What is important in a digital image is the amount of information, that is the amount of pixels in width and height. The resolution determines the size of the image when it is printed.

Click here to download pdf-file Serie 1:X

Serie 1:X has the same amount of pixels in width and height and also the same document size ( in kb ) but the resolution is changed from 500 ppi to 36 ppi. When just the resolution is changed you just affect the size of the printout, the amount of information = number of pixels, is not changed. What really happens is that the pixels are spread over a larger area as resolution decreases. For this to work properly the option "resample image" must be unchecked so that "Image size" is greyed out. See the pictures of the dialogbox.

A calculation demonstrates how it works:

Image 1:5, the number of pixels in width is 510, the resolution is 100 ppi
510 / 100 = 5.1 inch = 12.95 cm, just as it says in the screenshot of the "Image size"-dialog

Click here to download pdf-file Serie 2:X

Serie 2:X has been made in a different way.

I started with a rather large image, 1955 pixels in width and 2383 pixels in height, the resolution is 400 ppi to start with. This time the resolution is changed BUT the option "resample image" is checked, then the resolution decreases at the same time as the number of pixels decreases.

The size of the printout is not affected! But the quality is! Though it can´t be seen until the resolution is very low, because of that a normal printer can´t use more than about 150 ppi anyway.

On a highresolution printer used to print films for platemaking it will make a more significant difference.

Compare image 2:1 and 2:3, the resolution is just half and so is the number of pixels in width, but still the images print at the same size.
The filesize on the harddrive changes faster because the number of pixels are half in both width and height, 3.3 Mb of 13.3 Mb is 25 %, because of that you should not use images bigger than needed, it will take a lot more harddrive space! If the image is double as big in width and height it will need 4 times the space on your harddrive!