If you’re an author, you should be comfortable with words, but you’re probably less familiar with images, especially technical issues such as resolution and DPI (dots per inch). For most authors, this isn’t a problem most of the time. But if you’re dealing with images in print books or on merchandise, you may come across a phrase such as “Images must have a resolution of at least 300dpi”. This article will help you understand these terms and their importance, so that you can select suitable images for print books and printed items such as bookmarks.
Digital Image Sizes
Digital images are made up of pixels, and so their size is given as pixels wide and pixels tall. By zooming in, you can see the individual pixels that make up the image, and instead of looking smooth, edges start to look rough and jagged.
See the images above. The first one is the image viewed normally, the second one shows the top of the letter “e”, zoomed in so that individual pixels start to become visible.
When printing, each pixel in the digital image becomes a dot of ink on the page, hence the term “dots per inch”. A lower resolution image has a smaller number of dots to distribute, and so has less detail. When printing images that are to be viewed at a close distance, such as book covers and bookmarks, the resolution should be at least 300dpi. In other words, there should be 300 pixels for every inch in the printed image.
As an example, an image that is 300 pixels wide and 600 pixels tall, printed at a size of one inch wide and two inches tall, would look good. In this example there are 300 pixels for every inch of printed image. If it were printed at two inches wide and four inches tall, the 300 pixels of width would be spread over two inches, so each inch would only have 150 pixels. Viewed at a typical reading distance, the image would appear indistinct and low quality. If you’re printing something like a poster that will be viewed from a greater distance, you may be able to use a lower resolution, but your printer will be able to advise on this.
Taking book covers as another example, a common size is six inches wide and nine inches tall. For the cover image to be 300dpi, it needs to be (6 x 300) pixels wide and (9 x 300) pixels tall. This equates to 1,800 pixels wide and 2,700 pixels tall. If the image was 600 pixels wide and 900 pixels tall, it would have a resolution of only 100dpi when printed at 6” x 9”.
It may seem like a simple matter to take a small image and resize it to make it large enough to print at high resolution. Image editing software makes this easy, but won’t always give good results. A small image simply doesn’t have the same amount of information as a larger image. When resizing in this way, the software adds pixels in a process called interpolation, but the results are often of poor quality.
On the other hand, if you have a large file such as a print book cover, you might want to resize it to a smaller version for posting on the web. This works well, but make sure that you use “Save as” to save the small version as a new file. It’s important to keep the large original because you can’t use the small version to recreate the large one.
Also, note that JPEG (.jpg or .jpeg) files are lossy. Every time you save a file in this format, some information is lost and cannot be restored. For this reason, always keep the original file and work from that.
My thanks to Henry Hyde for checking the technical details, and Glory Ralston for checking the advice was understandable. Any remaining issues are my responsibility.