The score measures how much starlight the whole frame collects in the longest exposure you can take before the stars start to trail.
- Exposure time, from the 500 rule:
t = 500 ÷ (crop factor × focal length). The crop factor is 43.27mm divided by the sensor diagonal.
- Light collected:
sensor area × t ÷ f-stop²
- Score:
100 × √(light ÷ baseline light). It uses the square root because image noise improves with the square root of the light collected, so the score tracks how clean the image will be.
Baseline: a 14mm f/1.4 on full frame scores 100. That's the widest, fastest full-frame combination currently made.
Limited to real lenses: on each sensor format, the controls won't let you pick a setup that's both wider and faster than a lens that actually exists. The limits come from our database of cinema and stills lenses (fisheyes excluded). So the fastest aperture depends on the focal length: on full frame you can reach f/1.4 at 14mm, but f/0.95 only from 35mm.
Capped at 100: no lens currently made, on any format, collects more light than the baseline, so no setup scores above 100. Custom sensor sizes have no lens list, so only the cap applies to them.
- One stop faster: score ×1.41
- Half the focal length: score ×1.41
- A bigger sensor gains twice: more area, and a longer exposure at the same focal length.
- Enter the T-stop if you know it. It's a truer comparison between lenses.
- Enter the aperture you would actually shoot at. Many fast lenses need stopping down for clean stars in the corners.
Not scored: fisheye lenses (their projection breaks the field-of-view maths), sensor noise, sharpness, coma and vignetting. The 500 rule is an approximation; on high-resolution sensors stars may trail sooner.
Based on the Lonely SpecMark by Ian Norman of Lonely Speck, simplified to sensor, focal length and aperture only.