Print size, and why DPI isn't it
Three modes, one honest readout, and the number that doesn't do what people think
Three modes, one honest readout, and the number that doesn't do what people think
The short version. Width and height — or the sheet — decide how big the print is. DPI does not. DPI only decides how many pixels fill that size. And because alt processes are contact-printed, the negative comes out the size of the finished print: an 8 × 10 print needs an 8 × 10 negative.
Print size is a single control with three answers. Pick one and only that mode's settings are shown, so nothing on screen is adjustable but ignored.
Say what paper you've loaded and SaltPrint places the image as large as your printer can actually print on it. This is what most people want.
It asks the printer for its own unprintable margin rather than assuming the paper's edge — no desktop printer can reach the edge of its own paper, and assuming otherwise is how images get clipped. That margin is shown, along with where the number came from, and you can override it: measure the white border on a real print with a ruler and type what you measured. Trust your ruler over the driver.
A sheet is a space the image is placed in, never padded to. The paper is the border, so a mismatched aspect ratio just leaves more paper on two sides.
4 × 5, 5 × 7, 8 × 10, whole plate, 11 × 14, or a size you type. Here the image is composed onto a canvas of exactly that size, so an aspect ratio that doesn't match gets a border with a tone.
Note these are image sizes, not paper sizes — the two are separate lists on purpose. Offering "A4" as an image size would promise a 210 × 297 mm print that no desktop printer can make, because none of them print to the edge.
The pixels as they are. The printed size is then whatever those pixels come to at your chosen DPI — which SaltPrint works out and tells you.
In every mode, a line reads “→ prints W × H”, in millimetres or inches. That is the real size of the finished print, including in Don't resize, and it accounts for whatever rotation or crop you've applied.
It's there because without it the only control with a visible effect is DPI — which leads people to change DPI over and over trying to fix a size, and get the same wrong print every time. If the readout says what you want, the print will be that size.
DPI multiplies pixels. It does not resize the print.
And it multiplies them quadratically — double the DPI and the file is four times the size. On an 8 × 10:
| DPI | Pixels | Uncompressed 16-bit TIFF |
|---|---|---|
| 360 | 10 Mpx | ~21 MB |
| 720 | 41 Mpx | ~83 MB |
| 1440 | 166 Mpx | ~332 MB |
| 2880 | 664 Mpx | ~1.3 GB |
A printer can't use more resolution than it lays down, so past a point you are paying in file size, memory and time for a print that looks identical. SaltPrint warns you when you head somewhere expensive and then lets you do it — 720 is genuinely useful on some pigment printers, and we can't ask your printer what it's really capable of.
Since an over-specified DPI never gives you a wrong-sized print, only a large file, it's a safe thing to get wrong. A wrong width is not.
There's no enlarger in alt-process printing. The negative lies directly on the sensitized paper, so the negative is the print, dimensionally. Getting the size wrong doesn't cost you a reprint — it costs you the coated paper, the chemistry and the exposure too.
Which is why the readout is worth reading before every unfamiliar print, and why the margin is worth measuring once, with a ruler, on your own printer. See also positives and negatives.