What is so Special about 300?
Most designers I have talked to about resolution have been told that they are supposed to scan everything at 300dpi, but they do not really know why. In fact, there is a formula that can be applied to any project which shows that 300dpi is not, in fact, the magic number for everything, all the time.
There are two factors involved in determining how much of the original detail an image can hold. One is the dpi, which determines how many pixels there are per inch of image. The other is the physical size of the image as it will appear on the page. If you have only one of these pieces of information then, excuse the pun, you do not have the whole picture.
For example, I am often asked to send images here and there, and told that they are needed at something like “roughly 10 x 12 inches.” As I explained in Chapter 7, the file size of a 10 x 12in image at 72dpi is very different from a 10 x 12in image at 300dpi. Unless I am told the resolution as well as the size, I have no way of knowing exactly what is required. Sometimes Photoshop is not available to the person receiving the images, so if I send something that is seventeen times bigger than needed —and perhaps in the wrong color mode, too—they have no way of dealing with it themselves. Also, as I do not like to send images out without the appropriate calibration, it is a great help to be told what kind of process will be used for final output.
If the final output method will be offset litho, we have to look a little closer to figure out what the dpi value really needs to be. What kind of paper is being used? If it is an uncoated paper, then the line-screen value of the halftones probably does not need to be higher than 133lpi (Note: that is “lpi”—lines per inch—not “dpi”), otherwise detail will be lost as the dots spread into each other. If, however, you are printing on a coated paper you can usually go straight up to 150lpi. Some magazine covers go as high as 175 or even 200lpi, but this is less common. The smaller the dots are, the more fiddly they are to print, and the more often the printer has to stop the presses to clean things up. So, values higher than 150lpi are used only if they are considered necessary for the quality of the result. In terms of visual effectiveness, most of us pretty much cannot see the dots at 150lpi, anyway.
The rule of thumb is to double the line-screen value that you are going to end up with and use that as the dpi at which you need the image. This is where the magic number of 300dpi comes from—it is twice the line-screen value of a halftone intended for printing on coated paper. If you have the image at that resolution, then—so long as the physical size is large enough—you are covered whether you end up printing on coated or uncoated paper, because the actual line-screen of an image is applied by the imagesetter printing the film. So it does not really matter if you have the image at a slightly higher resolution than you eventually need. All it means is that you merely have a little more detail available within the digital image than you can actually reproduce. It will not be a problem. The problem is when you have an image at a resolution that is too low, and therefore you pick up less detail than you could potentially print.
Once you have decided on the correct resolution, you need to deal with the size. Of course, it is quite likely that you will not know at the outset what size you want a particular image to be. This happens to me all the time. And, if I put very many high-resolution CMYK images into a page layout, things start to slow down. If the project is a book, it may eventually mean that I have to sit and wait for the screen to redraw every time I do anything at all, which is a serious state of affairs when a deadline looms. Fortunately, there is a solution.