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DPI vs. Pixels & the Printed Page

We often find ourselves explaining the same thing over and over to clients and friends. Finally, we decided to start making comprehensive tutorials to help people understand the world of Photoshop and other such things. It is so easy to obtain and learn these software packages, but too often people don’t get the real understanding they need. So here is the first lesson in what will hopefully become a series explaining some of the basics.

Dots Per Inch / Pixels Per Inch

First, plain and simple, “Dots per inch” and “Pixels per inch” are the same thing. I am not sure how it got out there that they may be different, but they are not. A digital image is made up of a grid of square pixels. These many many pixels, when put together, represent the whole image. On a computer screen, they are square pixels of color (represented by the monitor as even smaller units of either red, green or blue, but that is another lesson). On the printed page, they are represented by tiny dots of blue, red, yellow or black, and when viewed further away, they form the whole image.

An image 480 by 360 pixels, at a screen resolution of 72 dpi.

The same image, 480 by 360 pixels, but at a print resolution of 300 dpi.

A close-up of a screen representation of the image and the printed page version.

The Image Size Palette in Photoshop

In Photoshop, under Image -> Image Size you get the palette that starts to explain all this. The first two numbers are the Pixel Dimensions... this is how many pixels are in your image. You only have a finite number of pixels in an image to deal with, and the Document Size is the way that those pixels get dispersed. So the Document Size is the physical size of the output of those pixels (expressed as a measurement... inches, cm, picas, whatever).

The Pixel Dimensions vs. Document Size at 72 dpi.

The Pixel Dimensions vs. Document Size at 300 dpi.

A 480 pixel by 360 pixel image can manifest itself in a few ways. On screen, which is 72 dpi, the image is 6.67 inches by 5. In print for a magazine, however, I would want the image to be 300 dpi, so I open up the Image Size palette and make sure that the “Resample Image” box is unchecked. Then I plug in 300 to the Resolution box. Notice how the Pixel Dimensions do not change, but the physical inch size or Document Size does change to 1.6 x 1.2.

So the best way to change the resolution of an image is to use the Image Size palette with the Resample Image box UNCHECKED. When you do this, Photoshop will tell you how big (inches) your image can print at 300 dpi (or any other dpi for that matter). Below, is why you should not resize an image with the Resample Image box checked.

Very Important: If you learn nothing else, learn this!

IT IS VERY IMPORTANT that you understand that new pixels should not be created when you want to enlarge an image. So many times, someone takes an image that is low resolution (72 dpi) and just changes it to 300 dpi with the Resample Image box checked. This means Photoshop is creating new pixels where once there were none. Notice that the image is now 2000 pixels by 1500. Where do these new pixels come from?

The image at 72 dpi close up. Notice how crisp the edges are.

Changing an image’s resolution from 72 to 300 with the Resample Image box checked. Notice that the file size is increasing from 506.3K to 8.58 Megabytes. Photoshop is creating 7 and half megabytes of new information.

The same image, now 300dpi. Notice how there are more pixels (they are smaller), but the detail, or crispness, has been lost.

When Photoshop resizes images, as an example, it takes one pixel and makes it into 4. It decides what color the other three pixels will be by looking at the pixels around it. Then it makes an educated guess as to what colors the new pixels need to be. It does a good job, but as it is spreading the original information thin by making other pixels like it, the image comes out fuzzy or blurry. PHOTOSHOP CAN NOT CREATE NEW DETAIL. This is why we do not suggest you resize an image when your original is not large enough. Anything beyond 20% of the original image size will result in a blurry edges and a general loss of detail. The best thing to do is rescan your original, or resign yourself to the fact that your 600 x 480 pixel image from your digital camera can not print full bleed on a printed page.