Digital BW, The Print

Steve,

To try and answer what happens when Channel Mixer settings are over 
100 or less than zero, lets assume we have a single color image whose 
pixels have RGB values 113, 57, 179.  In Photoshop, if you open up 
Curves and mouse over this image you will see it has a tonal value of 
87.  Curves calculated this as follows  .30(113) + .59(57) + .11(179) 
= 87.

If you then create a Channel Mixer adjustment layer and set it to 
monochrome, to retain the same luminance value, you will need to set 
the red slider to 30% and the green slider to 59% and the blue slider 
to 11%.  Changing these sliders to anything else will change the 
resulting tone.  Which is ok, because black and white is all about 
tone.

Now, lets assume we change our Channel Mixer red slider to 200% and 
the green slider to -50% and the blue slider to -50%.  This is how we 
are telling Photoshop to calculate our tone  2.0(113) - .50(57) - .50
(179).  Our resulting tone would be 108.  Which would give us a 
lighter image since 108 is lighter than 87.  You cannot make Channel 
Mixer calculate a tone greater than 255 or less than 0.  Anything 
over 255 is collapsed to 255 and anything less than 0 becomes 0.  
Almost all the experts agree that when using Channel Mixer to keep 
the total percent as close to 100 as possible.

Regarding constant, constant allows us to add or subtract black from 
the image.  A negative constant adds black and a positive constant 
subtracts black.

Now, the second half of this email is opinion.  I personally do not 
like using the same adjustment layer to both convert my image to 
black and white and to adjust tone.  I have several reasons for this 
but one of the major reasons is this forces the tonal change to be 
more of a global change (e.g. keep the channel mixer as close to 100% 
as possible, which means you change one, you need to change 
another).  I usually like to selectively change tone.  If you care to 
learn more about how I convert to black and white, you can go here, 
but be forwarned, it is a long read.  
http://www.zuberphotographics.com/page_TMSIntro.htm.

Finally, we need to be conscious of what channel mixer does.  Many of 
us look at our images and what to darken the sky or lighten the green 
foliage.  So we open up Channel Mixer, Levels or Curves and start 
working on the channel level.  What we need to be aware of is when we 
think blue sky or green foliage, we are thinking on the color range 
level.  Whereas Channel Mixer works on the color channel level.  In 
our example of the RGB image with values of 113, 57, 179.  Its color 
range is blues.  However, it has information in all three channels.  
So when we use Channel Mixer, or Levels or Curves to adjust channels, 
we are affected ANY pixel that has data in the channel being 
adjusted.  When we work at the color range level, we are only 
affecting pixels whose dominant color is in that color range.  The 
Hue/Saturation adjustment works on the color range level.  But I 
rarely use it to adjust tone.  Instead, I use Levels or Curves in a 
two step process that basically selects the color range and then 
applies the Levels or Curves adjustment. You can read more about it 
here.  
http://www.zuberphotographics.com/page_PSComparison.htm#ColorRange

For the first half of this email, I hope it helped.  For the second 
half, thanks for listening.

Thomas