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RGB CMYK LAB and Dan Margulis !

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  #51  
Old 11-16-2007, 03:54 PM
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Markzebra Markzebra is offline
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Re: RGB CMYK LAB and Dan Margulis !

Oops sorry edgework,I duplicated your answer there. Yes Its about shadow detail density. In CS3 You can of course use a Smart filter on your new K layer to sharpen it - you could for example apply a blend mode to the K layer, maybe multiply? sort of like the opposite of a shadow highlight, bottom end density control with a scrubby slider.
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  #52  
Old 11-16-2007, 06:19 PM
pixelzombie pixelzombie is offline
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Re: RGB CMYK LAB and Dan Margulis !

definitely something to master but i can't justify doubling a file size everytime i need to add black to an RGB image, at least not at my current gig...
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  #53  
Old 11-16-2007, 07:57 PM
transoptic transoptic is offline
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Re: RGB CMYK LAB and Dan Margulis !

I had two questions:

first, I like your tip edgework on sharpening the black plate. How would you do this? I tried to copy it on a new layer, run high pass, and change to overlay blending. It looks interesting that way. Is there a superior method you use?


Second question is, I'm fully aware of how each mode has its own seperate gamut, but I've never come across color shifts unless printing or uploading to web, etc. Except when switching modes with lots of layers and masks. So can someone give me reasons, or better yet examples where switching modes under normal circumstances are bad?
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  #54  
Old 11-16-2007, 10:34 PM
edgework edgework is offline
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Re: RGB CMYK LAB and Dan Margulis !

Quote:
Originally Posted by transoptic View Post
I had two questions:

first, I like your tip edgework on sharpening the black plate. How would you do this? I tried to copy it on a new layer, run high pass, and change to overlay blending. It looks interesting that way. Is there a superior method you use?
You're thinking too hard. I meant sharpen the K channel. Literally. Go to your channel's palette, activate only the k channel and sharpen it. Works particularly well for faces since there is virtually none of the textural detail in the black plate to get grunged up. Eyes and hair can take a heavy hit and always look better for it. Do this on a duped layer to blend as desired.

Quote:
Second question is, I'm fully aware of how each mode has its own seperate gamut, but I've never come across color shifts unless printing or uploading to web, etc. Except when switching modes with lots of layers and masks. So can someone give me reasons, or better yet examples where switching modes under normal circumstances are bad?
Not bad, necessarily, but the reality is that CMYK is incapable of producing many of the colors you take for granted in RGB. Find a picture of a vivid blue dress with lots of variations in blues and greens for detail. You'll find that with the default conversion in Photoshop, which is Relative Colormetric, all those subtle shifts in tone will pretty much turn to mud. Blues and greens suffer particularly, as well as vivid purples, turquoise and oranges and virtually all pastels; they are simply out of the normal printing gamut for CMYK. (Reds fare better and Yellow is actually purer in CMYK.) Photoshop converts colors that are in gamut precisely, and the stuff that's outside the range gets crunched at the edges. Hence the blue mud. The other useful conversion mode is Perceptual which recreates the relationship between all the colors but which will necessarily alter all of them in order to do so. It's useful when you're not going to get the original RGB color anyway and at least want to maintain detail and a reasonable facsimile of the original intent. Other options would be to pull out the lightness channel from LAB, or one of the RGB channels that captures the needed detail, copy it before converting, then layering it in on top of the image in luminosity mode. You'll need to play around with moves like this, and you'll send your colors way south, but, again, the colors weren't there to begin with. If it's a toss-up betwen bad color and bad detail, or color that's off but which has clear detail, go with the second.

Of course, not everything goes to a printing press these days, but if you need 1000 copies of something, you'll need to navigate the mysteries of conversion. (Just remember that every beautiful image you've ever seen in a book, magazine or poster has managed to get around these limits, so all is not lost.).

Last edited by edgework; 11-16-2007 at 10:39 PM.
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  #55  
Old 11-17-2007, 09:06 AM
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Re: RGB CMYK LAB and Dan Margulis !

Yes -everything that edgework has said is 100%. Having some sort of production experience helps a lot in knowing what a good CMYK file is, and that just converting an Adobe RGB file to PhotoshopSWOP default is a no-no. The key is the black plate. Each press varies immensely in how it handles the black ink: whats the ink limit? what kind of GCR do you need? ie how HEAVY is the black plate as it stretches into the lighter tones of the image. Images vary too, more monochromatic images can benefit from a stronger K plate which helps their detail and highly saturated images a lighter K.

By the way Margulis is a good teacher in these areas because thats his background I reckon. Edgework has given some good advice there about moves on the L channel (which can now be a smart L) and a lum blend on this, thats great technique which I use. Luminosity blend uses LAB calculations fairly non destructively without any conversions actually you can just use adjustments with a Luminosity blend with tremendous power.
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  #56  
Old 11-25-2007, 12:23 PM
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amica999 amica999 is offline
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Re: RGB CMYK LAB and Dan Margulis !

Hi guys and girls,
I am reading my way through the LAB book and found some difficulty following the calculation instructions....it states that in the calculation screen choose green channel and it should be applied to the whole picture (all other channels if none is selected). In CS3 I do only have the option the choose red, blue, gray etc. Would that in conclusion mean that I have to dup the layer twice and apply the green channel in the first dup to the red channel, and in the second copy to the blue channel? Wouldn't a channel mixer be more appropriate? Please let me have your thoughts, as I am lost here. Thanks
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  #57  
Old 02-05-2010, 11:18 AM
katheo katheo is offline
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Re: RGB CMYK LAB and Dan Margulis !

I have been reading Margulis' color correction books and have a few questions. I hope this an appropriate place to post them; if not, please direct me to a better forum.

Q1-- CMYK corrections sent to Epson RGB printer
I am learning to color correct in CMYK. I would like to print to an Epson 1280. Epson's materials say if you edit in CMYK you should convert back to RGB before printing. Is this necessary or advisable for every print out?!? I am just learning, so I tend to make a few curve adjustments, print, adjust, repeat... How much am I losing by staying in CMYK when I send the print command from Photoshop?

Q2--Appropriate image resolution.
Margulis recommends making sure that the image has normal resolution (1.5-2 times the screen ruling at 100% magnification). My Epson 1280 has a spec of max 2880 dpi x 720 dpi. My Sony A100 camera produces images 3872 x 2592 pixels @ 240 pixels/inch, Adobe RGB 1998 @ 8 bits/channel.

I have read that file pixels/inch should equal 1.5-2 times the printer LPI (is this 720 LPI for the Epson 1280? or 320 LPI?). What should be my 'normal resolution' to make sure that I am viewing USM corrections on the screen at the right resolution?

If I want to print at 5.35 inches x 8 inches, do I need to resample my image before doing the corrections? It seems like I wouldn't want to lose any information if I don't have to.

2592 pixels x 3872 pixels to be printed at 5.35 inches x 8 inches = 484 pixels / inch resolution without any resampling. Is this advisable? What would be better to view USM adjustments ideally?

Can anyone help?
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  #58  
Old 02-05-2010, 04:35 PM
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Re: RGB CMYK LAB and Dan Margulis !

Thanks to all above for the interesting info, always put off learning and using Smart Object layers, and from what I have read here its well worth my time to put them into use. I created an action I use at the very start of my workflow, converting the image to CMYK, and then a double USM sharpen directly on the black plate. Image is then duplicated and sent back to my original RGB file and set to "Luminosity" and masked away from out of focus areas.

I find it a great little trick to sharpening an image shot on film and then high res scanned, as they come back to me with a desperate need of selective sharpening and colour correction. So I will put this Smart Object into use...

Last edited by MacBurg; 02-05-2010 at 10:29 PM.
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  #59  
Old 02-05-2010, 08:54 PM
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Re: RGB CMYK LAB and Dan Margulis !

Katheo, Welcome to RetouchPro. I will try to answer some of your questions.

1. The Epson 1280 is an inkjet printer not a printing press. So firstly Line Screen is not applicable to it. Secondly, while it uses Cyan, Magenta, Yellow, and Black inks, its print drivers and assoc S/W are meant to handle a RGB image. All of the conversions factor in that it is being fed an RGB image and one of the RGB color spaces. It does not do very well if you feed it a CMYK file - sorry but that's just the way it is.

2. For most printing 1440 dpi should produce very good results on the 1280. If you print at 2880 (Best Quality) the printer is incredibly slow. While the printer resolution is 2880 your image does not and should not be 2880. That's because each dot printed is really made up of multiple sub dots. If you read your manual or check Epson's site you will see more info and explanation. Ideally 240 dpi or 300 is max you need to use. If you want to keep all of the orig pixel info and print at 484 dpi, that will work but the printer driver will just interpolate / resize anyway when it gets the data. You are better off having a number that divides evenly into 1440 or 2880 or 720 (like 240) so that you get the sharpest image (less round off).
Regards, Murray
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  #60  
Old 02-05-2010, 10:17 PM
doctorontop doctorontop is offline
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Re: RGB CMYK LAB and Dan Margulis !

When Chris Cox did the webinar he hinted that more things will be happening with smart objects in CS5 it will be interesting to see what Adobe come up with.
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