Understanding DPI

and Pixel Resolution

Though the terms DPI (dots per inch) and Pixel Resolution are used to describe the resolution (or clarity) of an image, they really describe two different, yet very important aspects of the printing process.

What is DPI?

DPI (dots per inch) refers to the number of dots that a specific printing process uses to recreate an image on paper. For example, some inkjet printers spray (or jet) between 72 and 300 dots of colored ink per linear inch. A high-quality photographic printer can generate up to 400 dots of color per linear inch.

It is important to know what the optimal DPI is for the specific printing process that you plan to use. Ask your printer. They have the minimal and optimal DPI for every printer memorized.

What is Pixel Resolution?

Pixel Resolution refers to the number of pixels within a digital image. Pixel Resolution is normally expressed by the number of pixels along the X (horizontal axis) and Y (vertical) axis.

For example, a photographer may reference that an image has a resolution of 4800x3600. This means that the image is 4800 pixels wide by 3600 pixels high.

It should be noted that when we discuss the quality of a digital image we don’t mention its’ physical size but its Pixel Resolution. Digital files don’t have a physical size until we print them.

I realize that the difference between DPI and Pixel Resolution seems subtle since both describe units of color within an image. The big difference is that Pixel Resolution defines the total number of pixels available within an image and DPI defines how many of those pixels will be applied per inch during the printing process.

In order to determine the quality of the final print output of a given image you need to know:

     1) Printer's recommended DPI

     2) Final Output Print Size (width x height)

     3) Digital Image Pixel Resolution

Optimally, you desire a 1:1 ratio between dots and pixels. In other words, for every dot of color that a printer can print you want one pixel of color. This will provide the highest quality print for the specific printer and medium that you plan to use.

(DPI x Print Width) x (DPI x Print Height) = 

(w x h) Optimal Pixel Resolution

For example, you’ve likely seen an inkjet printer at work. An inkjet printer begins at the top of the paper, the print-head scrolls from left to right and systematically moves from top to bottom, printing hundreds of individual lines of color dots that eventually form a complete image.

For example, let's take a printer that prints 72 dots per inch (DPI) vertically and horizontally. Thus an image that is 10” high x 8” wide will require 720 rows (10x72) from top to bottom that are made up of 576 color dots (8x72) from left to right.

(72 DPI x 8") x (72 DPI x 10") = 

 576 x 720 Optimal Pixel Resolution

In this example, the desired or optimal pixel resolution for the source digital image is 576 pixels wide x 720 pixels high. If we were printing the same image on a high-resolution photographic digital printer that generated 400 DPI we would have a different set of optimal pixel resolution specifications.

(400 DPI x 8") x (400 DPI x 10") = 

 3,200 x 4,000 Optimal Pixel Resolution

Same print size – two different printers – requiring two different pixel resolutions.

Ask your photography source for the original pixel resolution of any desired image. This is the pixel resolution of the image before it has been resized.

So, what happens when the Pixel Resolution doesn’t match the required DPI?

This is a constant challenge. Most printing software will automatically size the Pixel Resolution of a digital image to satisfy the required DPI for the printer. This process is called digital interpolation.

Digital interpolation software attempts to multiply or combine pixels in order to achieve optimal pixel resolution. If there are too few pixels the software will generate new pixels based upon the color of surrounding pixels. If there are too many pixels the software will merge pixels together generating a single pixel whose color is representative of the combined pixels.

Normally, reducing the pixel resolution of an image to meet the required DPI of a printer tends to maintain quality. The software is lowering the quality of the high-resolution digital image to be on par with the lower DPI requirement of the printer.

Problems tend to occur when we print a digital image whose pixel resolution doesn’t satisfy the desired DPI of the printer. Most printers can manage a modest increase in interpolated pixel resolution. Too great of an increase in image resolution can create color blocking or stepping. In order to mitigate blocking the software will attempt to blend or blur pixels creating a soft image.

This becomes a bigger issue when we begin looking at large format printing. Let’s say that we want to generate a large-format wall mural that is 12 feet wide by 8 feet high. More than likely, we would use an inkjet printer for final output with a required DPI of 72 dots per inch.

(144” x 72) x (96” x 72) =

10,368 x 6,912 Optimal Pixel Resolution

In this scenario an inkjet with a print output specification of 72 DPI will require a digital image with a pixel resolution of 10,368 x 6,912. This exceeds the pixel resolution of most digital camera technology.

Large-format printing can be more forgiving since viewing distance plays a major role in the visual experience of the audience. Still, it is important to ensure that the desired photographic image for reproduction was captured using high-resolution digital technology and has a pixel resolution that meets the DPI requirements of the printer.