Printer Resolution– Measured In Dots-Per-Inch (dpi)

An inkjet printer reproduces an image's square pixels by squirting round dots of ink at the paper– the number of dots laid down along an inch of paper is called printer resolution and is measured in dots-per-inch (dpi). Most quality photo printers usually have different dpi resolution settings to allow optimal output when using different papers and inks. Settings of 720 dpi and higher allow pixels to be defined sharply and their color accurately reproduced– ink dots at higher resolutions also blend together a bit more seamlessly so you don't see them as separate specks.

But using a high dpi setting on your inkjet cannot improve the basic quality of your image; only more pixels in the image can do that. Remember the stretchy rubber photo? If you try to enlarge your picture too much, pixels get bigger and bigger and begin to be noticed; a high dpi setting on your printer may just make them look more well-defined– so you can see them at their worst. You must have enough pixels in your image to begin with and they must be close enough together so they can't be seen individually.

Printer Settings For Best Results

How close should those pixels be? Epson says you'll get the best print quality if you have an image resolution value of 240 ppi when your printer resolution is set to 720 dpi; 300 ppi is recommended for 1440 dpi or 2880 dpi settings. Higher ppi values than 300 are not necessarily better; if pixels are too close together, unsharp images may result. However, you can often print at a lower ppi values than those recommended above and your photos will look fine– let your eyes be your guide. If your camera doesn't have high enough resolution to produce lots of pixels, you'll usually have to settle for smaller photos if you want to print with more pixels-per-inch.

Why? Because as we've seen before, picture size and the number of pixels-per-inch are related. Let's say you want to print an image at 8 x 10 inches and you enter those values into the size fields of your imaging program. Don't click "print" yet; the ppi field now shows that your photo will output at only 150 pixels-per-inch– probably too low for a good 8 x 10 print. Well, why not just up it to 300ppi?

If you input that new ppi number, the dimensions of your picture will jump down to a smaller size. That's the program's way of saying there aren't enough total pixels available to have them line up shoulder-to-shoulder at 300-per-inch to form the larger image. Remember: the number of pixels-per-inch is automatically re-calculated by your imaging program with each change of picture size. And if you input a ppi value of your choice, your photo will be automatically re-sized to print with that number of pixels-per-inch.

Digital Magic

There is a way to get both your desired image size and higher ppi value if you so desire. You can use a bit of digital magic ­called interpolation­ to tell your imaging program to add more pixels to the image (within reason) till the ppi number gets high enough to produce a smooth-looking print. Simply unlock the picture size and ppi relationship (usually by checking a box called Resample Image in your image editing program) and then type in both the image size and the ppi value you want.

Interpolation won't give you as good a picture as if all the pixels were there to begin with (because the program is spinning them out of thin air and making an educated guess as to what colors they should be), but with a little image sharpening after they've been added, you'll usually be pleased with the result.

The concept of camera, image, and printer resolution is not the easiest to grasp; part of that's due to the confusion generated by manufacturers (and even professionals) who use the wrong terms. But once you understand how these various resolutions inter-relate, you'll have complete control over the quality of your prints.

NOTE: For permission to reprint this article, contact arthur@dpcorner.com.

Page 2 of 2