Hey gang!
Earlier, a discussion came up in the Birds Talk section about ppi and how it relates to cropping and preparing an image to post on the Web, specifically to how it relates to a 100% crop. There was some disagreement, as does happen, and a point was made that 72 ppi does come up as an "internet standard" and that also the Photoshop default for your "Display Resolution" is 72 ppi.
OK, then! We've had so many discussions, and have repeatedly told people that "ppi doesn't matter for Web posting", but that second point about the Display Preferences tweaked my interest, especially as to whether there was any effect on the Actual Pixels view (I suspected not) compared to the Print Size view (I knew there was an effect).
I didn't want to get the thread itself tangled up in a distracting topic (about cropping bird photos), so I decided to post here where it would be, well, more appropriate and hopefully a bit educational and maybe entertaining:
Here's a little test I ran -- it was in a copy of Elements on my laptop, but the same "stuff" was there.
I loaded a pic that has pixel dimensions of 900x600.
I set the Photoshop Display Preferences to the default of 72ppi (Edit/Preferences->Units and Rulers).
I then set the Image/Image Size resolution to 72 ppi. I didn't click to resample the image, so the 900x600 has a "print size" (in inches) of 12.5x8.33 inches. So, two things you would expect: you would expect the View/Print Size to show an image of about 12x8 inches and then if you View/Actual Pixels the same image should be shown.
Well, the two views were in fact the same, but they were not about 12x8 inches. In fact, they are both quite close to 9x6 inches.
What this indicates is that because the Print Size view resolution is "mapped" to the ppi resolution set in the display Preferences resolution. So, that has the effect of doing a "100%" display of the image, although it was not an accurate size for a 72ppi image!
In fact, though, I noted that the actual size of the image at "Actual Pixels" as well as the Print Size being mapped to "100%" was just about 9x6 inches -- in other words, just about 100 ppi, the number that I take to be a close approximation of your typical monitor resolution.
So, I was inspired to set my Display Resolution to 100ppi!
And then, I went to View the Actual Pixels, and the image/size did not change -- in other words, the Actual Pixels view is an accurate 100% pixel view at the actual monitor resolution, not at the Photoshop Display Resolution setting!
Then, when I clicked to View the Print Size, guess what? The image size on screen actually came out to just about 12x8 inches! In other words, it confirmed that the monitor resolution is in fact quite close to 100 ppi and that there is a significant difference between viewing at the 100 ppi monitor resolution and the 72 ppi default PS display resolution "Preference"! The takeaway being, you are better off setting that Preference to an accurate figure if you want to view a "real" print size!
Then I played a bit more -- I set the Image Size ppi back to 100ppi, still keeping it at 900x600 pixels, and again, the Print Size view matched the Actual Pixels size (9x6 inches)!
So then, I set the display Preferences resolution to 300 ppi! Again, the Actual Pixels didn't change, but Photoshop turned the display dimensions of the Print Size into, well, multiplied those 100 pixels per inch to 300 Display pixels, so blew up the 9"x6" image to 27"x18" (too big for my monitor)!
Anyway, the thing to bear in mind: the Display Preferences Resolution does make a difference to how the Print Size is rendered, which can be meaningful...in fact, if you are, say, preparing a 100% crop or an image resized for the Web you can use it to get a "quick check" of your image size, assuming you have the image ppi set for the same as the PS Preferences ppi...
But for practical purposes I'd rather use the Actual Pixels view for Web posting. And then, if you want to use the Print Size to view the "actual" print size, make sure that Preferences Resolution is properly set to a real resolution value! 100ppi is quite close to a typical display and so can work, or you can get more "exact" by taking your monitor's horizontal or vertical resolution and dividing it by the physical inches/dimension of the display!
