Digital BW, The Print

Steve,
The sum of rgb adding to 100% just preserves the luminence of the orignal. You don't 
have to stick with that if you like the effects of another sum. Red skies can get torn 
up as you're only using 30% or so of the info that is recorded in the red pixels. It 
often helps to add some additional info, such as from the L channel to your red using 
channels as layers for a smoother outcome.
The constant slider is described by Adobe thus: "Drag the slider or enter a value for 
the Constant option. This option adjusts the grayscale value of the output channel. 
Negative values add more black, and positive values add more white. A -200% value 
makes the output channel completely black, and a +200% value makes the output 
channel completely white. "
Sarah

snip>
> --- In DigitalBlackandWhiteThePrint@yahoogroups.com, "Steve Kale" 
> <stevekale@b...> wrote:
> > 
> > I typically use the split channels method to render B&W from colour 
> but have recently 
> > begun playing with the channel mixer.  I guess I need some help as 
> to what the sliders do 
> > - I am simply fiddling in the dark at the moment.  Should the sum 
> of the Red, Green and 
> > Blue slider percentages ideally add to 100?  What is the purpose of 
> the Constant slider?  If 
> > a channel is raised beyond 100% what does this mean? (I found that 
> some quite dramatic 
> > effects can be produced by say raising the red to 200% and having -
> 50% for the other two 
> > channels but it seems that skies can get torn up by doing this.)
> > 
> > Thanks
> > 
> > Steve