Introducing the Image Size Dialog Box | Photoshop CS For Dummies

As mentioned earlier in this chapter, you have three image attributes — size, resolution, and dimension — all vying for your attention and all affecting each other. These attributes, in fact, are like three points on a triangle. Change any one of the points, and at least one of the others has to change proportionately. If you decrease the file size (number of pixels), for example, either the physical dimensions (printed size) or resolution (number of pixels per inch) must also decrease. If you want to increase the physical dimensions, you have to increase the file size — add pixels, in other words — or decrease the resolution.

Avoiding mistakes with the Image Size dialog box

Thinking about all the possible permutations can drive you crazy, and besides, they aren’t the least bit important. What is important is that you understand what you can accomplish with the Image Size dialog box (refer to Figure 4-3) and that you know how to avoid mistakes.

Remember

So, now that I’ve provided all the background you need, it’s finally time for me to offer a modicum of fatherly admonitions and avuncular advice:

Admonition: Don’t change the file size! Changing the Pixel Dimensions (file size) values can be disastrous, as I explain in the section, “Changing pixel dimensions.”

Advice: To avoid changes to file size, deselect the Resample Image check box at the bottom of the Image Size dialog box. When you deselect the option, the Image Size dialog box changes, and the Width and Height options in the Pixel Dimensions portion of the dialog box become unavailable to you. A link icon also connects the Document Size portion’s Width, Height, and Resolution text boxes, showing that changes to one value affect the other two values as well.
More advice: You want the image to look the same on-screen after you get done fiddling around with the Image Size command. If your image looks different on-screen after you change the Resolution or Width and Height values and exit the Image Size dialog box, you’ve done something to change the file size.
Admonition: Don’t resample unless you have to! Turn off the Constrain Proportions check box by checking the Resample Image check box. If you make changes to the image Width and Height values, Photoshop resamples (adds pixels by averaging pre-existing pixels) the image.

Advice: If you have to resample, resample down. If you lower the Width and Height values, you’ll probably be okay. But if you try to raise the Width and Height values, you’re likely to muck things up.

Admonition: Don’t muck up your image! Above all, make sure that none of the changes you make permanently affect your image.

Tip

Advice: You can undo just about anything bad you do. If you manage to mess up everything and change one or more settings in the Image Size dialog box to settings that you don’t want to apply, you can return to the original settings by Alt+clicking (Option+clicking on a Mac) the Cancel button. Pressing Alt (Option on a Mac) changes the word Cancel to Reset; clicking it resets the options. Now you have your original settings back in place so that you can muck them up again. If you already pressed Enter (Return on a Mac) to exit the Image Size dialog box, choose Edit→Undo or press Ctrl+Z (z+Z on a Mac) right away to undo your changes.

Admonition: See, I told you not to muck up your image! If you performed another action after you erroneously resized, you will find, to your dismay, that you can’t undo the resize by pressing Ctrl+Z (z+Z on a Mac). That command lets you undo only your very last action.
Advice: Don’t panic. You can undo your mistakes by using the magnificent and powerful History palette (see Chapter 14).
More advice: Whatever you do, be sure to use the Bicubic setting in the Resample Image drop-down list box. I’d tell you what bicubic means, but you don’t want to know. Suffice it to say that it keeps Photoshop running smoothly.
And still more advice: If you want to change the unit of measure that displays by default in the Image Size dialog box drop-down list boxes, choose Edit→Preferences→Units & Rulers (Photoshop→Preferences→ Units & Rulers on a Mac) and select a different option from the Units drop-down list box.

Remember

You may think that changing the image size is something that you never want to do. But you may, in fact, want to reduce the image size on some occasions — to get the image to print at a certain size, to enable your computer to handle the image, or to allow the image to display on the Web properly. The following steps show you how to reduce your image size.

Using the Image Size dialog box

The Image Size dialog box is used to change the dimensions of an image and set image resolution. Use the following steps to resize an image and set the image resolution for your needs. For instance, if you are creating a Web graphic and the image file is set at 300 ppi, you’ll use this dialog box to set the image resolution down to 96 ppi.

Before you follow these steps, choose File→Save As to save a backup copy of your image. The steps result in Photoshop tossing away pixels, and when you delete pixels you can’t get them back after you close the file. (Though you can always undo the action or use the History palette to revert to an earlier history state while the image is open. Check out Chapter 14 for more about the History palette.)

To resize an image and set the image resolution, follow these steps:

Open the image at the highest resolution possible.

For example, if you’re opening a Photo CD image, select the 2048 x 3072 option from the Resolution drop-down list box.

If you are opening a non-photo CD image, such as a scanned image, you don’t have a choice in resolutions.
Choose Image→Image Size to open the Image Size dialog box.
Note the values in the Pixel Dimensions Width and Height text boxes.

You may want to write ’em down — they’re important.
Make sure that the Resample Image check box is selected.
Enter your desired print width and height in the Document Size text boxes.

If you want Photoshop to retain the original proportions of your image, make sure that the Constrain Proportions box is checked.
Enter your desired resolution in the Resolution text box.

Check the sidebar, “Yeah, okay, but what resolution should I use?” earlier in this chapter for some suggestions on acceptable resolution values if you need help.
Check the Pixel Dimensions values.

Did either of the values get bigger? If so, you need to reduce your Output Size Width and Height values or lower the resolution. Otherwise, Photoshop adds pixels to your image, and you probably won’t be happy with the results (though minute increases may not change the way the image looks).

If the Pixel Dimensions values got smaller, on the other hand, proceed to Step 8.
Make sure that the Bicubic option is selected in the Resample Image drop-down list box.
Click on the OK button.

Photoshop resizes — or, if you prefer, resamples — your image in accordance with your perfect settings. If you don’t like the results, press Ctrl+Z (z+Z on a Mac) or choose Edit→Undo immediately to put things back to the way they were. Again, if you perform another action after resizing, the History palette (see Chapter 14) is available for undoing.