I like to teach it this way:
Pixels have no size in mm or inches or whatever.
They are simply mathematical equations until a human comes along and wants to see them.
At this point the human being "OUTPUTS the image and determines the size of the pixels in the output medium. Tiny pixels give very fine pictures while big, blocky pixels give coarse images.
If I use use 10 DPI, then the pixels will be 0.1 inch square. If I use 100DPI then the pixels will be 0.01 inch square etc., You can make the pixels appear any size you want but the bigger they are and the closer you get, the worse your picture will look.
I do this experiment:
I project an image 1000 X 1000 pixels onto a screen and go to maximum magnification in PS. I then measure the projected pixels and they are around 1/2 an inch square. As I zoom out, and the picture gets smaller, so the pixels decrease in size until we can no longer see individual pixels. I then measure the image size and determine the pixel size by dividing by 1000. If the image is 10 inches long then the pixel size is 0.01 inches. so there will be 100 of them per inch.
It's taking a reverse look at resolution
Herschel
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