The Bortle scale explained: how light pollution changes what you can photograph

2 November 2026 · 3 min read

The Bortle scale rates how dark a site's night sky is, from 1 (the darkest places on Earth) to 9 (an inner-city sky). The amateur astronomer John E. Bortle published it in Sky & Telescope in 2001, and it's still the quickest way to describe your sky.

The nine classes

Class	Name	What you see
1	Excellent dark site	The Milky Way casts shadows; zodiacal light is obvious; airglow visible
2	Truly dark site	The summer Milky Way shows detailed structure; clouds look like black holes in the stars
3	Rural sky	Some light domes on the horizon; the Milky Way still looks complex
4	Rural/suburban transition	Light domes in several directions; the Milky Way is visible but lacks detail
5	Suburban sky	The Milky Way is faint or invisible near the horizon; clouds are brighter than the sky
6	Bright suburban sky	The Milky Way only visible near the zenith, if at all
7	Suburban/urban transition	The whole sky has a grey-white glow; the Milky Way is invisible
8	City sky	The sky glows orange or white; only bright constellations are recognisable
9	Inner-city sky	Only the moon, planets and a handful of bright stars are visible

Bortle described each class by eye. Many people now use a sky quality meter (SQM), which measures sky brightness in magnitudes per square arcsecond. Higher numbers are darker: very dark sites read about 21.7–22, suburbs about 19.5–20.5, and city centres below 18.5.

How to find your class

Look up your site on a light pollution map, such as lightpollutionmap.info, which estimates sky brightness from satellite data and labels it with a Bortle class. Then sanity-check it on a clear, moonless night: can you see the Milky Way? How faint are the stars you can see overhead?

What light pollution costs you

Light pollution adds a bright background to every frame, just like moonlight. The faint target is still there, but it sits on top of a brighter sky, which brings more noise. To get the same quality image you need more total exposure.

The cost grows fast. Every magnitude of extra sky brightness is about 2.5 times more sky glow. A Bortle 7 sky can easily be 10 times brighter than a Bortle 3 sky, and for faint broadband targets you'd need roughly that much more exposure to match the dark-site result. That's why an hour under a dark sky can beat a whole night in the suburbs.

What works from a bright site

You can still do excellent work from Bortle 6–8:

Narrowband emission nebulae. Hα, OIII and SII filters pass only a few nanometres around each emission line, blocking most streetlight. This is the single biggest help for city imagers.
Dual-band filters on colour cameras. The same idea in one filter: the Orion, Rosette, Heart, North America and Veil nebulae all respond brilliantly.
Bright clusters and the brightest galaxies. Globular clusters and the cores of M31, M81 and M82 work with longer integration.
Light-pollution filters help a little against older sodium streetlights, but modern white LED lighting covers the whole spectrum and broadband filters can't remove it. They also dim galaxies and change their colour.

Save faint galaxies, reflection nebulae, dark nebulae and dust for trips to a darker site.

Set it in RigTonight

On the Tonight tab, set Light pollution to your site's Bortle class. RigTonight lowers each hour's score for brighter skies, so you can compare a night at home with a night at a dark site, and its filter advice for each target tells you when a narrowband or dual-band filter will make the difference.