Archived posting to the Leica Users Group, 2003/11/03

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Subject: Re: [Leica] My Leica M digital solution
From: Clive Moss <>
Date: Mon, 03 Nov 2003 18:17:05 -0600
References: <003801c3a23f$d23da300$6401a8c0@CCA4A5EF37E11E>

B. D. Colen said the following on 11/3/2003 1:22 PM:

> Most digicams with a bw mode achieve it by simply doing a 'forced dump'
> of the color information...from rgb to gray scale. That's what Photoshop
> does if you go to Image - Mode - and cuts the file size by
> about 2/3 throwing out the color info...And the resulting image does not
> look like "real" black and white....But it is possible to use the
> channel mixer in Photoshop to make an RGB file look like bw..there are
> some commercially available actions that do this extremely...
> I hope that explanation is clearer.....

I am still not getting it. My Canon G3 also produces an RGB image if I 
use the B/W when I load it into Photoshop. If I load a B/W image from 
the Canon and switch it to Greyscale using Image>Mode, it does not 
change its appearance, but the histogram changes and the document size 
get reduced to a third of what it was, as you said. What I do not 
understand is if I am losing information. In RGB mode, R=G=B for all 
points - that is why it is gray. In greyscale mode, is it still 8 
bits/pixel, or 24? I assume 8, because it does get smaller. What 
information is being lost?

I do usually use the channel mixer to go to B/W, because I can play with 
the balance after the fact -- one can change filters after the exposure, 
a luxury we did not have with film. I have never used the commercial 
actions, because I do not want to superimpose artificial film grain over 
  natural digital noise :-)

I am not sure what you mean By a "forced dump". I assumed that convert 
to greyscale used the L of Lab, or the L of HSL.

So, I did a search, and found
(which I have read, but do not claim to understand completely)
has some interesting recommendations about how to convert to gray scale:
"You can convert your picture instantaneously in gray scale pictures see 
even in a black and white pictures as a magician. To do so, you just 
need to convert your RGB values into the Y component. Actually, Y is 
linked to the luminosity (Y is an achromatic component) and X and Z are 
linked to the colorfulness (X and Z are two chromatic components). Old 
softwares used Rec 601-1 and produced:
     Gray scale=Y=(299*Red+587*Green+114*Blue)/1000
     With Rec 709, we have:
     Gray scale=Y=(213*Red+715*Green+72*Blue)/1000
Some others do as if: Gray scale=Green (They don't consider the red and 
blue components at all)
     Or Gray scale=(Red+Green+Blue)/3
But now all people *should* use the most accurate, it means ITU standard:
     Gray scale=Y=(222*Red+707*Green+71*Blue)/1000
     (That's very close to Rec 709!)
I made some personal tests and have sorted them in regard with the 
global resulting luminosity of the picture (from my eye point of view!). 
The following summary gives what I found ordered increasingly:
     |Scheme                       |Luminosity level|
     |Gray=Green                   |        1       |
     |Gray=ITU (D65)               |        2       |
     |Gray=Rec 709 (D65)           |        3       |
     |Gray=Rec 601-1 (C illuminant)|        4       |
     |Gray=(Red+Green+Blue)/3      |        5       |
So softwares with Gray=Rec 709 (D65) produce a more dark picture than 
with Gray=Green. Even if you theorically lose many details with 
Gray=Green scheme, in fact, and with the 64-gray levels of a VGA card of 
a PC it is hard to distinguish the losts."

Yes -- I do not know if B/W is spelled grey or gray -- that's what 
happens when you are an immigrant.
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In reply to: Message from "B. D. Colen" <> (RE: [Leica] My Leica M digital solution)