Grad School Knowledge Saved Me! Working with Grayscale Images for Print

Did you know grayscale images should be saved at 600 DPI for print?

Save your artwork at 600 PPI when working with grayscale images for print.

I was working on some grayscale images for an upcoming publication and asked upfront if raster images were okay. The answer? Yes. So, I completed the images—only to have the printer request everything in vector. 😱

If you've ever tried using Adobe Illustrator’s Image Trace to convert a raster file to a vector, you know how bad the results can be. Vector art must be planned from the beginning. Shapes, gradients, and values must be intentionally constructed—not haphazardly assembled in a last-minute conversion.

I tried to import my Photoshop paths to apply fills and strokes, but the process was quite time-consuming. When rendering a raster image, the pixels shift as you adjust, distort, move, and liquefy. As a result, the final image does not adhere to the constraints of the original paths.

Before panicking, I sent one print-ready image and asked the printer, “I worked in 600 PPI. Will this reproduce well?” I created the images at 600 PPI in Photoshop, recalling information from graduate school about grayscale imaging over a decade ago.

The answer: Yes!!! A resolution of 600 PPI was acceptable for their press, which means that vector graphics were not necessary after all.

Internal female reproductive anatomy. The Empowered Hysterectomy - Internal Female Reproductive Anatomy. Created by Carol Hrejsa, CMI for Hachette Book Group, © 2025 Hachette Book Group

Why does resolution matter?

Color prints at 300 PPI because CMYK inks overlap at different angles. The blending of the colors occurs in the "in-between space," making it appear to be a smooth transition to the eye.

Grayscale needs 600 PPI since tonal variations rely on dot size rather than color mixing. This reproduction depends on a basic optical illusion: the human eye interprets the patterned areas as varying tones. Higher resolution makes the transition of tones indiscernible by the eye, resulting in a perception of gray even though only black ink is used.

A visual explanation of how the eye views Color vs Grayscale at different resolutions.

Black and white line art should be created at a higher resolution of 1200 PPI! Since it consists of only two colors—black (ink or toner) and white (no ink or toner)—there’s no need for varying dot sizes to simulate tone. A higher resolution helps to keep the edges crisp and clear.

In the end, the client preferred the raster version, so I was fortunate that it was accepted for print, and I didn't have to redo everything. Raster art tends to feel more natural, while vector art can look too clean or too mechanical.

Most of my work is in color, and much of it stays digital. Understanding the importance of print and how resolution impacts output can prevent many issues and future headaches.

Lesson learned: Always ask for print specifications and make sure to get confirmation directly from the printer for the final say.

Bonus question:

How does Spot Color Work?

Using a single, pre-mixed ink in place of (or in addition to) CMYK standard colors is known as spot color printing.

An example of a printed image using grayscale and spot color for accents. © 2017 SAGE Publishing

Spot colors use a particular ink formula (e.g., Pantone 2995 for a brilliant blue) in contrast to CMYK, which combines tiny dots of cyan, magenta, yellow, and black. Using spot colors with only black ink can be cost-effective, creating gray-tone images enhanced by a splash of color for visual interest. This approach reduces the expenses of running all four process color screens for large publications.

This is an example of using two channels to create a specialty ink color, known as a spot color. © 2017 SAGE Publishing

Using Photoshop to Preplan Spot Colors

You can configure a file for spot color printing even though Photoshop is generally a raster-based application. Here's how:

If the spot color is an accent with grayscale images, work as you normally would with a grayscale render.

Apply Spot Colors to Spot Channels

Go to Window > Channels to open the Channels panel. Choose the New Spot Channel option after clicking the menu icon. Select custom or Pantone ink color and adjust the opacity accordingly. The places where the spot color should be shown should be painted or filled in. You can use opacity with your brush and even erase. This spot channel remains a distinct ink layer in the Channels panel.

Focus on Clean Edges

Steer clear of effects like feathering and gradients unless you know how they will work with spot color printing. Using halftones, a dot pattern that mimics tonal fluctuation, is a good alternative when gradients and shading are necessary.

Use the Correct Format for Saving

To preserve separation for printing, spot colors must be in a PSD, PDF, or EPS.

You may flatten your artwork in the final Photoshop rendering if you don't want to give the client layers, however make sure the spot color remains a separate Channel in the Channels panel.