A photograph's brightness follows the reciprocity law: shutter time, ISO sensitivity and aperture area all trade off against each other. This simulator computes a relative exposure index from your three settings and feeds it into the render's exposure, while the shutter speed separately controls how long light trails (from the moving aurora and the slowly rotating night sky) accumulate on the sensor.
Exposure index: E = (t · ISO) / (100 · N²)
t = shutter time (s), ISO = sensor sensitivity, N = aperture f-number
Doubling t, doubling ISO, or halving N² each roughly doubles E
Star-trail onset: visible trailing once t exceeds ~15-20 s
(Earth's rotation smears point sources during long exposures)
- Shutter speed — longer exposures gather more light and let the aurora's motion blur into longer streaks; too long and stars start to trail.
- ISO — brightens the image but adds visible sensor grain/noise at high values.
- Aperture — a wider aperture (lower f-number) lets in more light per second, the same way a bigger shutter time does.
- White balance — tunes the colour cast; a Kelvin setting far from the scene's true colour temperature shifts the whole shot warm or cool.
- Manual focus — mirrors the article's key technique: zoom into a bright star in live view and adjust focus until it is a tight point, not a soft blob.
- Aurora activity (Kp) — higher Kp storms are brighter, move faster, spread further from the pole and are visible from further south in the UK.
Real-world application: this is exactly the workflow used by night photographers in Scotland, the Lake District and Northumberland to capture colour and structure the naked eye cannot see.