What Is The Best Resolution For Printed Grayscale Images?

Back in the day, we had to calculate the resolution of our images in order to make them as small as possible to fit on a floppy disk. Not anymore. Storage media has progressed to the point that we don’t need to worry as much about how to store our files. So, how many dots per inch are required to print our grayscale images at their best?

Yesterday and today, the imagesetter or platesetter is the measurement by which photographic resolution is determined. The typical highest resolution of these devices is 2540 dpi. So, a photographic quality image is considered by most to be an image with a resolution of 2540 dpi or greater.

The general rule for modifying photos with the intention of printing them is to keep them at the highest quality possible. This might be the quality they came off your digital camera for example. Their are two problems with this; The first is the limitation of storage space available to you, ie., the space on your flash drive or hard drive. The second, is the amount of time it takes for your images to print.

Avoid reducing the quality of your original images from either a scanner or camera at the time of capture. Set the scanner or camera to the highest possible setting for quality. Never resize the image so it is smaller. You shouldn't try and make it larger either, just keep it at the original size, so that the image is not distorted. This is true for color and grayscale images.

To print a black and white image, a printer should be set to use only the black ink. If all four colors are used, you will not have true black in the image. If only black ink is used, there are really no true shades of gray printed. The printer uses a pattern of dots to represent each pixel of the

image. In order to simulate variable-sized halftone dots in computer printers, dithering is used, which creates clusters of dots in a "halftone cell." The more dots printed in the cell, the darker the gray. As the screen frequency gets higher (more cells per inch), there is less room for dots in the cell, reducing the number of shades of gray or color that can be generated.

In low-resolution printers, there is always a compromise between printer resolution (dpi) and screen frequency (lpi), which is the number of rows of halftone cells per inch. For example, in a 300 dpi printer, the 8x8 halftone cell required to create 64 shades of grays results in a very coarse 38 lines per inch of screen frequency (300 dpi divided by 8). However, a high-resolution, 2400 dpi imagesetter can easily handle 256 shades of gray at 150 lpi (2,400 /16).

I maintain that the maximum resolution required for printing a black and white (or grayscale) image is 160, in order to provide a minimum of 256 shades of gray. I obtained this by taking the original 2540 dpi of the imagesetter and dividing by 16. The result is 158.75 lpi. Since we cannot have a fraction of a dot or line, I round up to 159 and to 160, in order to provide an even number of dots or lines. For most users, there is no need to distinguish between lines per inch and dots per inch, in order to provide the desired results.

Left: Halftone dots. Right: How the human eye would see this from a sufficient distance.

The bottom line; The maximum required resolution of black and white (or grayscale) images, in order to provide the best possible image quality without excessive file size, is 160 dpi.