Factor = 2^Δf

Δf = ln(factor)/ln2


Example:

#8 yellow = K2 is a 1-stop filter with factor 2

Factor = 2^1 = 2

Δf = ln(2)/ln2 = 1 stop


For the light yellow K1 (#6)

Δf = ln(1.5)/ln2 = .58 stop (usually rounded up to 2/3 stop)