Image sizing problem

AM
Posted By
Anthony_Manno
Oct 22, 2006
Views
279
Replies
6
Status
Closed
When I open an image that is 2048 X 3072 pixels and 8.5 x 12.8 inches and 240 dpi and then resize it with the image size window to be 4×6 inches but keep the dpi at 240 dpi the physical dimensions as measured by the PS rulers and as displayed by Windows default picture viewer show the dimension to be 12×15 inches. When I change it to 72dpi then the dimensions really do become 4×6 inches.

In fact when I look at the image when I first bring it up PS says that it is 8.5 x 12.8 inches, but it really is three times larger than that unless I change the dpi to 72.

So, I am figuring that there is something that I don’t understand here. Isn’t the definition of dpi or dots per inch suppose to be how many dots every inch? That means that if I choose to put more dots for every inch then the image has more information and detail in every inch. Why would the amount of inches change as I change the dpi? Or am I missing some kind of fundamental thing with dpi?
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EH
Ed_Hannigan
Oct 22, 2006
Inches have little meaning inside a computer. Pixels dimensions are what’s important until you print.

The dpi number (actually should be ppi, pixels per inch) is misleading. If you increase the resolution without Resampling the "physical" (inches) dimensions will shrink, but there will be the same number of pixels. If you have Resampling checked you will add pixels but NOT detail.

You absolutely cannot add detail that was not there to begin with. Photoshop can make "educated guesses" on what the new pixels should be but it cannot add what was never there.

Take an image and do some resizing with Resampling checked and unchecked until you get a handle on how it works.
AM
Anthony_Manno
Oct 22, 2006
Thanks for your response. I figured that the dots were pixels but I wasn’t sure. I was struggling for a good way to explain what I meant without using pixels but I guess I picked the wrong terminology by using detail since I realize that you can not "add detail that was not there to begin with". That was a mistake.

I just rediscovered the "Print size" option for zooming. I am guessing that it is shrinking the amount of pixels so that the image will fit in the new size. All those different sizes can get confusing, especially when I don’t print much. I just display it on the screen and post it on the net. But, it still gets confusing that when I choose a smaller size for image size it looks smaller when displayed with PS and Windows picture viewer, but that is different than what the print version is. I guess you have to remember that they look different.
Y
YrbkMgr
Oct 22, 2006
Great site to get your head around the issues is here:

<http://www.scantips.com>
CP
Charles_Potter
Oct 23, 2006
I want to change pixel size in all JPG images in a file to a standard size and rebuild file.
EH
Ed_Hannigan
Oct 23, 2006
Charles,

Why?
RG
Rags Gardner
Oct 23, 2006
Charles,

There is no standard pixel size for images displayed on a monitor, the usual target for JPG images. It depends on both the current screen resolution settings and the physical dimensions of the screen. Both 72 PPI and 96 PPI are common JPG defaults. But few applications are aware of the true screen dimensions.

Cheers, Rags 🙂

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