Light from a small source falls off with the square of the distance. Doubling the distance cuts light to a quarter — two stops. Enter the old and new distance.
How it works
I₂/I₁ = (d₁/d₂)²; stops = 2·log₂(d₂/d₁).
FAQ
Does it apply to the sun?
Not in practice — the sun is so far away that moving a few meters changes nothing.
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