Re: [Openexr-devel] magic numbers

[Peter Kümmel]

Anders Dahnielson wrote:
> 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.)

Good hint, so I would say:

For a normal photographic subject with a 5 stop
brightness range 18% is about middle gray.

How good were 1950 the printers for making gray cards?

Peter