Tool · 21-step test chart
Free 21-step test chart, ready to print.
A 21-step test chart you can download freely, as a 300 DPI PNG, negative or positive. Same idea as the Stouffer 21 step, to read by eye or with a densitometer. No sign-up: the file is computed in your browser.
Generate your 21-step (Stouffer-compatible) target now
21-patch target (3×7 grid + continuous gradient), equal steps of about 5 % in grey value, from white to black. Pick your polarity below, download, print on inkjet transparency with colour management off, expose onto your sensitised paper.
Downloaded file at 1680×668 px (300 dpi), free, no sign-up. In the app, a 25-step grey grid and its gradient print straight onto a corner of your negative (Target → Show the target tab) and get the same curve as the image.
Target computed like the app’s: evenly spaced grey steps and a continuous gradient. On a scan, the app reads each step as perceptual lightness, L* (CIELAB).
Download
The file you download.
- Format
- PNG, 1680 × 668 px at 300 DPI — a strip of about 14 × 5.7 cm once printed.
- Layout
- 21 steps in 3 rows of 7, from white to black at equal intervals, plus a continuous gradient in the last column.
- Polarity
- Negative or positive, to pick in the generator. No watermark, no sign-up.
Format choice
Why 21 steps, and not 25?
In the app, Calibration Flow uses a 25-patch chart (5×5 grid, steps of about 4 %) that prints straight onto your negative, read on the L* CIELAB scale. This 21-step chart is the generic version: it follows the principle of the Stouffer 21 step, the reference scale of labs for decades.
21 steps is a compromise between granularity and visual legibility. On a Stouffer, each step is worth 0.15 optical density, or half a stop; here the steps are equal in grey value (5 % each), and your printer turns them into densities. That is enough to see where a process blocks up in the shadows and loses the highlights.
If you want a numerical calibration with automatic L* reading and .acv export (Pro mode, €9.90/month), the 25-patch chart of the Calibration Flow app is more precise. If you want a paper chart to print and read visually or with a densitometer, this one fits.
Usage
How to use this chart
- 1
Print without colour management
Print the PNG in greyscale, with no ICC profile applied, on transparent film or diffusing paper. Any software correction skews the reference linearity.
- 2
Expose on your sensitised paper
Place the chart on the sensitised paper, expose under your UV source. Note your parameters — time, distance, source — to reproduce.
- 3
Read the bounds
Identify the first visible step (practical Dmin) and the last step distinct from black (practical Dmax). You get the usable tonal range of your process.
25 patches, L* reading, exported curve.
In Calibration Flow, the 25-patch chart prints straight onto your negative, and the app measures your print once photographed or scanned. Free with sign-up on the web, no credit card.
Calibrate with the 25-patch chart of the app