The Bortle scale, created by amateur astronomer John Bortle in 2001, rates night-sky darkness from class 1 (pristine) to class 9 (inner city). For night photographers, it predicts what the Milky Way will look like.
The classes
| Class | Sky | What you'll see and photograph |
|---|---|---|
| 1 | Excellent dark site | Milky Way casts shadows; airglow visible |
| 2 | Truly dark | Rich Milky Way structure; clouds appear black |
| 3 | Rural | Light domes on the horizon; Milky Way still detailed |
| 4 | Rural/suburban | Milky Way clear overhead, weak near the horizon |
| 5 | Suburban | Milky Way faint; sky glows in photos |
| 6 | Bright suburban | Milky Way only near the zenith |
| 7–9 | Urban | Milky Way invisible; only bright stars |
SQM readings
A Sky Quality Meter measures sky brightness in magnitudes per square arcsecond. Bigger numbers are darker: about 22.0 at the darkest sites, 21.5 in good rural skies, 19.5–20.5 in suburbs and below 18 in cities. Convert readings with the SQM to Bortle converter.
How it affects photos
Light pollution adds a bright, often orange-gray glow that reduces contrast and limits exposure time. You can partly correct color in processing, but faint detail lost in the glow doesn't come back. Each step darker on the scale improves contrast noticeably.
Finding dark skies
- Use the light pollution checker for any coordinates — it also shows which towns cause the glow and in which direction.
- Every location page on this site lists an estimated Bortle class, and our Milky Way by state pages rank the darkest spots.
- Certified dark sky places protect their skies with lighting ordinances.
- Shoot away from the direction of nearby cities, and go higher: thinner air above haze is darker.
About 1 min read · Updated 2026-10-20