I've been in the midst of reformatting all my paperbacks to sell on Amazon. The books are essentially the same except for the newer cover. However, when I import the original photos into InDesign, I sometimes had to stretch the photo to fit the frame.
This was the first time I got the message, that went something like, "IngramSpark has found problems with your images being lower than the recommended....."
"What?" I was outraged. How dare they say that my images are less than 300 PPI. That's outrageous. I'm a photographer, I'd never let my images get less than 300.
While that is absolutely true, IngramSpark looks at "effective" PPI when printing your book.
After scratching my head, googling stuff and playing with image sizes in photoshop, and scratching my head some more, I figured it out.
It doesn't matter how big your photo is. If you stretch it to fit a frame you are lowering the "effective" PPI. Imagine what happens when you stretch slime. You can still see it, but it gets thinner and you can start to see "through" it.
That's what happens to the image. You can see the effective PPI if you look at the "Links" window when you are on an image in InDesign.
I've attached an example. My image is at 300 PPI. If it's larger than the frame and it's shrunk to fit the frame, the PPI will stay 300 or go higher.
But if your frame is much bigger than the image, and for this example, I had to stretch it far, the effective PPI dropped to 168 but you can see that the image PPI remained at 300.
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