Exposure Calculator

The exposure triangle consists of three camera settings that work together to control exposure: ISO (sensor sensitivity), aperture (f-stop, the lens opening size), and shutter speed (how long the sensor is exposed to light). When you change one setting, you must adjust the others to maintain the same exposure. Understanding this relationship is essential for astrophotography where lighting conditions are challenging.

Exposure Value (EV) is a number representing the combination of shutter speed and aperture that produces a given exposure at ISO 100. EV 0 is defined as f/1.0 at 1 second. Each +1 EV halves the amount of light (one stop darker), while -1 EV doubles it. The formula is: EV = log₂(f² / t), where f is the f-number and t is the shutter speed in seconds. Astrophotography typically requires very low EV values, from -6 to -2.

Equivalent exposures are different combinations of ISO, aperture, and shutter speed that result in the same overall brightness. For example, ISO 1600, f/2.8, 15s is equivalent to ISO 3200, f/2.8, 7.5s — doubling ISO allows halving shutter speed. This matters because you can trade off settings to optimize for your specific needs: shorter shutter for sharper stars, lower ISO for less noise, or wider aperture for more light.

For Milky Way photography, ISO 3200-6400 is typically the sweet spot for modern cameras. This provides enough sensitivity to capture the dim Milky Way while keeping noise at acceptable levels. Many Sony, Canon, and Nikon cameras perform well up to ISO 6400. Start at ISO 3200 and adjust based on your specific camera's high-ISO performance and the darkness of your location.

For astrophotography, use the widest aperture your lens allows (f/1.4 to f/2.8 ideally). Wider apertures let in more light, which is critical for capturing dim celestial objects. While some lenses show better corner sharpness when stopped down to f/4, the significant light loss (half the light per stop) usually isn't worth it. Fast prime lenses like 14mm f/1.8, 20mm f/1.4, or 24mm f/1.4 are excellent choices.

For sharp, pinpoint stars, use the NPF Rule calculator on this site to determine your maximum shutter speed based on your specific camera and lens. Generally, you want the longest shutter speed that doesn't cause star trailing. For a 24mm lens on full-frame, this is typically 10-15 seconds. For star trails, use 30+ seconds or stack multiple exposures. Longer exposures gather more light but may cause stars to streak.

Star trails require either very long single exposures (30 minutes to several hours) or stacking many shorter exposures. For stacking: use ISO 800-1600, f/4-f/5.6, and 30-second exposures taken continuously. Then combine them in software like StarStax or Photoshop. This method prevents noise buildup and allows for battery changes. For single exposures, use Bulb mode with a remote trigger and very low ISO (100-400).

Night sky exposure is tricky because camera meters can't reliably measure starlight. Use the presets in this calculator as starting points: Milky Way (ISO 3200, f/2.8, 15s) for a typical clear night. If too dark, increase ISO or use a longer shutter (within NPF limits). If too bright (light pollution), decrease ISO or shutter speed. Always shoot RAW for maximum flexibility in post-processing.