This tutorial covers the basics of image resolution, the amount of detail in and quality of an image. This is highly relevant to preparing a document for either print or web. This tutorial covers:
What is resolution?
To understand resolution it will help to have a basic understanding of pixels. A pixel is the smallest unit that can display a certain color in an image. It is helpful to think of it as an empty box that can be filled with any color in the spectrum. The more of those boxes that are within an inch, the more colors that can be displayed, and thus the more detailed and clear the image can be. Resolution, then, is the number of pixels per inch within an image. The greater the resolution, the greater the detail within an image.
No matter what the final destination for an image is, resolution is important. It determines things like file size and the overall quality of the image. However, image resolution depends upon the final destination for the image. An image going online will need a different resolution than one going onto a large poster.
First, to find the resolution of any image that is open in Photoshop, click on Image > Image Size (or shortcut Option+Command+i) and a dialog window will appear. One of the values listed will be resolution, which is listed as a pixels per inch value (ppi). This value can be any number, but the following values can be used as guidelines depending on the final destination for the image:
- 72 pixels/inch: A resolution of 72 ppi is good for images that will be appearing online, or on a screen.
- 150 pixels/inch: A resolution of 150 ppi is good for images that will be printed on the plotter printer in the MDL or the Science Building. This keeps images detailed enough for the size, but small enough to print speedily.
- 300 pixels/inch: A resolution of 300 ppi is good for general purpose images. These can be things that will be printed on regular, letter sized paper or even slightly larger sizes like 11 x 17 in. paper.
Resolution between documents
The last thing to remember about resolution is that it can vary from image to image. If multiple images are going to be combined onto a single poster, it is generally a good idea to set them all to a consistent value. If they are not consistent then the images will change in size when moving between documents. What this means is that an 3 x 5 inch image in a 150 ppi document will become 6 x 10 inches when added to a 72 ppi document. Setting all images to the same resolution value avoids this problem.