Image Resolution Terminology
Before getting into the resolution you need for your work, it’s essential to understand the language used. The first term is “dots per inch” (DPI). This measurement, sometimes called “pixels per inch” (PPI), is a standard way to describe the resolution of an image.
DPI and PPI are numerically equivalent (e.g., 300 DPI = 300 PPI). However, DPI refers to printing (Output or Print Resolution)—the number of ink dots printed on a page. PPI refers to digital devices (e.g., computer monitors, smartphone screens, image scanners, digital cameras), and is the Input or Image Resolution.
When a graphics requirement is “high resolution”, it means that there are more dots per inch than in an image with a low resolution. In lower-resolution images, the pixels are larger, which creates a more jagged-edged look.
Example: Suppose you have a 1″ x 1″ image with a resolution of 72 DPI. This image contains 72 x 72 = 5,184 dots (or pixels). If this same image had a resolution of 300 DPI, it would contain 300 x 300 = 90,000 dots (or pixels). The more dots or pixels in the 1″ x 1″ space means higher resolution or crispness. Some people recommend limiting image file size to about 125 KB (or about the same number of dots or pixels). This might be something like 350 DPI on a side in a square image. However, the human eye can’t resolve much beyond 300 DPI.
Computer screens are generally optimized for viewing at 72-100 PPI. That’s why images on the internet look sharp on your monitor but print poorly. Line art usually requires 600 DPI resolution for optimum quality.
The advantage of higher-resolution images is that they reproduce more detail and subtler color transitions than lower-resolution images. The DPI used should be done in the original photograph or scanned image. An image with 72 DPI can’t be printed at 300 DPI, unless the image is worked on with software. Trying to increase resolution electronically or digitally merely spreads the available information across more pixels. What’s done is called “interpolation”, in which dots or pixels are added to the image. This might look okay … but it might not.
Common Image File Types (alphabetically)
GIF or gif (Graphics Interchange Format) image files are compressed in order to reduce transfer time. These work well for line-art and clip-art in e-books.
JPGor jpg(Joint Photographic Experts Group or JPEG) images are optimized for photographs and similar continuous-tone images that contain many colors. It can achieve great compression ratios, while maintaining very high image quality. JPG images work well for photographs.
PNG or png (Portable Network Graphics) files are in a lossless storage format. In contrast with TIFFs, the software looks for patterns in the image that it can use to compress file size. The compression is reversible, so the image is recovered exactly. These work well for e-books and p-books.
TIFF or tif (Tagged Image File Format) software is used almost exclusively as a lossless image storage format that uses no compression. These are used for p-books.
Resolution for e-books
If your book is intended for viewing on a PC monitor, on a tablet, or on a cell phone, then an image resolution of 72 DPI is fine. If your image has too low a resolution, you may end up with what’s called “pixilation”, an image with large, coarse-looking pixels, jagged edges, and will look fuzzy. Images using JPG or GIF formats usually work well for e-books, where the compressed files are useful for transmission efficiency.
Resolution for p-books
If your book will be printed, then an image resolution of 300 DPI is required. That’s because the printing process allows for much greater detail. In fact, if the images don’t meet this requirement, it’s possible that a book printer may reject your project.
The preferred format for p-books is TIFF (or tif). It’s the highest resolution of all the different available graphics formats. However, because a TIF-file holds much more detail, the file size is larger. But the printed image will have cleaner, crisper definition. To make transmitting these files more efficient, they can be compressed using ZIP software.
If you’re planning to do both an e-book and a p-book, understanding this concept means that it makes sense to work on the p-book design first. The resolution can be reduced for the e-book. However, going the other way (having to add data to a picture) is not very effective.
Note: If you use too high a resolution, you’ll end up increasing the file size and slowing the printing of the image. What’s worse is that the device being used won’t likely be able to reproduce the extra detail contained in the higher-resolution image. Besides which, as noted above, the human eye can’t resolve much more than 300 DPI.
How to Know the DPI of Your Image
Here’s a way to determine the resolution of your picture. Go to your image in your File Manager. Right-mouse click the image. When you do, you’ll see something like the following box open:
Along the top of the window, you should find a “Details” tab. Scroll down to find the information about your “Image”. You can see its dimensions and resolution. I made a simple graphic in CorelDraw and exported it as a jpeg, both in 300 dpi and 72 dpi. Because you’re viewing it on an electronic device, they’ll look the same. The big difference comes when you print it. The word “HELLO” on the one with 72 dpi is going to look fuzzy. What I wanted you to see was both the difference in dimensions and resolution.
When I exported these two graphics out of CorelDraw, it showed that they both are about 7″ x 10″. Yet, the dimensions in that Detail window have changed because there is less information to share. They both will still print at 7″ x 10″, just different quality.
Hint: If you plan to print an image, don’t take it will your cell phone. Most provide you with 72 dpi. Get a good camera, most which default to 300 dpi.
If you’re planning to have graphics or images in your book, be sure to have files with enough resolution to print at the size you want in the finished product. Higher-quality (higher resolution) improve the look and feel of a book, while lower-quality (lower resolution) may take away from the quality of the content.
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