Digital BW, The Print

Thanks a lot Thomas!


> From: rgb2bw <jnk0941@...>
> Reply-To: <DigitalBlackandWhiteThePrint@yahoogroups.com>
> Date: Sat, 19 Feb 2005 14:44:58 -0000
> To: <DigitalBlackandWhiteThePrint@yahoogroups.com>
> Subject: [Digital BW] Re: Understanding channel mixer
> 
> 
> 
> 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
> 
> 
> 
> 
> 
> 
> 
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