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send CMYK filesCMYK for photographers

Problems and pitfalls - some first steps

At Northlight we have a strict policy of only supplying our images to commercial clients in RGB colour spaces (such as Adobe98 or sRGB for web)

Indeed, the company only supplies in CMYK formats if the client convinces us that they know what they are doing, and appreciates that the conversion will be a chargeable part of our work.

Why such a strict policy?

Keith Cooper has written this introductory article with UK colour management consultant Ken Thomson to address some of the issues that as a photographer, you should be wary of when supplying images in CMYK.

It's intended as a guide for photographers who might have clients who want work in CMYK.

A second article covers print related issues for larger scale printing.

It's not that difficult, but there are a number of potential difficulties along the way and it's all too easy for staff at clients to 'blame the photographer' when their necks are on the line for some bad image quality in the company's new brochure.

Could you write an article for our web site? More details

Of course, as colour management consultants, we're happy to give basic advice - something Northlight offers free to clients using our work. For more complex work in the print side of things, we're happy to pass on details of people who's knowledge and expertise we trust, such as Ken...

Using CMYK

I'm going to assume that you've got your monitor properly calibrated, and if you print images, you've got good lighting to evaluate your prints and are printing using printer ICC profiles.

Not to worry if such things are new to you, just I'd suggest reading some of our introductory colour management articles first

Color Management bookI often get asked for suggestions about learning more about the nuts and bolts of Colour Management.

My usual suggestion is Bruce Fraser's Real World Colour Management. My own copy is well thumbed. It's my first port of call if I'm asked a question and I feel I don't quite understand an issue well enough to be absolutely sure of an answer.

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If you print your own work on an inkjet printer, then probably your only exposure to CMYK is the fact that your colour printer contains Cyan (C), Magenta (M), Yellow (Y) and Black (K) inks.

It may have other colours to improve print quality, but essentially you can print full colour with just these four inks.

What confuses many people is that the printer driver software for your printer converts your RGB (Red/Green/Blue) images into the appropriate CMYK ink levels for you, so the printer is best viewed as a RGB device (like a monitor) rather than CMYK.

So, for printing your own work on inkjet printers, there is no need to go anywhere near CMYK.

  • Note: if you print via a RIP, then your printer profiles are usually CMYK ones, but you can still send RGB images to the RIP for printing and never have any involvement with the CMYK side of things.

The difficulty comes when you need to send your work to someone who will be using large scale printing presses, whether for posters or books or magazines.

If you've looked at working in different colour spaces, then you'll be quite aware that the range of colours you can capture with your camera can exceed both what can be fitted into a particular space (say Adobe98) and what can be displayed on your monitor.

There is then the problem of what colours can be printed on your inkjet printer and how the colours in your image are mapped to what the printer/paper/ink combination can cover - the printer's Gamut.

These limitations are often addressed in ways such as soft-proofing and rendering intents when using printer profiles.

So, what about CMYK?

comparison of SWOP and Adobe 98Think of a CMYK colour space as you might an ICC printer profile - it represents what a particular press can manage on a particular paper (or 'stock').

The spaces are generally smaller than what you might choose as a working space.

The example at the right give an idea of the differences, shown with the OSX ColorSync utility.

This example compares aspects of Adobe98 and a space that often seems to get picked by designers for no good reason: U.S. Web coated (SWOP) V2

  • Do note my usual health warning about such diagrams. They may illustrate a point, but often have all kinds of subtle biases built into them - if you see an article with lots of them and no explanations then assume they are there to look good ;-)

CMYK space represent rather more than your usual inkjet printer profile, since they usually have the concept of 'ink limits' in them.

The four CMYK 'numbers' are usually in percentages in the range of 0 to 100%

There is not a big press made that won't leave a sticky mess on the paper if you use 100% of all four inks, so there is the concept of a total ink limit, that depends on the stock used and press type. i.e. a 320% limit means that C+Y+M+K must add up to no more than 320%.

It's also useful to use more black ink in the mix where possible (it's cheaper and there are less image registration issues), so there are things like Under Colour Removal (UCR) and Grey Component Replacement (GCR) to consider.

If you've thought that creating printer profiles for your desktop printer was a bit involved, then that pales into insignificance once you decide to start profiling presses.

Indeed, it takes a lot of experience in this field to get the best out of this type of printing, which is why when a client is asking about aspects of their workflow relating to press issues, I'll have no hesitation in giving them Ken's details.

One key thing to remember is that you lose a lot of information about an image, once you convert it to CMYK. It's a one way trip.

If you consider presses with more than the four inks, then profiles get even more complex and less amenable to subsequent change.

The Client who asks for 'Standard CMYK'

Just send CMYK files at 300dpiWhen a new client phones up and says that they would like images supplying in CMYK, the alarm bells start ringing.

First up, I'll ask 'What sort of CMYK?

If the answer suggests surprise that there is anything other than 'standard CMYK' then I know I'm dealing with a company that is in serious need of colour management knowledge as well as my photography...

All too many graphic design studios have the unfortunate approach of just converting any RGB images they get into the default CMYK space that their copy of Photoshop is set to.

Unfortunately, the Photoshop defaults don't actually match to many presses (and vary between Photoshop versions and locations)

Additionally, your images as the photographer, have just been mangled and might not look anything like what you saw on your monitor before sending them.

If the client knows what space they want me to use, then at least we're getting somewhere, however I'm still suspicious of why they want me to do the conversion. I do however at least have a chance to see how things might change in my images.

The best clients know exactly how they want the images supplied and will give me some guidance as to the final output medium, so that I can send them files that won't look drastically different when printed. Such clients are well aware of the capabilities of their different output mediums and the print companies that they are using.

Unfortunately, the number of design companies falling into this last category is still relatively small.

Guidelines

One document/link I regularly send out to clients is the UPDIG guidelines. The guidelines were created to establish photographic standards and practices for photographers, designers, printers, and image distributors.

Very useful for showing that a consistent and colour managed workflow helps get things 'right first time' more often. Almost always, when I run into resistance to change, it's on spurious cost grounds.

Materials such as the guidelines help demonstrate the benefits of actually having a clue about what you are doing ;-)

Conversion to CMYK

In some ways it's a bit too easy in Photoshop - you just change the mode to CMYK

photoshop convert to cmyk

The problem is that there are loads of different CMYK colour spaces available and just converting to one at random is potentially going to cause difficulties for reproduction of your image down the line.

You might wonder why the bother, surely the CMYK profile can be fixed later, with another conversion? It's a bit more complex than that, and CMYK to CMYK conversion has its own set of pitfalls, particularly what can happen to blacks.

Oh, and the blame when it goes wrong? it won't be taken by the graphics design people who've just chosen any old CMYK because that's what they've always done...

In Photoshop CS5 I notice that there is at least a warning...

warning about conversion profile

You can use the convert to profile option as well. This gives even more conversion options, such as choice of rendering intents.

convert to profile dialogue

I'll not go into all the different conversion considerations in this article, just give a few pointers.

The rendering intents can be thought of as how the colours from one space are squeezed into another - whether just out of gamut colours get clipped, or there is a more widespread alteration, to preserve the 'look' of an image.

The choice is usually between Perceptual and Relative Colorimetric.

As to using dither and the conversion engine - if you need features like this then your level of colour management knowledge is probably a bit beyond what we're covering here...

Black Point Compensation (an Adobe developed feature) allows for differing blacks in the source and destination profiles - for Relative Colorimetric it can make the results look better.

BPC is an optional modification of relative colorimetric conversions, but because it is currently applied to all steps from source space to destination space, it also affects the black point in the proof.

In principle limited to scaling the L channel, BPC moves the black point of the space you are converting from, up or down in lightness to match the black point of the colour space you are converting into, which can avoid clipping of shadow details. Because this may expand the simulated lightness range in the destination space, you should disable it for soft-proofing and proof-printing.

Proofing - or an idea what it will look like

If you want to see what sorts of parts of your image will change when in print, then there is always the option of 'soft proofing', where the display changes to show an impression of what things will look like.

There are lots of potential sources of error here - you need to know the destination profile, and it helps to know where the print will be reproduced.

That said, it can offer some useful hints.

The image below is from the Datacolor test image that I regularly use for print testing - it's in the Adobe98 colour space, and the screen shot below shows the soft proof of picking a US SWOP profile (one of the ones that gets picked as a default sometimes)

Move your mouse over the image to see the effect of the conversion (remember that you are seeing a screen grab, converted into sRGB for web display)

soft proofing of cmyk conversion

You can also show a gamut warning - I've set this to flash red.

Move your mouse over the image to see areas where there is a mismatch

gamut warning shown

With this approach, you can also set the display to simulate the actual ink and paper colours.

Be careful though, if you have other items on the screen (palettes, windows etc.) the brightness of these can make the proofed image look very poor.

The next step is to simulate a print using your inkjet printer and appropriate paper.

In the Photoshop print dialogue, there is the option to do a proof print.

Note that I've still got SWOP selected, you might want to select a more appropriate profile...

print proofing dialogue

Once again there is a whole big area of work concerned with accurate proofing, and I'll leave details of some of the things to look for until our next article.

soft proofing exampleAt Northlight we've a controlled viewing system that I use for evaluating prints - if you are producing hard proofs, you really do need to consider something like this.

Summary

Be careful when a client asks to receive images in CMYK

Unless you know how the images are going to be used and they give you suitably complete conversion information be aware that things could go wrong with your image further on in the workflow.

If you do need to convert your files to CMYK, then try out soft proofing at least to see how your image could be affected by the conversion.

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Our second article looks at more of the considerations if your work is being produced for books and magazines, and some aspects of colour management in the press environment.

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