Digital images for print and Web use
Working with digital images is THE most difficult area of desktop publishing to get one’s head around. It’s a very complex subject. If you’re struggling with it, it’s only natural -- everyone struggles with it at first. The subject comes with a steep learning curve.
The information below is intended for people who need to share digital images with designers, printers, or other people involved in publishing, but it's also great information for people beginning to learn about manipulating and editing images in applications such as Adobe Photoshop.
With digital images, the WHY is more important than the WHAT. The WHAT will be different for every single image you encounter. (The HOW is another matter entirely that takes a lot of hands-on experience to master.)
That said, there’s an easy rule of thumb for distinguishing images appropriate for screen use from those appropriate for print use:
Standard screen (Web) resolution is 72 ppi (pixels per inch).*
Standard print resolution is 300 ppi.**
*While you can technically post images of any resolution on the Web, the reason why 72 ppi is standard is because a) the computer monitors with the lowest resolution (Mac) are or were at one time 72 ppi and b) using such a low resolution keeps file sizes small, which is significant if not important because larger files take longer to download (appear onscreen when a Web page is loaded).
**Print resolution is standardized at 300 ppi not necessarily because it’s an optimum resolution for all images, but because it’s the lowest resolution, generally speaking, that one can use before an image begins to degrade in quality. Prepress and printing companies are hard pressed to accept images with a resolution lower than 300 dpi because they know lower-resolution images won’t print well. This is the rule for FINAL images handed off to printing companies, but it’s a fair rule of thumb for sharing images, as well.
Resolution, however, is only part of the story.
When you’re sharing images with designers or desktop publishers for use both online and in print, the best thing to do is to send images AS CLOSE TO ORIGINAL AS POSSIBLE. The best, most ideal images in hardcopy are the hardcopies themselves, which designers/desktop publishers can scan to their specifications. The best, most ideal digital images are copies of the files that came straight off the camera. With digital images, that frequently means that an image could contain a resolution of 72 ppi, but the dimensions could be enormous; for example, 15” x 24” -- much too large to use in the layout of an 8.5” x 11” page with text, etc. Because the image needs to be reduced in size, the resolution can be increased at the same time by a professional without harming the image. It’s not difficult to reduce the sizes of images without losing clarity.
Enlarging images, however, comes at a cost. The lower the image’s resolution, the harder it is to enlarge. There’s a point at which an image begins to pixelate or break up into squares if it’s enlarged too much. To enlarge an image, you want to make sure that the image contains a high resolution (300+ ppi) to begin with and that you’re enlarging it at maximum to a point just before it begins to pixelate.
Compression also affects the integrity of an image. There are two types of compression: Lossless and lossy. Lossless compression reduces the size of an image’s file without loss of image data. Lossy compression reduces the size of an image’s file by discarding image data irrevocably. Somehow, JPG format has become a standard output format for digital cameras, but it is not an ideal format for source images because it uses lossy compression. With experience looking at umpteen compressed and uncompressed images, you can easily tell if an image has been saved with lossy compression because you will see what’s known as "artifacting" in areas of the image where two colors abut. Artifacting is a kind of visual “dirt” in the image — with high compression, it can appear as a variegated halo around the border of the darker color, but at lower resolutions, it can look like clumps of squares or individual squares of inappropriate colors pockmarking the image.
Artifacting can also be described as irreparable damage. In screen/Web use, it can be tolerated or even invisible because of the low resolution of monitors, but artifacting wreaks havoc on images in print. Print exaggerates artifacting and makes it more visible. There’s a tool in Photoshop that can be used to clean up a lot of artifacting, but it’s far from ideal and damages images in other ways.
This explanation is not even the tip of the iceberg, unfortunately, but it should give you a basis for figuring out what images you have and can share with designers and desktop publishers. I hope it helps. Feel free to ask me further questions.