When you place a digital image into Adobe InDesign, the info palette displays a much higher actual ppi value (for example, 288) than the expected 72ppi resolution.
- Your image was captured at high resolution and with low compression. For example, Large format or high pixel values and fine quality.
- Your image was not edited and/or saved from Adobe Photoshop.
- Your image was saved at a resolution of 72ppi.
Solution 1: Use photo editing software to save the image at the desired size and resolution before placing the image into InDesign
Some digital cameras omit data that applications use to generate a preview for the image. In some cases, ommission of the preview data results in a preview that is saved at a ppi resolution of 72ppi. High resolution images typically contain millions of pixels. When those pixels are mapped to a low pixels-per-inch ratio, such as 72ppi, the physical dimensions of the image preview are quite large.
For example, if your image was captured at a pixel depth of 2560 wide by 1920 high and then those pixels are mapped to a ppi resolution of 72ppi, the physical dimensions of the image become 35.5556" wide by 26.6667" high.
When a high resolution image mapped to a low ppi resolution is placed into InDesign it will usually need to be resized. Resizing an image in InDesign does not resample the image. This means that no pixels are lost when the image is decreased in size after placement into InDesign. Since the image contains the same total number of pixels (eg: 2560 X 1920) when you reduce the size of the image each inch of image will contain more pixels.
For example, if you reduce your 35.5556x26.6667" image in InDesign by 50% so that it is 17.7778x13.333", the number of pixels per inch doubles to 144ppi. If you reduce your orignal image to 25% of its original size the image dimensions will be 8.8889 X 6.6667 and the ppi resolution will be 288ppi.