It is a known fact that images viewed on computer monitors don’t always match what comes out of inkjet printers. This is because the color pixels captured by digital cameras are defined quite differently than the pixels portrayed on the computer monitor and the monitor’s pixels differ quite significantly from the ink patterns that are literally sprayed onto the paper.
But even though both inkjet printers and printing presses both use CMYK inks, the images printed on inkjet printers usually don’t produce the same appearance when printed in publications. This is quite true, but why?
Color images are displayed differently on each device because the technologies for each medium use different processes; monitors (left), halftones (middle), and inkjet (right).
The answer to this mystery eludes many of today’s magazine publishers and even many publication printers. This is a problem that the digital imaging community (photographers, image editors, and pre-press operators) have struggled with for decades. Color Management Professionals (CMPs) undergo rigorous color science studies to understand how to maintain the same look in color images that are reproduced on different substrates and a variety of printing processes. Since you may want to produce your images in print, we’ll look at a synopsis of what the challenges are and some surefire ways to produce the results you’re looking for.
First and foremost, cameras and monitors capture and project color images as RGB light but all ink-based printers must convert these RGB colors into CMYK colors behind the scene! Even though you send RGB files to your inkjet printer, the printer doesn’t rely on RGB inks to produce all the colors in the prints. RGB colors are for projecting colors while CMYK colors are used to print colors.
Projected colors are always viewed in RGB while printed colors are always produced from some formulation of CMYK inks. That’s simply how color science works. Printers don’t print the RGB colors directly. While you send RGB images to your inkjet printer, it converts those colors into some form of CMYK during the printing process. Even when you send an RGB file to your eight-color printer, the base CMYK colors are augmented by slight amounts of Photo Cyan, Photo Magenta, Red, and Green colors. However, there has been one printer (the Oce´ LightJet) that produced color prints from RGB, but it didn’t use printing inks… it was a photographic printer that exposed photographic paper and film using RGB light. This printer is no longer manufactured.
Each printing process utilizes a unique pattern to express the variable tones between solid and white.
Viva le difference
The inkjet printing process is completely different from the print reproduction process. As a matter of fact, the two systems are overtly dissimilar. If your images are headed for print and you are not sure of which printing process will be utilized, you might be headed for trouble. Here’s why.
The possible surfaces for inkjet printing vary wildly and include everything from paper to wood, from metal to fabric, and on virtually every surface and texture in-between. To accommodate this range of printing applications, inkjet “inks” are liquid rather than solid, so they can be applied to varied surfaces and substrates.
Dots versus spots. The peanut butter consistency of press inks and the well-defined shapes of the halftone dots used by the printing industry differ significantly from the liquid inks and less defined “micro-dot” dithering used by the inkjet printing process.
The color spots produced by inkjet printing systems may include more than a dozen colors and are liquid to accommodate almost any surface. Printing press dots are well-defined symmetrical shapes and are much thicker consistency to accommodate the high-speed transfer to paper. Both inks are translucent because they must blend to create other colors.
The extremely small inkjet droplets appear more like a mist than a defined pattern; each pixel value (0-255) creating a metered amount of microscopic spots so small that the human eye perceives them as continuous tones. Due to the smoothness of the tones and graduations of color, inkjet images require a bit of sharpening to deliver detail (detail remember is a product of contrast, and contrast is not a natural inkjet strength).
Dot structure of halftone images (left) and color dither pattern (right).
Both the inkjet and publication systems convert the RGB (red, green, and blue) values of each pixel into equivalent CMYK (cyan, magenta, yellow, and black) values before printing those colors onto paper. However, after the color conversion, the two processes take decidedly different paths to deliver ink on paper.
While printing presses use grid-based, well-defined dots that are impressed into paper surfaces, inkjet printers utilize micro-dot patterns sprayed onto surfaces. The same image may appear in several different forms during the reproduction process. Original image (far left), digital pixel (near left), printed halftone (near right) and Inkjet dither (far right)
Publications use the geometric structure of halftone dots to interpret pixel values as tonal values on paper surfaces. Each pixel produces up to four overprinted color halftone dots. These halftones dots translate darker values of each color into large dots and lighter values into smaller dots. The full range of darkest-to-lightest tones produce dots that vary in size depending on the press and paper being printed.
To avoid the visually annoying conflict that occurs when geometric grids collide (called a moire pattern), each CMYK grid pattern is set on a very carefully calculated angle. The positive advantage that inkjet images have over halftone images is that the image resolution required for inkjet prints is significantly less than the resolution required by the halftone process employed by publication images.
However, the most important issues to address with print have to do with color fidelity and tonal reproduction. The difference in the way inkjet images and publication images are prepared makes a huge difference in the way the images appear when they come out the delivery end of the process.
Inkjet printers are like ballet dancers while printing presses are more like Sumo wrestlers; not unlike chamber music versus thunder roll. One is quiet, graceful and articulate, the other noisy, violent and powerful.
The biggest difference between the two processes can be seen in the highlight and shadow areas. Inkjet inks are sprayed onto substrates through a very controlled matrix of 720-1440 spots per inch using a slow and measured inches-per-minute process. Publication presses smash ink into the paper under extreme pressure, at speeds measured in images-per-minute, translating the entire tonal range into a limited geometric matrix of just 150 variable-size dots per inch. Publication presses are huge, high-speed, rotary rubber stamps.
Inkjet printers carefully step the paper through the machine in an extremely precise manner while the printing press shows no such restraint. Presses display an amazing ability to control the placement and transfer of images in spite of the blazing speed of the process.
You might be able to dress a Hippopotamus in a tutu, but you can’t expect it to pirouette. There are simply physical limitations. At production speeds, the shadow details suffer, delicate highlights tend to drop-off rather abruptly, and the middle tones print darker. The printing industry is aware of this dot gain issues and compensates for them with G7 process controls and compensation plate curves, but the beast remains a beast.
There’s a pretty good chance that both color and tonal detail will be unwittingly lost in the printing process if nominally prepared images are sent to press. Having spent many years of my career in both photo labs and the pressroom, I can assure you that detail in both the lightest portions and darkest areas (and placement of the middle tones) will need special attention to transfer all the detail on the press. Highlights get flattened, and shadows get closed more easily because of the high speeds and extreme pressures involved.
This means that images destined for print must exhibit more internal contrast in the quarter tones (between middle tones and highlights) and three-quarter tones (between middle tones and shadows as well as a slight adjustment to the middle tones to reproduce at their best. I’m sure I will hear some disagreement about this from some publishers, but as a former pressman, I know that images that do not get some special attention usually print somewhat flat.
The image on the left might look good as a print, but it would reproduce poorly on a press. The shadow areas would get even darker and lose all detail. The image on the right will darken slightly in the lower tones producing an excellent result in print. White balance is also critical in publication printing. Compensating for the unavoidable effects of the press always pays off.
There is a cardinal rule in printed publications that states that even areas of the whitest whites and darkest darks must contain dots. The only “paper white” should be specular (light reflecting from glass or chrome) and even pure black doesn’t print solid black; everything contains dots. Unlike inkjet printers, printing presses cannot hold (or print) dots smaller than 2-3% value (247). Dots smaller than this never make it onto the paper. This is why additional internal contrast is needed on both ends of the tonal range.
Photographers certainly know their way around cameras and software (Lightroom or Photoshop), and they understand color and tonality as it relates to mechanical prints. They are also accustomed to references to RGB (red, green, and blue) colors and may even understand how inkjet printers work, but very few are familiar with the behavior and limitations of huge printing presses. The analogy of ballet dancers versus Sumo wrestlers is an accurate one.
Photographers understand fine art prints and image editing software though few see their photos through the eyes of pressmen. But perhaps they should!
There is a significant difference between preparing photos for inkjet printers and preparing images for publication presses. The publication RGB-vs-CMYK conversion thing differs significantly from inkjet conversion in color gamut, image saturation, and tonal reproduction.
When an image is captured, it can potentially possess more than 4000 tones per (RGB) color. That’s a whole bunch of possible colors. But the sobering factor is that all printing processes reduce those possible 4000 tones down to a mere 256 tones per RGB color before any ink hits the paper. Obviously, the post-processing tone and color shaping of camera images are super-critical! Simply put, how the photographer shapes all that data before it is ready for print determines how much detail and clarity will get printed on the pages of the magazine.
Once again, the top picture would print great on an inkjet printer but would lose very critical detail on a press. Compensation for the unavoidable effects of the press is always advised. In Part 2 of this series, I’ll show you exactly what adjustments were made to this photo. Additional sharpening also helps compensate for the slight blurriness of the halftone process.
The old adage “start with the end in mind” comes clearly into focus here. No matter how much data is captured by the digital camera, the publication press is the ultimate arbiter of tones and colors, and deserves the loudest voice in the conversation. The color gamut of CMYK conversion is even more restricted than the basic sRGB gamut of Internet images, making this post-processing exercise perhaps the most precarious scenario of them all. If you ignore the special attention needed for magazine images, don’t expect the images to pop off the page. Ignore the press’s advice, and you’ll pay the price in both detail and color reproduction.
In the follow-up article entitled “Preparing Images for Publication Part 2,” I’ll reveal the literal “trade secrets” for producing great publication images.
