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ukzembla
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March 23, 2023
Answered

Grayscale PDFs for print, dot gain and profiles

  • March 23, 2023
  • 1 reply
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Hi, I work on a lot of books for print that just are K plate only. So far I've generally been asked by various printers to manually prevent TAC going over 80–95% depending on the images and the press, and I've  always done this using curves in Photoshop on a grayscale image with max + min highlight / shadow. Is there a better way than doing this manually?!… I haven't read a lot on here about people working in black and white so much but there must be lots in the same situation as me. I believe that InDesign just ignores whatever profiles are attached to a grayscale image when it's imported, but you can still export to greyscale with whatever dot gain you want – is that going to be better / worse / more / less work than tweaking things in Photoshop? Thanks! I would ask the printer but … they are quite rarely clear or helpful on profiles.

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Correct answer rob day

which I've also been told can be caused by too high an ink density.

 

Total Ink drying problems are only a problem with 4-color printing—there’s no way black only at 100% should cause drying problems—if it did you wouldn’t be able to print 4-colors.

 

CMYK profiles limit ink on a color conversion—typically coated sheetfed profiles limit total ink to between 300% and 350%. US Sheetfed is 350%, US Web Coated SWOP is 300%. Uncoated profiles are usually less, Uncoated GRACoL is 280%. The limit is only enforced on a color conversion—you can color correct a CMYK image and force the total ink over the limit.

 

Also, InDesign does ignore embedded Gray profiles, and with Overprint Preview turned on, InDesign displays the grayscale as it would print on the CMYK black plate—the preview of a grayscale would change depending the ID document’s CMYK profile, and how the profile is soft proofing the black plate, but the gray output values do not change.

 

If you are color correcting to Gray Gamma 2.2 in Photoshop, the soft proof of the grayscale is going to change when it is placed in a document with the default US Web Coated SWOP and Overprint turned on. Gray Gamma is a Display class profile, not a press output profile. It might coincidentally profile the press, but it’s not an Output profile for any press.

1 reply

rob day
Community Expert
rob dayCommunity Expert
Community Expert
March 23, 2023

So far I've generally been asked by various printers to manually prevent TAC going over 80–95%

 

Limiting the grayscale black point percentage tops my list of printer’s advice to ignore. That conventional wisdom originates from the 90s when there was no reliable way to soft proof profiled grayscale images. The idea is you can prevent images from going too dark from dot gain or "filling in", but all you are really doing is limiting the already limited dynamic range of a black only image printing on an offset press.

 

Dot gain is most noticeable in the mid range values, and a Gray profile will soft proof the gain through out the scale. Consider this scan of a black only image printed on coated stock from a 4-color press running to the Coated GRACoL CMYK profile:

 

 

The patches at the bottom are in 5% increments. There’s plenty of range between 80 and 100%, and there is a clear difference between the 95% and 100% patch. Limiting the black point of this image to 85% would only wash it out

 

 

 

Which gets to the other printer problem:

 

I would ask the printer but … they are quite rarely clear or helpful on profiles.

 

In fairness to the printers, most probably don’t know what the profile for a black only press run is—the profile could change depending on the amount of ink density the press person decides to run. But the Gray profile you assign to a Grayscale image in photoshop soft proofs dot gain so there’s no need to put a limit on the black point. Black Ink profiles tend to be more accrate through out the scale:

 

 

Black Ink - US Sheetfed Coated compensates for more gain than Black Ink GRACoL Coated, or Dot Gain 20%

 

More on Black Ink profiles here

https://community.adobe.com/t5/indesign-discussions/exporting-rgb-cmyk-and-grayscale-to-print-pdf/m-p/10990825#M179498

 

 

 

 

    U
    ukzemblaAuthor
    Known Participant
    March 23, 2023

    Thanks for such a detailed and thought-out reply, Rob! Does it make a material difference that it's books we're working with, and so things move though the press pretty fast, and are pressed + bound not long after? I should perhaps have said too that they're typically made on uncoated paper. There's also the problem of ink transfer onto the facing page (I've temporarily forgotten the term for this, sorry), which I've also been told can be caused by too high an ink density. Do you have experience working with book printers too?

    rob day
    Community Expert
    rob dayCommunity Expert
    Community Expert
    March 30, 2023

    Hi Rob,

    I don't disagree with anything you've posted. Fundamentally I just don't like the way Photoshop displays the shadow end of grayscales, the blacks are too contrasty against the white of the sceen and you can see clear differences between 98% and solids whereas in Print the Black would be dark grey and there would be nowhere near the contrast you are seeing on screen.

    Your screenshot with CoatedGRACol 2013 as the Simulation Profile is a much more reasonable expectation of 1 colour Print than anything you've screengrabbed from photoshop.

    A Custom Dot Gain curve doesn't fix the problem of the Black point being viewed as black as RGB can go and AFAIK the white point too.

     

    So you end up needing to paste the Grayscale into the black channel of a profiled cmyk.

    And whilst that should work nicely for most Coated stocks with a similar white/yellow point, for that to be meaningful with Uncoated you'd need to have a profile for the specific stock and process that you are printing on.

    And even if you go to all this effort and expense there is nothing to stop you falling out with Printer A and Printer B giving you a different result because their RIP is set up to handle cmyk Profiles differently.

    Not sure about any of it any more. Is there anyway to set Photoshop's White and Black points but still use a greyscale?


    Your screenshot with CoatedGRACol 2013 as the Simulation Profile is a much more reasonable expectation of 1 colour Print than anything you've screengrabbed from photoshop.

     

    Sorry, I posted the wrong capture. You can save out a custom Dot Gain curve as an .icc profile and use it in Acrobat as the Simulation Profile—for Acrobat to see the profile it needs to be saved in:

    ⁨Library⁩ ▸ ⁨Application Support⁩ ▸ ⁨Adobe⁩ ▸ ⁨Color⁩ ▸ ⁨Profiles⁩ ▸ ⁨Recommended⁩

     

    Here the Simulation profile is my Custom Dot Gain profile. The bottom image has no correction—95% is outputting as 95% and its soft proof is a similar value to 100%:

     

     

    The top image has my correction applied 95% is corrected to 87%:

     

     

    The Photoshop Grayscales are displaying the same with my Custom Dot Gain profile assigned:

     

     

     

    Fundamentally I just don't like the way Photoshop displays the shadow end of grayscales

     

    Gray .icc profiles do have a limitation in that they can’t specifiy the "color" of black ink the way a CMYK profile can—100% gray is always displayed as absolute black, which wouldn’t happen on an uncoated sheet, so it’s a problem with Gray profiles not Photoshop.

     

    As you suggest we can overcome the gray profile 100% display problem by working in the Black channel of the CMYK file, but for 1-color only printing where there’s no CMY on the sheet, I think it can be dangerous to assume the press is going to run exactly to something like Uncoated GRACoL where the profile might assume a lower black density. With black only I think the press operator might push the black in order to overcome the "grayness" of one color on an uncoated sheet. For color work where there’s CMY supporting the black, the tendency might be to hold back on the black density because the blacks are going to be richer.

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