Resolution is meaningless in an image until that image is given a physical size (on the web, images are measured in pixels and the size of a pixel on your monitor depends on the physical size and resolution of the monitor). When you save an image in an editing program like Photoshop you assign a physical size and resolution, which is the physical size you choose divided by the number of pixels in that dimension. You can cange either the physical size, or the resolution, without resampling, and the other value changes in inverse proportion, but the actual pixesl in the image are not affected (and the appearance in Photoshop willnot change at all).
InDesign reads the the physical size and resolution information from the saved file when you place it, and reports that resoution under the link info as "Actual PPI." If you click and release to place the image, unless it is extremely large at a low saved resolution, ID will use the physical dimension saved in the file to place it at "100%." Very rarely ID will scale an image on placing this way becasue the physical dimensions are too large, or no information was recorded in the image -- typically this happens with images straight from the camera.
The Actual PPI figure is actually of no importance. Remember, scaling an image in ID does not alter the pixel count, it only changes the realtive size of how those pixels will be rendered ON A PAGE, like inflating or deflating a printed balloon. The resolution of the image as it is scaled in ID is callled the effective resolution and is also listed in the link info as "Effective PPI." this number is the one that you need to watch.
Different output methods and viewing distances require different effective resolutions. A 300 ppi image on a book page viewed from 100 yards wouldn't even look like a single dot, but you can read a billboard with a resolution well below 1 ppi because you are very far away. there's a pretty good discussion of this at Distance-Resolution Formula.
Presuming, though, that you are making layouts for viewing at aor around arm's length, there are some "rules of thumb" that you can use to determine if the image will output in a satisfactory way. The first is the resolution should be 2 x lpi in your half-tone screen. Since most commercial printing was at 150 lpi when that rule evolved, it give you the standard 300 ppi value you see used in many places. You can, in fact use lower resolution, down to about 1.4 x lpi, without much degradion for most images, and images without a lot of detail, especially on uncoated paper, can go even lower in a pinch. Desktop printers typically output in the range of 100 lpi, so it's usually safe to stay in the range of 180 - 200 ppi effective resolution if you paln to print at home.
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