RE: filmscanners: creating correction curves from scanned calibration chart?

["michael shaffer" <rarewolf@roadrunner.nf.net>]

Ned writes ...

> There is a problem here, and that is your assumption
> that the 18% grey would be at RGB 117/117/117.
> ... The correct value for mid grey would be... whatever the RGB
> value you have measured off your monitor that matches mid grey. ...
> ...
> Basically - the only way to do this successfully is to get one of those
> monitor measuring devices and profile your monitor, and calibrating your
> workflow.

  Because my system is calibrated, I am not assuming.  If my monitor is
truely gamma calibrated, and I use a gamma=2.2 working color space, then a
18% gray card's RGB value should be R=G=B=117 (see formulas below, they
aren't mine).  I ask you ... under what circumstances would an 18% gray card
be anything else? (you may also assume I have ambient illumination under
control.)  However, gray balancing is one thing ... involving the color
patches and color space gamut is another, and therefore the reason form my
#1 question.

  However, I'll probably satisfy myself having gray balanced (as Bruce
Fraser implies ... "you're 95% there if the grays are accurate").  The more
important question is still #2 ... "how do I do it?".


shaf  :o)


> >From: "michael shaffer" <rarewolf@roadrunner.nf.net>

> >gray_value == (0.18^(1/2.2))*255 = 117 .  Similarly,
> >
> >logD (18%) = .745 implies (1/(10^logD))^(1/2.2)*255 = 117

>(2)  What is the best way to create the curves?  For example,
> I would like to use the target for the "curve's" gray eyedropper
> to change the curve such that 115/118/116 becomes 117/117/117,
> ... and for a different gray, 155/159/154 becomes 157/157/157
> ... but the gray eyedropper doesn't seem to work this way(?)