In the world of offset printing, the route to the highest quality black-and-white printing is pretty clear: print the images as duotones (black ink plus one or more gray inks) or as 4-color process. But what about digital printing? I wanted to know the best way to print a black and white photo book, so I did some tests.
Because offset press inks are translucent, black is never really black, but a dark gray. Given this translucency, black and white photographs printed just in black ink can appear washed out, without the full dynamic range visible in a silver print or on screen. In offset printing, the solution to this is to use additional impressions of ink to boost the density of the shadow (darkest) areas of the image. This can be done one of two ways:
- The duotone process, which uses two ink colors—black plus a gray—to increase the density of shadows and the overall dynamic range of the image. A tritone uses three ink colors, a quadtone four, etc.
- Print the black and white images using the process colors (CMYK), boosting the density of shadow areas of the print by adding in cyan or magenta. Only black is allowed in the midtones and highlight areas of the image, because CM and Y can cause very visible color shifts away from a neutral gray.
CMYK Color Shifts
On both offset and digital presses, the light-midtones and highlight areas of images are extremely susceptible to the slightest variation in density of cyan and magenta. A light gray, for example, is almost impossible to reproduce neutrally and is usually shifted toward the cyan or magenta. Consequently, for printing black-and-white images using process colors, the non-black colors are kept out of the midtones and highlights, either by editing the CMY curves in Photoshop or by using an ICC printing profile that suppresses them.
Duotones and Tritones on Inkjet Printers
The tritone technique can also be used on some high-quality inkjet printers, like my Epson 3880. This printer uses nine inks, including four different blacks: photo black, matte black, light black, and light light black. Using different settings on the printer driver, a monochrome photograph can be printed in many different ways, one of them using photo black plus the two grays (a tritone). Note that production inkjet presses sometimes used to print books use CMYK and do not have the ink options of desktop printers like the 3880 or some wide-format inkjet printers. So far, these production printers, while incredibly fast, don’t come close to the quality of toner-based digital presses or desktop inkjet printers.
But what about the commercial digital presses used to print shorter runs of fine art and other photo books? These presses almost always print either black-only or CMYK. Rarely, they may be set up to print a fifth color, but that is typically a brand-specific PMS color or a metallic, and not a gray that could be used to print duotones. So duotones or tritones are not an option; the choices are to:
- Print in black only.
- Print in CMYK and use tone-mapping techniques used in offset printing to boost the shadows.
In most cases, these presses use toner, not ink. Would a toner-based digital press be similar to an offset press, where printing a monochrome image in CMYK can extend the tonal range, or does the digital press have a different kind of response to these techniques?
I decided to do a test on our Xerox color press, printing a monochrome image with good dynamic range both as black-only, and as CMYK with an appropriate ICC profile to boost shadow densities while avoiding color tinting issues in highlights. I did not do a test on our Océ black and white printers because they do not have the capability to print CMYK. The Océs print black and white of high quality, but print with a smaller dynamic range than the Xeroxes to begin with.
To create the image for the test, I took a 36M image on my Sony A7r of an old grain elevator in southeast Minneapolis, converted the images to black and white, optimized it as far as sharpness and dynamic range, and placed it in an InDesign file. I created a PDF from the InDesign file according to our standard procedures, not converting to CMYK in the process but leaving it in RGB. Then Sarah Purdy, our color manager, ran the PDF three ways on our Xerox 1000 digital color press:
1. Black only.
2. CMYK with our normal CMYK production ICC profile, which is optimized for color images.
3. CMYK with an ICC profile with the cyan, magenta and yellow filtered out of the midtones and highlights to prevent color shifting.
Then Sarah and I and Tim Navin, another staffer who is a photographer and has been involved in commercial color printing for decades, evaluated the prints visually.
Good dynamic range and sharpness.
CMYK with normal ICC Profile
Good dynamic range, somewhat less sharpness than the black-only, and some color shifting toward the magenta. Color artifacts in the sky areas, as expected using the standard ICC profile with a black-and-white image.
CMYK with ICC Profile Tuned for Black and White Reproduction
Good dynamic range, somewhat less sharpness than the black-only. No evidence of color shifting in the highlights and midtones, but also little if any increase in shadow density or dynamic range over the black-only print.
The Conclusion to the Best Way to Print a Black and White Photo Book
On our toner-based digital presses, the best black-and-white photo reproduction is achieved by printing black-only, as opposed to printing in CMYK in an attempt to boost tonal range. Printing black-only is sharper, and has just as much tonal range as the CMYK print.
Please note that we can print black and white either on our one-color Océ 6320s, or on our CMYK Xerox 1000s. The Océs are significantly more economical to print on, but the Xeroxes have much richer blacks and better dynamic range, even when printing with black only. If you’re requesting a quote please let us know if it is for a fine art photo book and we will price it for printing on the Xerox.
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Don Leeper is founder and CEO of Bookmobile, which has provided design, printing, eBook and distribution services for book publishers since 1982.