Your first calibration, step by step

The first calibration is the step that takes your alternative process from "it roughly works" to "it works reproducibly". You concretely measure how your chemistry responds to light on your paper with your UV source, and you get a correction curve that you'll apply to all your future images so they print the way you saw them.

Plan on one to two hours start to finish, including the time the sensitized paper takes to dry. Active work in the app runs fifteen to twenty minutes. The rest is waiting — chemistry drying between steps.

#What you do

Step one — Download the test chart. Open Calibration Flow, go to the Test Chart module, choose the variant suited to your process. For a cyanotype, an Aquaprint, a charbon, a bromoil or a gumoil: take a negative chart. For a résinotype: a positive chart. For the version: start with the plain one. If your printer renders flat tints poorly, take the high-resolution A4 landscape version with dithering. The downloaded file is a high-resolution PNG, ready to print.

Step two — Print on transparency. Print the PNG on an inkjet transparency with no colour management at all — that's the absolute rule, without which your calibration will be wrong even if everything else is flawless. The Printing a test chart without colour management page explains how to turn off your printer's automatic corrections.

Step three — Sensitize your paper. Depending on your process, prepare your sensitized paper as usual — brush-coated sensitizer for cyanotype, pre-sensitized gum for Aquaprint, and so on. Nothing specific to Calibration Flow here, your usual chemistry workflow applies.

Step four — Expose your chart. Lay the transparency on the sensitized paper, expose for the time set for your process with your usual UV source. Then dry your exposed paper following your routine. The whole usual chemistry chain applies. Once dry, you physically have a printed test chart — 25 grey patches from paper white to the densest black your process can reach.

Step five — Import into the app. Photograph the dried chart with your iPhone or scan it with a flatbed scanner. Import the photo into Calibration Flow through the Curve module. iPhone HEIC, JPEG, PNG — all are read directly.

Step six — Analyse and adjust. The app automatically places the measurement zones on your scanned chart and computes your process's response curve. The Smart Analyzer proposes a reasonable starting point for the three sliders (black point, white point, midtone gamma). You adjust if needed — the preview and the curve update in real time. See Tonal analysis for the detail.

Step seven — Export the .acv. Once the curve suits you, export it in Adobe

.acv

#Why it's worth taking the time to calibrate

Without calibration, your process responds in its own way — shadows block up to black, highlights crush to white, the usable tonal range may be 8 to 10 exploitable grey levels out of the 25 your paper can technically produce. With calibration, you recover the full range — 20 to 22 distinct levels on your final prints. Visually, that's the difference between a "rough" print and a "faithful" one.

It's also what makes your work reproducible over time. A calibration saved on 19 May 2026 tells you exactly how your process responded that day. Six months later, you can run another calibration and compare — if the new curve is very close to the old one, your chemistry is stable. If they diverge, you know something has drifted.

#When to start later

If you're a complete beginner in alt-process. Get familiar with the process first over three or four prints — learn to sensitize, to expose, to wash. Once you pull a clean cyanotype on the first try, that's the moment to calibrate.

If you're making a one-off print with no intention of a series. Calibration takes an hour or two. If you just want to try a process for fun without making it a regular practice, you can skip this step — the result won't be optimal but stays valid.

If your process is still unstable. If your chemistry varies from one print to the next, or if you're still changing paper regularly, calibrating now makes little sense — the calibration will just capture a temporary state. Stabilize your routine first, then calibrate.

#Key points

#The test

Run your first calibration on a process you have a handle on (typically cyanotype). Once the

.acv