A calibration test chart isn't a single object: one version lives inside your negative, others run freely on the site. This page walks through them and tells you which one to pick for what you need.
#How it works
The app's chart lives in the Negative screen, not in a separate screen. When you generate a negative, the bottom options bar offers Format · Style · Effects · Target. That tab opens a sheet titled Control strip, subtitled Reference target. A toggle, Show the target, does exactly what it says: "Adds a 25-step grey scale at the bottom left" of your negative.
That scale is a 5×5 grid of 25 solid grey patches, from white to black, plus a 6th column with a continuous gradient — always present alongside the grid, not a separate option. It's drawn at 25% of the smaller side of your image, placed at the bottom left with a small margin: it covers that corner of your negative.
Two things matter for using it correctly:
- It's drawn before the curve and before the negative inversion. It gets exactly the same treatment as your photo. With no curve selected, you get the raw chart — useful for your very first calibration. With a curve selected, you get a control chart: it tells you whether that curve holds up on this specific print, on the same negative as your photo.
- It has no polarity setting of its own. It follows whichever Negative or Positive mode you chose for the whole negative — there's no separate switch. Generate in negative, and the chart comes out in negative with everything else; the sheet's thumbnail preview follows the same mode.
Multi-zone mode (1, 2 or 4 zones) "splits the image into zones so you can test several curves on a single print." In 2 or 4 zones, each zone gets its own small 5×5 chart + gradient, computed with that zone's own curve, plus a label showing that curve's name.
Turning on "Show the target" automatically turns off "Fill the paper": the two aren't compatible together. There's no Pro-mode lock on this chart — it's free, like the rest of negative generation.
Outside the app, three free generators run right in your browser, on the site, with nothing to install and no account:
- The 21-step chart — a 3×7 grid (21 patches) plus a gradient, polarity of your choice, as a 1680×668 px PNG at 300 dpi, no watermark.
- The same logic in 25 patches (5×5 + gradient, the same math as the app's chart) on the site's process pages — cyanotype, Van Dyke, platinotype, gum bichromate, carbon, screen printing — with polarity already set for that page's process. The screen printing one, positive, can be screened.
- The Color Venn chart, free on the gum bichromate and carbon pages, as an A4 PNG at 300 dpi: it checks how your inks overlap, not a grey scale.
#Which chart for which process
| Your process | Polarity |
|---|---|
| Cyanotype, Aquaprint (all variants), bromoil, carbon, gumoil | Negative |
| Resinotype, screen printing | Positive |
| Printer calibration alone | Negative |
Full detail on the rule and its origin: Positive or negative chart.
#Why it matters
The app's chart is the one you'll use most often, because it's already where you're working: no separate file to keep track of, no risk of using a chart printed with a different printer or paper than the actual print. Since it gets the same curve and the same inversion as your photo, a clean control chart tells you directly whether your correction holds on this specific paper, with no guesswork.
The site's charts answer a different need: testing or calibrating before you even have a photo to print, or keeping a standalone chart file on hand. They don't replace the app's chart, they complement it.
#When to avoid it
If you want a standalone chart file, independent of your photo. The app's chart is always printed inside the negative: it can't be exported on its own. Use one of the site's generators instead.
If your subject fills the whole frame with no margin. The chart covers about 25% of the bottom-left corner of your negative: it overlaps the image beneath it.
If you want a different polarity for the chart than for the image. Not possible today in the app: both follow the same Negative/Positive mode.
If you want your own patch count or chart size. No chart allows it: the app always draws 25 greys at 25 % of the shorter side, the site offers 21 or 25 patches. The workarounds are on Create a custom test chart.
If you're calibrating a color chain (Aquaprint Four-Color, Carbon Couleur). The Color Venn chart tests ink overlap, not per-channel tonal values: it doesn't replace a per-channel 5×5 chart.
#Technical details
Options-bar tab: Mire (FR) / Target (EN).
Sheet: title Bande de Contrôle (FR) / Control strip (EN), subtitle Mire de référence (FR) / Reference target (EN). Toggle Afficher la mire (FR) / Show the target (EN): "Adds a 25-step grey scale at the bottom left." Selector Mode Multi-Zone (FR) / Multi-zone mode (EN): 1, 2 or 4 zones.
5×5 grid = 25 solid patches, linear values (i/24 → 0-255), plus a 6th column with a continuous gradient, thin grey border. Size: 25% of the generated image's smaller side, placed at the bottom left with a small margin.
The chart is drawn on the canvas BEFORE the greyscale conversion, the curve, and the negative inversion — it gets the same treatment as the image. If the negative is halftoned (the Screen option under the Effects tab), that halftoning applies after composition, to the whole sheet: the chart is halftoned along with everything else.
Turning on controlStrip forces fillPaper off.
Resolution: the chart follows the negative's format (Format tab) and resolution (Effects tab: 300, 450 or 600 dpi); there's no standalone "chart only" file exportable in this flow.
No Pro-mode lock on this chart in the current code.
21-step chart (site): 3×7 grid (21 patches) + gradient, polarity of your choice, 1680×668 px PNG at 300 dpi, no watermark, continuous-tone rendering. The same mechanics, in a 5×5 grid (25 patches), power the site's process pages, with polarity already set for that process.
Color Venn chart (site, gum bichromate and carbon pages): A4 PNG at 300 dpi, computed in the browser.
#The test
Open a negative in Calibration Flow, go to the Target tab in the options bar, turn on Show the target, and generate without selecting a curve. Look at the bottom-left corner of the result: you should see a 25-step grey grid and its gradient. Then pick a curve and regenerate — the chart should change with it. Move on to First calibration.