What You Should Know about Pixels and Print Quality for Your Digital Photos - dummies

What You Should Know about Pixels and Print Quality for Your Digital Photos

By Julie Adair King

Generating a good print from a digital photo requires that you feed the printer a certain number of pixels per linear inch, or ppi. So the pixel count of a photo determines how large you can print the image without noticing a loss of picture quality.

The images below illustrate this issue. This image has resolution of 300 ppi.

300 ppi
A photo with an output resolution of 300 ppi looks terrific.

This next image has 150 ppi.

150 ppi
At 150 ppi, the picture loses some sharpness and detail.

This final image has a resolution of 75ppi.

Reducing the resolution to 75 ppi causes significant image degradation.

Why does the 75-ppi image look so much worse than its higher-resolution counterparts? Because at 75 ppi, the pixels are bigger, and the bigger the pixel, the more easily your eye can figure out that it’s really just looking at a bunch of squares. Areas that contain diagonal and curved lines, such as the edges of the coins and the handwritten lettering in the figure, take on a stair-stepped appearance.

If you look closely at the black borders that surround the images above, you can get a clearer idea of how resolution affects pixel size. Each image sports a 2-pixel border. But the border in the last image is twice as thick as the one in above it because a pixel at 75 ppi is twice as large as a pixel at 150 ppi. Similarly, the border around the 150-ppi image is twice as wide as the border around the 300-ppi image.

How many pixels are enough to guarantee great prints? Well, it depends in part on how close people will be when viewing the pictures. Consider a photo on a billboard, for example. If you could climb up for a close inspection, you would see that the picture doesn’t look very good because billboard photos are typically very low-resolution images, with few pixels per inch. But when you view them from far away, as most people do, they look okay because our eyes blend all those big pixels together.

Unless you’re doing billboard photography, however, people will be viewing your images at a much closer range, so a higher resolution is required.

The resolution you need to produce the best prints also varies depending on the printer, but 300 ppi is generally a good goal. For quick reference, this table shows you the approximate pixel count you need in order to produce traditional print sizes at that resolution.

Print Size (in Inches) Pixels for 300 ppi
4 x 6 1200 x 1800 (2 mp)
5 x 7 1500 x 2100 (3 mp)
8 x 10 2400 x 3000 (7 mp)
11 x 14 3300 x 4200 (14 mp)

The first set of pixel values represents the pixel dimensions (horizontal pixels by vertical pixels); the value in parentheses represents the total pixel count, in megapixels (MP). Your mileage may vary, however, so don’t panic if an image you want to print has fewer than 300 ppi. You may be satisfied with prints made with a lower resolution — just don’t go too low.

  • Select the resolution that matches your print needs before you shoot. Yes, some photo programs enable you to change the number of pixels in an existing image, a process called resampling. But adding pixels — upsampling — isn’t a good idea. When you take this step, the photo-editing software simply makes its best guess as to what color and brightness to make the new pixels. And even high-end photo-editing programs don’t do a good job of pulling pixels out of thin air.
Here you see the result of upsampling the 75-ppi image above to 300 ppi.

To create this image, the 75-ppi image was resampled the image to 300 ppi in Adobe Photoshop, one of the best photo-editing programs available. Compare this new image with the 300-ppi version above, and you can see just how poorly the computer adds pixels.

With some images, you can get away with minimal upsampling — say, 10 to 15 percent — but with other images, you notice a quality loss with even slight pixel infusions. Images with large, flat areas of color tend to survive upsampling better than pictures with lots of intricate details.

  • Use the highest resolution setting for pictures you may want to crop later. The numbers in the table above assume that you’re printing the entire photo. If you crop the photo before printing, you need more original pixels to generate a given print size because you’re getting rid of a portion of the image.

For example, the left image below shows the tightest framing that could be achieved with the camera, given the distance between the bird and the lens. Because the image was captured at a resolution of 24 mp, it could be croped to the better composition shown on the right and still have plenty of pixels to produce a good print. In fact, the cropped file contains about 5.3 mp, so that it can output a much larger print than fits on this page.

A high-resolution setting was used to capture the original (left), which enabled the cropping away of excess background and still left enough pixels to produce a good print (right).