Digital BW, The Print

> From: Ernst Dinkla
>
> Quotes:
>
> Can anyone tell me if there is a standard consensus about what Lab
> Luminosity value a Kodak 18% grey card should yield? I know there is a
> formula for calculating L* from density. I saw it once but couldn\ufffdt
> understand it. But seeing it showed me that L* could be directly
> translated into density. While testing my camera, which I suspect is
> slipping off the mark, I discovered that the best exposure of a Kodak
> Q14 greyscale target is not the same exposure that produces L*54 in a
> 2.2 workspace or L*61 in a 1.8 workspace after conversion through Camera
> Raw. Now I am wondering if the Kodak 18% grey card is applicable to
> digital capture. Is there another reflective grey value that is used to
> calibrate digital cameras and if so, is it standard? Or is something
> else going on?

The gamma has no effect on this relation. 18% gray is L=50 (more precisely
49.5).

> Bruce Lindbloom calculated that gamma 2.46 pegs 18% at L50

A gamma of 2.475 means that a midscale value translates into 18% gray. A
gamma of 2.44 means that a midscale value equals L=50. So if you accept that
18% or L=50 represents the eye's idea of medium gray, a gamma of somewhere
around 2.45 would be about optimum.

However, 2.2 is close enough for rock'n'roll. In addition, sRGB has a linear
segment at the low end, very much like the linear segment in the Lab curve.

--

Ciao,               Paul D. DeRocco
Paul                mailto:pderocco@...