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In addition to the aesthetic requirements of the cover image, we also need to consider its technical spec. When preparing a cover for print, never underestimate the usefulness of the telephone. Your commercial printer should be happy to give you advice on the technical requirements of your cover. The Resolution of the cover image should be at least 300 ppi, or Pixels Per Inch. This is the standard resolution for commercial printing. Sometimes you can get away with less, but your cover file may fail the printer's flight-checking program.
Some magazines, depending on how they are printed and on what type of paper stock, may call for a higher resolution, such as 350 ppi. You can check the image's dimensions in pixels using Bridge. It tells me here its Dimensions. This is the most important piece of information here, far more important than this. We want the image resolution to be 300 pixels per inch, but it is the absolute dimensions of the image that tell us its potential.
If we divide these numbers by 300, that gives us our potential image size, and that means that this image over here, even though its Resolution is currently 72 pixels per inch, it has enough pixels in it so that were we to change the resolution to 300, we would still have a document size big enough for a cover. Let's see what I mean by that. Here we're using Photoshop, and I'm going to go to the Image menu and to Image Size, where we see that the Resolution is 72.
It's important that when I do this, I have Resample Image unchecked so that the absolute number of pixels--the Pixel Dimensions--remain the same. You can see that if I change the image Resolution to 300, all that changes is the Document Size. Same number of pixels, same file size. When I click OK, file size remains the same, view size remains the same. With Resample Image turned off ,you can think of the document size/image resolution relationship as being like a see-saw. As one goes up, so the other goes down.
What you can't do and expect a good result is to work with Resample Image turned on and then either increase the. document size or the image resolution Doing that you'll be upsampling the image, adding pixels. Photoshop will let you do it, but the result would never be as good as if you had started with an original image that had the right number of pixels in it. While it's possible to split hairs about the difference between PPI, Pixels Per Inch, and DPI, Dots Per Inch, they are used interchangeably, and they are essentially the same thing.
If you need to evaluate the image's resolution in InDesign you can do this using two tools, either the Links panel--with the image selected in the Link Info it will tell me the image's Actual PPI and its Effective PPI. We see the same information on the Info panel, and if you don't have the Info panel open, you can get it from under the Window menu. The difference between the two, the Actual PPI is the resolution at the image's original document size.
The Effective PPI--which is by far the more important of the two numbers--is the image's resolution after any scaling has been performed. If the image has been scaled up, the Effective PPI will be smaller than the actual, and if the image has been scaled down, the Effective PPI will be larger then the actual. In this case, because the image is at less than 100%, the Effective PPI is larger then the Actual PPI.
All you really need to know is that this number needs to be at least 300 pixels per inch. You'll note that the Color Space of this image is RGB. It is intentionally RGB because I'm using a Color-Managed Workflow, and I'll say more about that in upcoming movies.
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