SaltPrint — Positives and negatives

The short version. Your photograph is a positive: bright things are bright. Alt-process printing exposes paper through a transparency, so the transparency has to be a negative — bright things dark — or the print comes out inverted. That inversion is what SaltPrint's Reversed tonal scale setting does.

What each one is

A positive is the image as you saw it. Snow is light, shadow is dark. Every photograph you open in SaltPrint is a positive, whether it came from a camera raw file, a scan, or a phone.

A negative is that image with its tones inverted. Snow is dark, shadow is light. It looks wrong on screen, and it is supposed to.

In the photograph	On the negative	UV through it	On the final print
Bright highlight	Dense, dark	Blocked	Light — paper stays pale
Midtone	Middling	Partly through	Midtone
Deep shadow	Clear film	Straight through	Dark — fully exposed

Why the inversion is necessary

Salt, cyanotype, platinum, van dyke and the rest are contact-printed. The transparency is laid directly against the sensitized paper and light — usually UV — is shone through it. Wherever light reaches the paper, the paper darkens.

So the tones arrive reversed by the process itself. Where the transparency is dense, little light gets through and the paper stays pale; where it's clear, the paper goes dark. Expose through a positive and you get a print with the tones backwards — a negative image on paper. Expose through a negative and the two inversions cancel out, leaving a positive print.

Two consequences worth knowing before you print anything:

The negative is the size of the finished print. There's no enlarger in the chain — the transparency sits on the paper — so an 8 × 10 print needs an 8 × 10 negative. That's why print size matters more here than in ordinary printing.
The negative's density is doing real work. It isn't a preview of the print; it's the thing that decides how much light each part of the paper receives.

What SaltPrint actually does

Reversed tonal scale, in the Make block, is the switch. On, and the tonal scale runs from paper white down to the scale colour, with your highlights at the dense end. Off, and you get a positive.

The preview always shows the file that will be written — so with the setting on, you're looking at a negative. That's deliberate. The negative is what you'll print through, so it's the thing worth judging.

What it is not doing is guessing. It doesn't examine your image and decide whether it looks like a negative already. If you feed it a scan of a film negative and leave the setting on, you'll get a positive transparency — correct behaviour, since it did what you asked.

When you'd want a positive instead

Turning the reversal off is a real option, not a mistake:

You're starting from a film negative. A scan of a negative is already inverted, so inverting it again gives you a positive transparency. Leave Reversed tonal scale off and print from the scan as it is.
You want a toned positive on paper. Everything SaltPrint does to the tonal scale works on a positive too — that's a print in a single colour rather than a negative to expose through.
You're making an interpositive as a step in a longer process.

Why you can't judge a negative by looking at it

A negative's job is to block UV. Your eyes judge visible light. Those are different questions, and the answers don't reliably agree: a colour that looks weak and washed-out on screen can block UV better than one that looks convincingly black, and a negative that looks dense can be passing more light than you'd guess.

So the density that matters is not the density you can see. It's measured by exposing a sheet and looking at the paper afterwards — which is what the tonal-scale test sheets are for. Judge from a print, every time.

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