Digital BW, The Print

Steve,
I believe when you turn off the a and b channel, PS displays the image based
on your working gray space.  The same thing probably happened when you
brought the L channel over, it got converted to your working gray.
Try setting your working gray to Lab grayscale and see if the results are
closer to what you were expecting.

Best regards,
John Moody

-----Original Message-----
From: DigitalBlackandWhiteThePrint@yahoogroups.com
[mailto:DigitalBlackandWhiteThePrint@yahoogroups.com]On Behalf Of Steve Kale
Sent: Thursday, November 10, 2005 6:00 AM
To: DigitalBlackandWhiteThePrint@yahoogroups.com
Subject: [Digital BW] Channel Mixer and Lab L Channel

I am puzzled by something I stumbled across yesterday.  I was working up an
image
converted to B&W with Channel Mixer (60% Green, 40% Blue).  I wanted to
sharpen the
image and tried a variation of the Smart Sharpen/Reduce Noise Sharpen action
I use -
specifically I copied the image, converted it to Lab Colour Mode (flattening
in the process)
and only sharpened the L channel.  I made a mistake while doing this and
brought just the
L channel of the copy over to the original as a layer.  This revealed
something I found
surprising:  the Lab Colour copy ( a flattened greyscale image) had
luminance information
in the a and b channels.  That is the luminance of the L channel alone was
different from
the luminance when all three (L, a and b) are displayed.  This would seem to
fly in the face
of what I thought the a and b channels represented.

I did another quick test.  I took the RGB original with the Channel Mixer
layer and flattened
it, leaving a greyscale image in RGB.  Converted it to Lab.  If I uncheck
either one of the a
and b channels the image stays looking the same.  But if I uncheck both the
a and b
channels (leaving just the Lightness channel displayed) the luminance of the
image
changes significantly.

I must be missing something basic but I don't quite get it.... Ideas?

Steve

PS:  I am using PS CS2.  (My RGB space is ProPhoto RGB.)






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