On 1/17/07, Peter Kümmel <address@hidden> wrote:
> I've figured it out, where 0.18 comes from:
>
> sqrt(x)/x = x^(-1/2) = middle
> x^(1/2) = 1/middle
> x = (1/middle)^2
>
> Because of this using 0.18 as middle value is
> equivalent to assume a contrast of only 1:31.
The 0.18 is the rounded value of a contrast
of 5 f-stops:
sqrt(32)/32 = 0.177
As a magic number 0.18 sounds familiar... 18% gray?
(Sorry, maybe nothing that contributes to the discussion; just wanted to share my two cents of associative talent.)