Polarization
A circular polarizer contains a linear polarizing layer plus a quarter-wave layer. Rotating it changes which orientation of linearly polarized light passes; ideal polarized-light response follows Malus’ cos² law.
When does a polarizer help—and when does it only lose photons?
Change how polarized the incoming light is and rotate an ideal polarizer. The lesson separates twilight/reflection use from faint-star work, where losing light is usually the wrong trade.
Interactive transmission
Transmitted light—
Exposure loss—
Physical model
An ideal linear polarizer transmits half of unpolarized light. A linearly polarized component follows Malus’s law, I = I₀ cos²θ. A camera CPL adds a quarter-wave layer for autofocus/metering compatibility, but it still costs light.
Milky Way and stars
For faint celestial targets, a CPL normally removes valuable photons without removing the dominant sources of light pollution in a controlled way. Start without it unless you have a specific polarized-reflection problem.
Twilight and reflections
Scattered skylight and reflections from water or foliage can be polarized, so rotation can change contrast. With very wide lenses the polarization angle varies across the sky, creating visibly uneven darkening.
Try it
Set polarized fraction to 0%: angle no longer matters and the ideal model loses 1 EV. Then set 100% and rotate from 0° to 90° to see Malus’s law. Real filters never reach the ideal extremes.
Related
What a CPL is doing—and why it can hurt night sky work
Reflections and blue sky
Non-metallic reflections can be strongly polarized, so a CPL can suppress glare on water, glass and foliage. Rayleigh-scattered skylight is most polarized roughly 90° from the Sun, so wide-angle skies can become uneven.
The cost
A CPL normally loses roughly 1–2 stops even when its effect is not useful. At night, faint stars and Milky Way light are precious photons, so unnecessary filtration usually worsens signal-to-noise.
When to use it
Use a CPL when the subject benefit is clear—daytime reflections, foliage glare or controlled sky contrast. Remove it for faint-star and Milky Way work unless you intentionally need to suppress a specific polarized reflection.