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Digital Printing Explained 2 – PPI vs DPI

Digital images are created by a grid of colored pixels.  The number of pixels in the grids x and y dimensions determine the resolution.  This creates and easy to use system as a 3MP image with a resolution of 2048x1536 will always have 3,145,728 pixels, regardless of the colors used in the image.  The Pixel Per Inch system gives a relation between pixels and print size.  This allows a certainty of print size as a 300ppi images will always represent 300 pixels per inch, so a 1000x1000 pixel image at 250ppi will print at 4” x 4".  Typically each pixel can be any of 16.7 million colors from either the Adobe RGB (1998) or sRGB colorspaces. The 16.7 million colors is based on 8bit RGB images where each pixel can have any color created by 255 shades of each Red, Green and Blue (255 * 255 * 255).

Digital Printing Explained 1 – MegaPixel (Image Resolution) vs Print Size

In this series we will be looking into the printing process, and how to get images; from the camera, into the computer, and printed.  This post will focus on the first part, how to get images from the camera and how the camera contributes to the printing process.

How many Mega Pixels do I really need?

Today, camera consumers are bombarded with spec based marketing, where bigger Mega Pixels (MP) implies better quality and demands more money.  The question we always get is, "How do these big MP numbers correlate to the final print?".  While there are some great reasons to have more pixels available, when printing, more does not always equal better.  The first and most useful aid is the standard print size to resolution chart.

While this chart is a useful guide, the final application of the print will determine what ppi is required.  The two biggest factors that determine minimum ppi are printer type and distance from the print (application).  A great example of this is the difference between an 5"x7" photo, 18"x24” poster and a 14’ x 48’ billboard.  The expectation is that the 5"x7" photo will be held at no more than 12"-14" away for viewing, while the poster will be several feet or more away and the billboard will be hundreds of feet away.  The human eye's ability to distinguish dot density (resolution) is based in part on distance from the print, so the further the viewer is from the print, the lower the resolution (ppi) requirements for a satisfactory print.