Accurate screen resolution vs web

Question

I have an aluminum 23in ACD (4years old) on a G5. Display Preferences have resolution set to 1920x1200 which looks correct and seems optimum. When I make a graphic that is 5inches x 3.5 inches in photoslop and view it at 100%, it is about 30% smaller than it accurately should be. In other words, if I take an actual ruler and measure on the screen, my 5inch wide graphic is 3.5inches. Why is this? And what does the web see that size as?
If I make the item 5x3.5 at 96ppi, its screen size is pretty much accurate, but I am told that it should be made at 72ppi.
Somewhat confused here.
Any comments appreciated, other than the usual "you ignorant...!!$#%..."
Message was edited by: Eric Gordon1

Answer 1

I'd say the 72 ppi standard is pretty much out the window at this time in history with respect to the web. When the original Macintosh came out with its 9-inch diagonal, black and white screen with 512 x 384 resolution, Apple went through pains to make sure it conformed to the typesetting standard of 72 ppi by making the screen 72 pixels/dots per inch. Thus WYSIWYG was born on the Mac whereby printed output on the 72 ppi dot matrix printer came out at the same size as elements on the screen. Everything matched on this original system.
Apple held onto 72 dpi screens for a while but the wheels started to come off the cart with the advent of notebook screens where there was a desire to have more on-screen pixels but the screen size was limited by cost and processing limits of the day. So 80 dpi soon showed up on Apple screens and up and up it has crept to the point where we now have a 17" MacBook Pro screen with 1920 x 1200 resolution at 133 dpi, nearly double the original 72 dpi.
Desktop publishing software applications have strongly adhered to the 72 ppi standard that is a part of their DNA which isn't going to go away. But the fact of the matter is that with screen resolution being all over the map, and resolution independence of displayed computer output still an unfulfilled promise, you can never hope your audience of viewers will all see what you have intended with your webpage at exactly the same size.
At this point in time, I'd say 96ppi is much closer to the average computer monitor display than 72ppi, so you are probably better off standardizing on that for web content meant primarily to be viewed on screen.