David,
The reason both images appear the same on your screen (and would on a Web page) is that the screen displays in pixels, not inches. 500 pixels takes up whatever proportion of your screen, depending on the screen resolution. Mine is set at 1024 x 768 so, horizontally, the 500 pixel wide Canvas takes up about 48% of the screen width. Vertically, it takes up about 65% of the screen height.
However, if you were to print these two images, here's the size you'd see (as long as you didn't tell your printer to "Fit to Page", or stretch the image to fill the paper's printable area):
500 x 500 pixels at 72 ppi (**500 divided by 72**) = roughly 6.94 x 6.94 inches
500 x 500 pixels at 300 ppi (**500 divided by 300**) = roughly 1.67 x 1.67 inches
You're right about the unfair comparison. Sorry David, I'm the one who needs slow down and think more. I did another test and instead of selecting the Shape in the 72 ppi image I dropped the Shape then selected the whole Canvas (to eliminate the anti-aliasing problem), copied it, and pasted it onto the 300 ppi Canvas as a Layer. Then I zoomed in to 500% and toggled the Layer off and on to compare the edges. They appeared to be the same just as you say they were.
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What does make a difference, in size as seen on the screen rather than resolution, is if you create the drawing on, for instance, a 300 x 300 pixel 72 ppi Canvas and paste it onto a 600 x 600 pixel 72 ppi Canvas. The pasted image covers one fourth of the larger Canvas.
600 x 600 pixels at 72 ppi would print 8.33 inches square
300 x 300 pixels at 72 ppi would print 4.17 inches square
The image that's larger on screen is still larger when printed because the ppi is the same.
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600 x 600 pixels at 300 ppi would print 2.00 inches square
300 x 300 pixels at 72 ppi, would print 4.17 inches square
Now the image that's larger on screen is smaller when printed because each inch gobbles up more pixels. The image that's smaller on screen is larger when printed because each inch takes up fewer pixels (it takes more inches to gobble up all those pixels).
Phew!
This is why it's best to experiment. There's nothing like experience to make things sink in. Intuition helps too.. but trying to make sense of it intellectually ain't so easy and it's hard to explain right, too. Thanks for giving me the practice and letting me know I was leading you down the wrong path. (Hope that doesn't happen too often!)
;
Jinny Brown
http://www.pixelalley.com
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