Explaining "Image Resolution" and View> Print Size

Image resolution as pixel size
The term "resolution" crops up in a number of different ways. There's image resolution, printer resolution, monitor resolution, scanner resolution.and we won't even mention your last New Year's Resolution.

Open an image, any image, in Photoshop. In Photoshop's Toolbox, double-click on the Zoom tool icon. That zooms your image to 100%. Now open the Image> Image Size dialog box. Uncheck the Resample box. Note that the upper part of the dialog box is grayed out -- any changes made will not alter the number of pixels in the image; only the Document Size fields are available. (Prior to Photoshop 6, that area of the dialog box was more appropriately named "Print Size.")

Double-click the Resolution field and enter a new value, any value, then click OK. Notice any difference in the image? Nope! None. No change at all. Photoshop is still working exclusively with the pixel dimensions. The actual content of the image file is unchanged when you change the resolution without checking the Image Size feature's Resample box.

Here's the deal:
--Each image consists of a given number of pixels. (Multiply the width in pixels by the height in pixels to get the exact number.)
--In Photoshop, you work only with those pixels.
--In Photoshop and on-screen, each pixel remains the same size, regardless of the image resolution.
--Image resolution determines the size of each pixel on the printed page.
--The size of the individual pixels determines the area on the printed page that the image will occupy. (Remember that the image consists of a set number of pixels.)

At 300 ppi (pixels per inch), each of the image's pixels will print at 1/300th of an inch square. At 150 ppi, each printed pixel will be 1/150th of an inch. At 72 ppi, each pixel prints at 1/72nd of an inch. An image that's 900 pixels wide and 1200 pixels high prints at a different size for each resolution.

It's that simple: Image resolution determines the size of each pixel when printed, and the number of pixels in the image determines the actual size of the image on the printed page.

The View> Print Size command
Photoshop's View> Print Size command purports to display an image on-screen at the actual print size. However, the command can't do that for one specific reason: Photoshop doesn't know your monitor size, nor the resolution to which it's set.

When the View> Print Size command is used, the image's size on screen depends on the size of the monitor and the resolution to which it's set. In this photo, you can see that the print size of the image in the Image Size dialog box doesn't match the physical ruler held to the screen.

In the above example, the monitor is set to a resolution of 1280 pixels by 1024 pixels. Compare that to the photo below, using the same image, the same ruler, and the same monitor, but with the monitor set to a resolution of 1024 pixels by 768 pixels.

Here's a simple test to see how much the Print Size view differs from the actual on-screen display of your monitor:
--Open any image in Photoshop.
--Use the menu command View> Print Size.
--Press Command-R (Mac) or Control-R (Windows) to show the Rulers.
--If the Rulers are not already set to Inches as the unit of measure, right-click (multi-button mouse) or Control-click (single button mouse) on one of the Rulers and select Inches from the contextual menu.
--Hold a standard ruler or tape measure near the screen (being careful not to scratch the screen) and compare the "real" ruler to Photoshop's Ruler.

It's easy to change the resolution of your monitor, should you so desire:
. Mac: Go to the blue Apple menu in the upper-left corner of the screen and select System Preferences. Double-click on Displays. Make the change in the Display tab.
.Windows: Go to the Start menu and select Control Panel. Double-click Display. Make the change in the Settings tab.