photoshop CMYK Color Correction question

[richard_5660]

This is a question that i have posed to many people in the industry, yet no one has ever been able to answer it. I thought it was a relatively simple question, but apparently it is not.

Fact: If you throw a gray card into a shot, you can always get perfect white balance adjustment later by going into photoshop, selecting levels, clicking on the middle/neutral gray eyedropper, then sampling the grey card in the shot.

What I have been told is that if you're 18% gray is right, then there is absolutely no reason to adjust the white and black levels, again sampling off a kodak color separation guide that I also throw into every shot. (I do product photography.)

I have also been told that if your 18% gray is right, then all the colors will be right... in other words, you have now color corrected the shot.

Incidentally, I know people who sample the white, black, and gray in their shot in photoshop and say they need to do it get all the levels correct. Everytime I sample, in addition to gray, also the white and black levels, it ruins the picture in my opinion.

Relatedly, I still don't the understand the difference though between white balance, color temperature, and gray levels. It appears that if your white balance is correct, then the other two are also correct.

So, here's the question:

I process CMYK. I have a Kodak color separation guide in all my shots that shows the CMYK reference patches.

Isn't there a way in photoshop to click on those reference CMYK bars that appear in the shot (just like one does with neutral gray in the prior example), and be able to color correct all four CMYK channels, albeit one at a time.

I have asked so many people this question and no one seems to know how to do it. In fact, I've spent hours researching this, and the only answer I've received thus far is that, again, you don't have to do any CMYK adjustment as long as you've done your neutral gray adjustment.

Any help on this would be greatly appreciated.

Finally, is there an industry preference out there for digital camera color settings. Adobe II is what I've been told b/c it has a wider range than Adobe I or Ia, but to my eye Adobe II produces some very unnatural-looking colors, especially reds and blacks. I've always preferred Ia.

I photograph oriental rugs that appear in national ads and catalogs. Color is everything to me. If, in real life, I throw the ad on top of the rug, the colors need to be identical looking. And, even if they are identical, they also need to be have the same brightness and saturation, which is a whole other issue altogether.

Thanks,

Richard

Again...

[paulr]

There are people here who know a lot more about color management than I do, but I think the short answer to your first question is pretty simple. A gray card is only foolproof if everything (your digital camera, your film, your scan, or whatever) is perfectly linear. In real life, curves can deviate, and color casts can be different in the shadows, mids, and highs.

[Henry Ambrose]

What paulr wrote is correct.

Further, whoever told you about the "fact" that clicking on a gray card in your scene is the way to get color balance has NO idea of what they are doing. Proper color management (you do have your all your gear calibrated and profiled don't you?) and careful attention to detail will get you close to accurate color - but only close - the rest of the way you have to do it manually in PS.

If you are working in a studio you should have lighting under control (like blocking all window light and matching your lights output for color) after that color correction should be a repetitive production line affair.

Buying a digital camera suddenly made you responsible for a lot more work than when you shot film and passed it off to someone else. Fun ain't it? What kind of large format digital back are you using or are you using one of the SLR digicams?

[Will Strain]

Sounds like you have some other confusion going on.

The white balance is the color temperature (and resulting color casts). You can set this with a pure white, or a middle grey pretty effectively.

The white and black point determine contrast range in the image. Tho if your shooting conditions are odd, setting the white point can affect color (by blocking out a portion of the yellow or magenta channels, for instance)

As far as using the CMYK patches you shot - I would be more inclined to take sample points on them, and keep an I on them as I make my levels or curves adjustments for the image. I would not try to do a one shot - balance all unless I decided to forgo control for the sake of speed. In which case, a proper white balance is the closest fast thing.

Adobe II can produce wonky colors, depending on how you are converting to CMYK. Ie. mode change, versus a profile change, etc. But that is a whole other issue... ie. do you have a profile provided by your press, or a calibrated proofer of some sort, monitor profiles, nothing funky in the conversion settings...

[Ellis Vener]

Why are you processing in CMYK instead of RGB? If you need to convertto CMYK becasue you use a RIp on your inkjet or becasue ou are runninga 4 colo press that is one thing but otherwise I'd like to understand: Why you are going that route?

[Dominique Labrosse]

Ellis, at the bottom of Richard's original post he mentions that he's shooting for magazine and catalogue reproduction (CMYK press work). It seems that he needs to deliver a CMYK file as a final deliverable.

Unfortunately Will I don't think a profiled workflow will work as Richard cannot profile for the multiple presses used to print the carpet advertisements. I would turn off my Photoshop colour management and colour correct manually using the numbers. If your target is in the shot, then you need to manipulate your curves so that the numbers line up with what they are supposed to be. Trust not your monitor, trust only the numbers. All you can do is get the numbers right and hope the pressmen are responsible in getting it right. You can also send them target numbers or a matchproof to compare against.

Be careful about your light sources. Make sure they are all consistent, make sure your flashtubes or hotlight bullbs are all the same age. Get a colourmeter, set your white balance manually, or shoot RAW and set it manually in the Photoshop conversion afterwards.

Colour correction is a entire discipline in itself.

Good Luck

[Ellis Vener]

Yes I saw that about his output needing to be in CMYK but it is my conviction (frome experience ) that he will be better off doing most of his color editing steps in RGB ratherthan CMYK. Hos camera(s) o scanners are RGB native devices, his data is natively RGB and hads to be converted to CMYK. He should probably choose a workspace will closely model CMYK gamuts (try Color Match RGB) but I think he will find it is easier to do what he is tryingto do in RGB.

Two very solid reference books for Richard will be "Real World Adobe Photoshop CS" by Blatner and Fraser and "Real World Color Management" by Bruce Frase Fraser.

Richard<

How are you photographing these rugs: which cameras and how are you doing your lighting? LAst year I did a lot of testing and visiting with the variousmakers of 22mp backs. From the three months of intensive research my conclusion is that the best 22mp back specifically for your applicatin (photographing finely woven textiles will be a Jenoptik Eyelike M22 used in 16 shot mode if you are using electronic flash as a lighting source or a Betterlight scanning back if using contant light. these combinations will produce the highest possible iamge quality

[richard_5660]

thanks for all the input. unfortunately i have to use cmyk b/c these are all going into magazines and catalogs.

still, can anyone tell me how to use the colors on the kodak color separation guide as a reference for color correcting the images? specifically. i know photoshop but i am not an expert.

best,

rich

[Ellis Vener]

richard,

I understand. I'm a comercial photographer by profession. Still you are better doing the bulk of your color correction work in RGB and then targeting specific CMYK conversions as out put and using Photoshop's (preferably Photoshop CS or CS2) "soft-proofing" tools to fine tune the conversions.

You are better off asking these questions at http://www.photoshopnews.com

[Dominique Labrosse]

Richard,

I'm not familiar with Ellis's reference books (I'm sure they are quite good) but I thought I'd better recommend "Professional Photoshop 6" by Dan Margulis. I've found it to be a very helpful guide to colour correction using Photoshop. I think the current version is now numbered 7. Certainly not a book that you will fully absorb in one reading but a great resource that grows with you and your colour correction skills.

Good Luck