Add README and LICENSE

README.md

@@ -0,0 +1,47 @@
+# Exposure Match Calculator
+
+A tiny static web app that calculates equivalent camera exposure settings between a **reference camera (A)** and a **target camera (B)**.
+
+## How it works
+
+The app uses exposure equivalence:
+
+`(N² / t) × (100 / ISO) = constant`
+
+Equivalent rearranged form:
+
+`t₂ = t₁ × (N₂² / N₁²) × (ISO₁ / ISO₂)`
+
+Where:
+
+- `N` = f-number (aperture)
+- `t` = shutter time in seconds
+- `ISO` = sensor sensitivity value
+
+## Usage
+
+1. Open the app in your browser.
+2. Fill all values for **Camera A (reference)**.
+3. In **Camera B (target)**, enter exactly two values and leave one empty.
+4. Click **Calculate Missing Value**.
+5. The app computes the missing target value for equal exposure.
+
+## Run locally
+
+No build tools required.
+
+- Clone the repository.
+- Open `index.html` in your browser.
+
+## License (GPLv3)
+
+This project is licensed under the **GNU General Public License v3.0** (GPL-3.0).  
+You are free to use, modify, and redistribute this software under the terms of that license.  
+If you distribute modified versions, you must also provide the source code and keep it under GPLv3.
+
+See the [LICENSE](LICENSE) file for the full license text.
+
+## Notes
+
+- Shutter input supports values like `1/250`, `0.004`, or `2`.
+- Calculator is for technical equivalence; real-world image differences (noise, lens transmission, dynamic range) still apply.