Cannot enter a decimal cmyk value in photoshop? Is rounding it up acurate?

Posted By
Feb 26, 2009
I know it’s a stupid question but in illustrator I can add in a decimal….in photoshop I am told that I cannot.

Why is this?

Thanks in advance.
Feb 26, 2009

AFAIK that is standard Photoshop behavior. Just checked in CS4 and that’s what happens. In print reproduction, fractions of a percent should not make any noticeable difference. You’ll get far more variation during the press run.

Feb 26, 2009
I seem to remember being told that the difference between the way that Photoshop and Illustrator handle CMYK is because Photoshop use LUTs which refer to L*ab; and Illustrator is unable to use the L*ab space.

But as Neil said, it really makes no difference in practical terms.
Feb 26, 2009
I’ve run into many situations where it does make a difference. One is where you are specifying solid tints in Quark or InDesign, where some of those builds use builds that are spec’d to the tenth of a percentage. It would be nice to be able to, when trying to match them exactly in Ps, spec at exactly the same resolution. Now I have to back to the layout app and modify the build to suit Ps.

The other is when printing to really good sheetfed presses that can hold and print a visible dot less than one percent. That can read as a zero percentage in Ps and yet still print as the slightest density on press. In the days of film generated plates, this was never a problem but it is today.
Feb 26, 2009
A tenth of a percentage really is "no difference." If you have a 50% dot versus a 50.1% dot, they both may give you a 70% on press and vary by 1 or 2 percent across the press anyway (or more, depending on the printing method and conditions on press). Spending time consolidating fractions of a percentage is a waste of time.
When working with 8-bit images, you can only have 256 discrete values in any one channel, so you may in effect have 2 values for any one percentage. Call them .5% if you want, but how this RIPs out may vary by more than differing fractions of a percent as well, depending on what kind of RIP/software/output you are using. And again, on press, the variations will cancel these differences out even further.
Seeing colours specifically spec’d out in fractions of a percent makes it really easy to pick out designers who don’t understand print.
Btw, there are no dots less than 1% that read as zero in Photoshop. There are three discrete values for 1%, and how they come out depends on the factors I mentioned above.
Feb 26, 2009
I’m not saying that you can see a tenth of a percentage, but you really can see five tenths of a percentage especially in the highlights. Actually I’ve seen situations all too often where even the difference between a dithered conversion in Ps to CMYK – even though the numbers read the same as the build in the layout app, in the shadows, is completely visible in print.

Yes, it’s true that you can only have 256 values in an eight bit image. Divide that into the on hundred steps in the CMYK ink percentage scale, and you’ve got many in between numbers, including steps below one percent that read as zero.

As far as designers being stupid – well some are, but a lot of them just spec the Pantone color as a CMYK build, relying on the app to generate the numbers, and the app generates in between numbers because the L*a*b values are out ( I believe) to the 1/100th of a value, even though we can’t spec those that way either.
Feb 26, 2009
There is definitely a problem when matching colours with decimal values from Illustrator, if you don’t round in Illy, you will get a noticeable ‘step’ between an Illy graphic and a so-called matching photoshop background, for instance. Photoshop does work in 256 levels not 100.

When you convert a sample to process in Illy, you should always round the decimals.

I can show samples where .2% was enough to require attention.

One percent cyan on my system is R250 G251 B253 so you can see there is plenty of numbers left to have values below 1%.

Actually I lightened that colour by half and it still read 1%C with RGB values of 252 252 254.

The difference is irrelevant to the eye, but critical to the printing plate.
Mar 13, 2009
Hi all, I’m a newbie to Illustrator and page layout programs, but I am struggling with a similar issue.

We use a spectrophotometer to measure spot colors, with X-Rite software that converts the spectral data to CMYK with one decimal point. While I agree that rounding down from .1 isn’t going to make much difference, I do think that rounding up or down from .5 will make a noticeable difference, especially if you have to do it 4 times for one swatch.

I understand that Adobe Illustrator can work with decimal values for CMYK. However, we need to create a multi-page document and I also understand that "tiling" in AI is not necessarily a happy thing.

This is my question: if I move from AI to a page layout program, is that program going to preserve my AI information as I entered it, or is it going to round off?
Mar 13, 2009
Measuring spot colors from a Pantone Swatch Book can be a bit of a snare and delusion.

Get hold of a bunch of them that were produced from different print runs and you will discover just how much a given color can vary from sample to sample.

And of course, you will not be printing on the same Press nor on the same stock that the swatch book was either.
Mar 15, 2009
In CS4, Illy will show decimals, Indesign and Photoshop will round up and not show the post-decimal values.

In Photoshop (CMYK mode), you can have more than one step within 1% (according to info panel) presumably 100% divided evenly into 255 steps and I can see 3 distinct levels between 0 and 1% dot (per channel).

These will often be a problem if you let them sneak in unexpectedly, so best to round off the .xx figures in Illustrator. I don’t know why Illustrator has had this option, seems a bit pointless to me and introduces a significant risk for printers and designers for no real gain.

That’s not to say there is no need for all 255 steps, I am not saying that, only that they are dangerous as ‘selectable’ values and should only be there for use in smooth blends and feathers etc.

I presume this is why Photoshop doesn’t let you pick on a % slider, it would jump along unevenly 0 then .392 .784 etc whereas the rgb slider can happily count from 0 to 255. The available tones are based on each channel being 255 steps in 8 bit per channel files.

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