You've got several ways to retrace your steps, including the lifesaving Undo command—just choose Edit→Undo or press ⌘-Z (Ctrl+Z on a PC). Unfortunately, the Undo command lets you undo only the last edit you made.
If you need to go back more than one step, use the Step Backward command—choose Edit→Step Backward or press ⌘-Option-Z (Ctrl+Alt+Z on a PC). Out of the box, this command lets you undo the last 20 things you did, one at a time. If you want to go back even further, you can change that number by digging into Photoshop's preferences, as the next section explains. You can step forward through your edit history, too, by choosing Edit→Step Forward or ⌘-Shift-Z (Shift+Ctrl+Z on a PC).
If you think you might someday need to go back further than your last 20 steps, you can make Photoshop remember up to 1,000 steps by changing the program's preferences. Here's how:
Open the Preferences dialog box.
On a Mac, choose Photoshop→Preferences and then, in the resulting dialog box, click Performance in the left-hand column to see the history preferences.
On a PC, choose Edit→Preferences and, from the resulting submenu, choose Performance.
Look for the History States field in the upper right corner of the Preferences dialog box and then pick the number of Undo steps you want Photoshop to remember.
You can enter any number between 1 and 1,000. While increasing the number of history states might help you sleep better, doing so means Photoshop has to keep track of that many versions of your document, which requires memory and processing power. If you increase this setting and notice that the program is running like molasses, try lowering it.
The History panel is like your very own time machine. Whereas the Undo and Step Backward commands let you move back through changes one step at a time, the History panel (see Figure 1-10) kicks it up a notch and lets you jump back several steps at once. (You can step back through as many history states as you set in Photoshop's preferences.) The History panel is much quicker than undoing a long list of changes one by one, and it gives you a nice list of exactly what you've done to your image—in chronological order from top to bottom—letting you pinpoint the exact state you want to jump back to. You can also take snapshots of your image at various points in the editing process to make it easier to jump back to the state you want (as explained in a moment).
Figure 1-10. Top: The History panel keeps track of everything you do to your images, beginning with opening them. The tiny pointer (circled) lets you know which editing step you're on. You can also take snapshots of your image at crucial points during the editing process like when you convert it to black and white and add a color tint. Bottom: If you take a snapshot, you can revert to that state later with a single click. For example, if you've given your image a sepia (brown) tint and later changed it to blue, you can easily go back to the sepia version by clicking the snapshot you took, as shown here, without having to step back through all the other changes you made. What a timesaver!
After you make a few changes to your image, pop open the History panel by choosing Window→History to see a list of everything you've done to the image, including opening it. To jump back in time, click the step you want to go back to and Photoshop returns your image to the way it looked at that point. The tiny pointer in the panel's left column (circled in Figure 1-10, top) shows which step you selected (and the step itself is highlighted). If you stepped back further than you meant to, just pick a more recent step in the list.
If you'd like to see a thumbnail preview at the top of the History panel showing what your image looks like every time you save your document, choose History Options from the History panel's menu, and then turn on Automatically Create New Snapshot When Saving. Clicking the thumbnail is a fast and easy way to jump back to the last saved version of your document without having to close and reopen it.
You can also get back to the last saved version of your document by choosing File→Revert (Revert Command).
Taking snapshots of your image along the way lets you mark key points in the editing process. A snapshot is more than a preview of your image because it also includes all the edits you've made up to that point. Think of snapshots as milestones in your editing work: When you reach a critical point that you may want to return to, take a snapshot so you can easily get back to that particular version of your document. To take a snapshot, click the little camera icon at the bottom of the History panel (shown in Figure 1-10). Photoshop adds the snapshot to the top of the panel, just below the saved-state thumbnail. The snapshots you take appear in the list in the order you took them.
History states don't hang around forever: As soon as you close your document, they're history (ha!). If you think you'll ever want to return to an earlier version of your document, use the "Create new document from current state" button at the bottom of the History panel. That way, you've got a brand-new document to return to so you don't have to recreate it.
The History Brush takes the power of the History panel and lets you focus it on specific parts of your image. Instead of sending your entire image back in time, you can use this brush to paint your edits away selectively, revealing the previous state of your choosing. For example, you could blur the heck out of a portrait with the Gaussian Blur filter (Skin Softeners) and then use the History Brush to revert to the original, sharp version of your subject's face, as shown in Figure 1-11. Of course, you paint away other edits, too, like color changes, filter effects, or any other changes you made in between.
The Art History Brush works similarly, but it adds bizarre, stylized effects as it returns your image to a previous state, as shown in the box on The Art History Brush.
Figure 1-11. By using the History Brush set to your image's Open state—see step 4 on this page—you can undo all kinds of effects, including a severe Gaussian Blur.
Open an image—in this example, a photo of a person—and run the Gaussian Blur filter.
You'll learn all about opening images on Opening an Existing Document, but, for now, choose File→Open and then navigate to where the image lives on your computer. Then choose Filter→Blur→Gaussian Blur. (Chapter 15 tells you all about using filters.)
In the resulting dialog box, enter a radius of 20 and then click OK.
Depending on the size of your image, this setting blurs your image pretty severely, giving you a lot to undo with the History Brush.
Grab the History Brush by pressing Y and then choose a brush from the Options bar.
Once you've grabbed the History Brush, hop up to the Brush Preset picker in the Options bar and pick a large, soft-edged brush (one that's pretty big and blurry around the edges). You'll learn about brushes in Chapter 12.
Open the History panel (choose Window→History) and then click a saved state or snapshot.
This is where you pick which version of the image you want to go back to. Since all you've done is open the image and run the Gaussian Blur filter, choose the Open state. Just click within the panel's left-hand column next to that state to pick it (you'll see the History Brush's icon appear in the column).
Mouse over to your image and paint over the person's eyes to reveal the unblurred eyes of the original image.
If you keep painting in the same place, you'll expose more and more of the original image (it's a gradual change). For example, a quick swipe over the eyes reveals traces of the original while a good scrubbing back and forth in one area reveals the original in its full glory.
If you've taken your image down a path of craziness from which you can't rescue it by using Undo or the History panel, you can revert back to its most recent saved state by choosing File→Revert. This command opens the previously saved version of your image, giving you a quick escape route back to square one.