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greyscale != grey?

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  #1  
Old 08-11-2005, 03:19 PM
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greyscale != grey?

Does anyone have a link to a good (true) greyscale filter? I have an image that has gone sepia with age and when I greyscale it in photoshop it has a distinct cyan colour cast that I cant seem to get rid of.
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Old 08-11-2005, 07:28 PM
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If you convert it to grayscale mode in Photoshop and have any colorcast at all, it's time for a monitor calibration.
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Old 08-11-2005, 08:16 PM
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and what doug said holds true for all other graphic editors i'm familiar with. putting any image into true grayscale means no color whatsoever. are you possibly referring to something that is desaturating to grays mostly, or are you using the true 'grayscale' conversion?

Craig
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Old 08-12-2005, 12:34 AM
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I've tried desatureate and mode->greyscale and if I take a colour sample each point has equal levels of magenta and yellow but higher cyan.
Surely true greyscale should have equal levels of all 3
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Old 08-12-2005, 08:56 AM
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Wrong. Convert your pallete to RGB and you will note all values are equal. This does not hold true with CMYK. For example 245R 245G 245B equals 4C 2M 2Y. 10R 10G 10B equals 70C 60M 60Y 85K. Bottom line Nancy, your greyscale images are in fact greyscale.

Cheers
Dave
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Old 08-12-2005, 12:32 PM
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Ironically Duv it was you who prompted me to check the CYMK values. In a previous colourisation I did you said the original image must have had a cyan colour cast that my colourisation hadnt got rid of.
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Old 08-12-2005, 01:19 PM
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Hi Nancy

This is very easy to test.

Open a new image and fill will 50% Grey
Change the eyedropper options to read RGB and CMYK

Results

R 128
G 128
B 128

C 52
M 43
Y 43
K 8

Looks like Dave is correct.

Ken
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Old 08-12-2005, 02:27 PM
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Hi Nancy

Not sure if I was misunderstood. Did you provide a before and after? If I was just looking at your colorization I may have been making an assumption that the original was a faded color pic not greyscale. Didn't mean to throw you for a loop.

Cheers
Dave
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Old 08-13-2007, 05:36 PM
leoinnyc leoinnyc is offline
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Re: greyscale != grey?

Nancy, the reason that your CMYK values aren't all even is that unlike RGB, which for all intents and purposes exists as a digital abstraction, CMYK numbers represent real ink on real paper. This is a highly non-linear system due to paper base color, ink impurities, varying apparent brightness, etc. The result is that in order to get "neutral" gray, you need to use unequal quantities of C, M, Y and K ink. The exact formulation of CMYK laydown is governed by the CMYK settings in your Color Settings dialog (or by the profile of your CMYK output device). Try going from heavy to light GCR and you'll see what I mean. If you do any output to process-color devices, you should really read up on this stuff. Hope that helps. - Leo
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  #10  
Old 08-13-2007, 08:32 PM
edgework edgework is offline
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Re: greyscale != grey?

If CMYK is your destination space, convert to RGB and desaturate. That will make everything dead even. Then convert back to CMYK and trust Photoshop to handle the blend. Even easier, convert to Greyscale then back to CMYK.

If you are going to send the image to a press, you might want to create a custom profile that makes your mix dominant black, particularly in the quarter-midtones. A normal SWOP conversion will put most of the weight into CMY, a good thing with skin tones in color, risky when greyscale is the desired result and a small shift in ink densities on press can whack the whole image with a cast. If you want to really get fancy, make two conversions, one with dominant black, one with the standard SWOP profile and blend them either using a mask or Blend If sliders to feed the SWOP values into the darkest shadows, leaving the rest with dominant black. The results are rich shadows and perfect quartertones and midtones, even if the pressman is having a bad day.
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