Bortle is a descriptive class (1–9) based on what you can see visually.
Sky brightness is a measurable number in mag/arcsec² (SQM). This is what your exposure math actually uses.
Typical rough mapping (varies a lot with direction, altitude, humidity, moon, etc.):
Bortle 2: ~21.6–22.0 mag/arcsec²
Bortle 4: ~20.8–21.3
Bortle 6: ~19.5–20.3
Bortle 8/9: ~18–19
If you’re sky-limited (which wide-field usually is), then the SNR improves like:
SNR ∝ √(total integration / sky flux)
So to get the same final SNR, required integration time scales directly with sky brightness (flux):
A difference in sky brightness of Δm mag/arcsec² changes sky flux by:
Sky flux ratio = 10^(0.4 × Δm)
And therefore:
Required integration time ratio ≈ 10^(0.4 × Δm) (to reach the same depth/SNR)
Bortle 6 (20.0) → Bortle 3 (21.5):
Δm = 1.5 → time ratio = 10^(0.6) ≈ 4× less time at 21.5.
20.0 → 22.0:
Δm = 2.0 → time ratio = 10^(0.8) ≈ 6.3× less time at 22.0.
So if you needed 10 hours from a ~20.0 sky, you might need only:
~2.5 hours at 21.5
~1.6 hours at 22.0
That’s why dark-site trips are “integration multipliers”.
For very wide field (fast lenses, big pixels, bright sky background), your sub length often hits a practical ceiling because the background climbs into:
clipped highlights,
lost dynamic range,
ugly gradients,
reduced star colour (stars saturate sooner as the background rises).
A simple rule:
Background ADU increases linearly with exposure time and linearly with sky flux.
So the maximum sub length before “sky fog too bright” scales like:
t_max ∝ 1 / sky flux
Meaning if you darken the sky by Δm:
t_max ratio ≈ 10^(0.4 × Δm)
Going from 20.0 → 21.5 (Δm=1.5):
t_max becomes ~4× longer
Going from 20.0 → 22.0 (Δm=2.0):
t_max becomes ~6.3× longer
So if your wide-field setup is limited to 15s before the background gets gross at ~20.0, then at:
21.5 you may manage ~60s
22.0 you may manage ~95s
(Exact numbers depend on f/ratio, pixel size, gain/ISO, full well, and how much headroom you leave for gradients and star saturation — but the scaling holds.)
Dark skies don’t just reduce noise — they let you run fewer, longer subs:
fewer files,
less stacking overhead,
less read-noise penalty,
easier gradients,
cleaner colour.
A good rule of thumb to use: A sky mag increase of 0.3 = 25% less total integration time needed!