G7 - Experts, Is it possible too . . .?

[jardo]

Hi,
I am doing some tests with G7. I work with non-standard presses, being that what I am trying to G7-ize is a high speed/volume inkjet web press. Now I understand that G7 works with the fact that you have inks and paper that are within the ISO specs. I will say upfront, I do not have paper and inks that are within ISO specs. I have followed the "G7 Calibrating, Printing, and Proofing by the G7 Method" PDF step by step as best as possible.

I have applied G7 to a inkjet press of ours and have seen good results that show positive CMY gray balance and test targets achieve better contrast. The catch is my Black ink. My black ink is casted very much towards the blue side of the spectrum. So when viewing my black and white photos that print in either black only, cmy only, or all cmyk, the black only looks very out of place because my cmy and cmyk are gray balance and look neutral but my black only photo looks very cool.

Is there a way to force my CMY curves to match the blue casted hue of my black ink without effecting the chroma of my 4color photo's drastically or in a bad way? I am using the Idealliance software, so if there is a magical button, preference setting that I need to make, I would prefer to do it in the software. Is this just not possible because I do not have ISO standard paper and Ink?

All thoughts and opinions are appreciated!

 

[Bret Hesler]

On the Create Curves tab, click the Gray Options... button. The gray balance is calculated from the paper white. At 50% it is half of the a* and b*, so you can set the Custom White to double the a* and b* of the midtone Black. I don't know how that would affect the highlight to quarter tone gray balance

I can understand keeping black type black-only, but is it necessary to keep a black-only image, black-only on an inkjet press? Would it be better to just convert it so it is neutral like the CMY and CMYK gray images?

Bret

 

[meddington]

I agree with Bret in that introducing color management might be a better way to go. I would guess that manipulating CMY curves to match a casted black (without regard to the paper white) could introduce some anomolies, particularly in the highlights... but too far remove here to make the call.

 

[jardo]

I am pretty sure that the end user (owner of the inkjet press) would object to converting B&W photos to 3 or 4 color, only because it would be costly in ink to them.

The solution that you have given, is it a for sure or is this a "based on experience and theory answer?" Please don't take it personal, as it sounds like you may not be sure.

 

[meddington]

I am pretty sure that the end user (owner of the inkjet press) would object to converting B&W photos to 3 or 4 color, only because it would be costly in ink to them.

The solution that you have given, is it a for sure or is this a "based on experience and theory answer?" Please don't take it personal, as it sounds like you may not be sure.

Depends on how different the paper white Lab values are from the artificial "doubled a* b* of the midtone black" on which you would base you gray balance curves. What are your paper Lab values and your black solid and 50% Lab values? Should be able to simulate the issues in Photoshop.

 

[jardo]

Pre G7 Values (Print #1)
My Paper Lab values are L=96.1 a=2.7 b=-12.4
My K 100% Lab values are L=36.91 a=-2.49 b=-5.98
My K 50% Lab values are L=63.53 a=-1.29 b=-9.14
My CMY 50/40/40% Lab values are L=67.31 a=-1.72 b=-4.39

Let me know what you think.

 

[Bret Hesler]

Well, I guess it is and experience and theory answer, because I did not write the software. However, midtone gray being 50% of the paper a* and b* is spelled out in the G7 documentation, and better be applied in the Curve software.

Don't worry, I'm not easily offended.

Looking a your Lab values, you are definitely starting out way outside the 12647-2 spec for paper color and black color. That being said, I have seen pretty good G7 gray balance on papers with a -8b*. Also, the L* for the black is pretty high at 36.91, equating to a density of 1.02. I am guessing that this is an uncoated paper.

I guess the question is which is more offensive, a blue casted neutral or spending a little more to produce a better product. Also, using an extremely heavy GCR profile can actually save you ink in the 4/c images and give you a neutralized black that is still mostly black but with a little magenta and yellow to neutralize it.

The drawback here is the dot size and dither pattern of the black potentially causing grain in the images.

Bret