This is the source code to the firmware powering the ΔStop 100 F-Stop timer. It is provided as-is under the terms of the MIT license. For more, visit: https://delta100.johnjonesfour.com/

ΔStop 100 is a dedicated enlarger timer that uses logarithmic, f-stop based timing instead of linear seconds. You set a base exposure in stops and every click changes exposure by a fixed fraction of a stop, so the math stays the same no matter the print size or paper type. That means faster test strips, clearer intent with every change, and less guesswork when a print needs a tiny nudge instead of a full second.

It behaves the way photographers already think: exposure as stops, not time. The interface is focused on repeatable decisions, so you can move from test to final with confidence and keep a consistent record of the choices that got you there.

Traditional timers measure time linearly, so the difference between 7 and 8 seconds is not the same change as the difference between 14 and 15 seconds. F-stop printing measures exposure in stops, where each step represents a doubling or halving of light.

Because each step is a constant ratio, your adjustments are predictable and comparable. Half- and third-stop changes feel natural, dodging and burning are easier to keep consistent, and notes from a previous session translate cleanly when you change paper size, contrast, or enlarger head.

ΔStop 100 applies this concept directly to the darkroom, without menus, math, or mental gymnastics. It gives you a repeatable workflow built on exposure theory you already use in the camera.