patch is plenty; 300–600 ppi over a typical strip gets you there without producing a file that is slow to work with. Very large scans are capped in resolution for processing anyway.
AltProcessDensitone compares the scan against the source image, and warns scan resolution low when the scan has fewer pixels across the print area than the source does. Scan at least as large as the source image, and the warning goes away.
Do not clip either end
This is the failure that looks like a working scan.
Keep paper white off the top of the histogram and maximum black off the
bottom. Both apps normalize to the extremes they find, so a clipped scan
silently costs you range at that end: StepWedgeDensitone’s Density range
reads — when the black end clipped to zero, and AltProcessDensitone warns
scan highlights at top of range — likely clipped and flags Print max
as clipped?.
If you see either, reduce the exposure and rescan. Headroom at both ends is free; a clipped end is unrecoverable.
Frame the whole print
For AltProcessDensitone especially, get the entire print in the scan, including a little of the paper margin and the coating border if there is one. Those are the anchors the app prefers for paper white and maximum black — without them, an endpoint has to be estimated from a percentile and is marked estimated in red, which is a much weaker measurement.
For StepWedgeDensitone, include the paper around the wedge so the detector has an edge to find.
Let the print dry first
Alt-process prints dry down. A damp print measures darker than it will ever look again, and you will build a correction curve for a print that no longer exists. Dry fully — overnight is not excessive — before scanning.
Then save the preset and never touch it
Every reading either app produces is relative. That is a feature: it means you do not need a calibrated densitometer or a reference target. But it also means consistency between sessions is doing the work that absolute accuracy would otherwise do.
So once you have a setup that scans flat and clips nothing, save it as a preset in your scanner software and use it for every print you will ever measure. A scan you cannot reproduce in six months is a measurement you cannot compare to.
Copy-shot with a camera
A camera on a copy stand works, and is raw by default. The extra thing to get right is the light: uneven illumination across the print reads as a tone gradient, which is a real measured error rather than noise. Two lights at 45°, equal distance, and a check frame of blank paper before you start. Manual exposure, fixed white balance, no automatic anything — the same rules as above.