DPI is completely irrelevant until you instruct a printer. All that matters is the number of pixels in the image. If you have a 1,000-pixel square image, that's a million pixels. Tell it to print at 100 pixels per inch, it gives you a 10-inch square print, but there's still a million pixels in the image itself. Tell it to print at 500 pixels per inch, that's a 2-inch square print and it's STILL a million pixels in the image itself.
When you go to the image resize dialogue box, if you uncheck the "resample image" box and change the dpi value, you will see the image dimensions change right in that box. But nothing is ACTUALLY changing in the image itself. All the pixels are still there.
But suppose you have an image that's 3,000 pixels on the long dimension, and you have a printer that prints at 300 dpi, and you want to print a 3-inch print. If you change the print size to 3 inches without resampling the image, it will be using all 3,000 pixels, effectively 1,000 ppi, and the printer can't use all that information. Many times the print willa ctually be WORSE because of the excess information. And the file size is larger, so it takes the print spooler longer to load the image up, and so forth and so on.
So you make new copy of the image, resampling to 300 dpi and 3 inches print dimension, and this is optimized for that size and that printer.
This is a gross oversimplification, but that's what's going on. Pay no heed to how many "dpi" the files are "displaying" at, it's completely irrelevant until you start sizing for print.
R.
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