18% gray card and color correcting

18% gray card and color correcting

Post by TonyHa » Mon, 02 Nov 1998 04:00:00



I'm new to this group and fairly new to Photoshop.   I hope I'm not asking a
question already covered.    It seems to me that if I take a photo of a gray
card in the same light as the image I'm photographing I should be able to solve
my color cast problems.   If this is true can someone tell me what the RGB
numbers should be for a 18%gray card?   By using the color info box as a guide
shouldn't I be able to adjust my color balance to the gray card?    Thanks for
your help!
 
 
 

18% gray card and color correcting

Post by Fit » Mon, 02 Nov 1998 04:00:00



Quote:>I'm new to this group and fairly new to Photoshop.   I hope I'm not asking a
>question already covered.    It seems to me that if I take a photo of a gray
>card in the same light as the image I'm photographing I should be able to solve
>my color cast problems.   If this is true can someone tell me what the RGB
>numbers should be for a 18%gray card?   By using the color info box as a guide
>shouldn't I be able to adjust my color balance to the gray card?    Thanks for
>your help!

I assume it shold be r046 g046 b046

That's assuming, of course, that an 18% grey card is actually 18/100 k
=====================================================
Always remember kiddies, never procrastinate tomorrow
when you can procrastinate today
=====================================================
          Fitz

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18% gray card and color correcting

Post by Steve » Mon, 02 Nov 1998 04:00:00



> I'm new to this group and fairly new to Photoshop.   I hope I'm not asking a
> question already covered.    It seems to me that if I take a photo of a gray
> card in the same light as the image I'm photographing I should be able to solve
> my color cast problems.   If this is true can someone tell me what the RGB
> numbers should be for a 18%gray card?   By using the color info box as a guide
> shouldn't I be able to adjust my color balance to the gray card?    Thanks for
> your help!

Go to levels. Double click the middle eyedropper icon to get to the picker to
set the colors. Set the RGB values to 128, 128, 128. Now using this eyedropper
in your image will set the sampled pixel to middle gray and adjust all other
colors accordingly. So clicking with this dropper on the gray card in your
picture would correct the cast.

--

(remove * if present)

 
 
 

18% gray card and color correcting

Post by Chris C » Mon, 02 Nov 1998 04:00:00





> >I'm new to this group and fairly new to Photoshop.   I hope I'm not asking a
> >question already covered.    It seems to me that if I take a photo of a gray
> >card in the same light as the image I'm photographing I should be able
to solve
> >my color cast problems.   If this is true can someone tell me what the RGB
> >numbers should be for a 18%gray card?   By using the color info box as a
guide
> >shouldn't I be able to adjust my color balance to the gray card?  
Thanks for
> >your help!

> I assume it shold be r046 g046 b046
> That's assuming, of course, that an 18% grey card is actually 18/100 k

NO!
The standard 18% gray card is 18% _reflectance_ -- which is what your eye
sees as halfway between black and white.  In other words, it is 50%
lightness.
So if you're using a working space with a gamma between 2.2 and 2.5, then
the 18% card should read about RGB = 128,128,128 or Lab = 50%,0,0.
Don't forget to average the gray card area to avoid picking up scanner noise.

Chris