Myth - 72dpi For Web Images
Every time I read articles or tutorials about saving images for Web pages or sending through e-mail they always mention saving the images with a resolution of 72DPI or Dots Per Inch. The interesting fact about this suggestion is that a computer monitor doesn’t deal with dots or inches, but rather with pixels.
The resolution of a monitor is indicated as two numbers: [number of pixels for width]x[number of pixels for height]. For example, resolutions are indicated as 800×600 or 1024×768. You notice no mention of dots or inches because a monitor only cares about the number of pixels. This means that when creating images for viewing on a monitor it is important to pay close attention to the size of the file in pixels.
Let’s see an example of how this works. Take the exact same image and resize down to 225×300 pixels. Set the DPI of the first image to 300 and the DPI of the second image to 10. Ensure that the image is not resampled so that the 225×300 pixels size remains intact.
Once both images have been created, open each of the images in a separate Web browser window and place the windows side by side. You will notice that regardless of the DPI, the images look identical. This is because a 225×300 pixel image will always be displayed as 225 by 300 pixels regardless of the screen size in inches.
Summary
Unlike printing, DPI as no affect on how an image is displayed on a computer monitor. When resizing an image for Web sites or e-mails it is important to resize the image based on the number of pixels, and not DPI.
Paul Salmon has been involved with computers for 20 years and currently works as a technical systems analyst. He has been involved in many aspects of computers including hardware, software development and quality assurance. He currently mantains Technically Easy, a blog that provides information about various technologies in use today.