Digital BW, The Print

Hi. I am just working up an image from my new digital camera and have come across 
something that is puzzling me.  The colour image looks great - no posterization and no 
apparent clipping in any channel.  From the RAW file I used Capture One Pro with high 
contrast setting and simply slid the RGB black point slider up to the left of the red 
histogram and then processed (no other adjustments).  In PS, I then used the split 
channels technique to work up a B&W version.  The image is of a sand dune against a dark 
(blue) sky.  My problem is that the red channel appears (on screen - I have not printed it 
yet) to be posterized in the sky.  I want to use the red channel because it is giving me the 
dark sky.  Any ideas as to why this would be?  I can go back to the original RAW file if need 
be. 

Steve 

(I have ordered the 3 channels as follows:  base Green at 100%, red on top at 90%, then 
blue with pinlight for extra contrast on top at 40%.  Although the posterization is evident 
just looking at the red channel.)