Resolution in an image determines how many pixels per square inch that image contains. This is expressed as ppi (pixels per inch).
For example, an image resolution of 72 ppi means that there are 72 pixels in a one-inch row and 72 pixels in each one-inch column of an image. Therefore, there are 5,184 pixels in one square inch (72x72=5,184). The necessary resolution will differ based on the desired output for the image. An image for a Web site can be between 72-96 ppi but the industry standard for the resolution of Web images is 72 ppi.
Resolution and File Size
The resolution of an image has no effect on the file size when saved in a Web image format. (However, the file size of the native Photoshop file will be larger.) When outputting images for the Web in Photoshop, the program will automatically convert the image resolution to 72 ppi. There is another Save option that will let you preserve an image's higher resolution but even that will not affect the file size. Consider the two images below.
|300 ppi, 60 kb||72 ppi, 60 kb|
Both have pixels dimensions of 200 x 200. One is at 300 ppi and the other at 72 ppi. There is no noticeable difference in quality and the file size of the images is identical.
Resolution and Print
It is in the domain of print that image resolution makes a critical difference. Different printers have different resolutions. Printer resolution is measured in dpi (dots per inch). Printed material is literally composed of dots of ink. The size and density of these dots determine the quality of the output. An image to be printed in a newspaper will be around 170 ppi. If you've ever held a magnifying glass over a newspaper, you could see the individual ink dots. Magazines are output at 266-300 dpi. High quality books are output at even higher resolutions. So image resolution is important when you're creating images for print but irrelevant as far as Web images are concerned.
Since this course is about producing images for the Web, we won't delve into the considerations for print too much. However, there are a couple of things to keep in mind.
- PPI versus DPI
- Technically, dpi applies to printer resolution but you will see it used interchangeably with ppi in different software and resource materials. For consistency, we'll stick to ppi in this course to describe monitor and digital image resolution.
- Print Size
- While image resolution has no effect on the size of an image displayed in a browser, it has a direct effect on the size of printed images. We'll touch on this more later when we discuss the resizing of images.
Image Resolution Summary
- Digital image resolution is measured in pixels per inch (ppi).
- The industry standard resolution for Web images 72 ppi.
- The resolution of an image has no effect on its file size.
- Most monitors have a native resolution of 72 to 96 ppi.
- Printer resolution is measured in dots per inch (dpi) which reflects the number of dots of ink printed on a one-inch line.
- Print graphics can range from 170-300 ppi or higher.
- Image resolution affects the print size of an image.
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