The Extra-Strength Tips for the Clients and Friends of Ben Willmore (www.digitalmastery.com):

It just dawned on me that we've never really talked about Photoshop's History palette (Window>History). This is one plum of a feature and especially invaluable to anyone who ever said the word, "oops!" For now, we'll start with some of the basics and then get more in-depth with future tips.

The History palette should also be known as the Undo's palette because that's what most people use it for. It simply lists the undo-able steps that you've performed since opening your image. With default settings, it will keep track of only 20 steps. If you were to perform 30 steps on your image, then Photoshop would remember how to undo just the last 20. Each one of these steps is known as a History State, simply meaning a previous state that your image was in that you can easily get back to.

You can change how many History States Photoshop keeps track of (and therefore how many Undo's you can use) by changing the History States setting in the Photoshop>Preferences>General dialog box (preferences will be under the File or Edit menu in other operating systems). In Photoshop 7, you can set it as high as 1000! I don't suggest you do that though. Photoshop has to keep track of all those previous states somewhere and it does it in a hidden file on your hard drive that's known as the Scratch Disk. This file is created when you launch Photoshop and is deleted when you quit the program. The more undo's you want to Photoshop keep track of, the larger that file has the potential of getting. If it ever fills up all the empty space on your drive, then you'll get a horrid message that says 'Could not complete your request because the primary scratch disk is full'. That simply means that it doesn't have space for any more undo's, so it won't let you do anymore to your image.

If you have more than one hard drive connected to your computer, then you'll want to choose Photoshop>Preferences>Plug-Ins & Scratch Disks and set the First Scratch Disk pop-up menu to the name of your fastest drive and the Second pop-up menu to the name of the second drive. That will let Photoshop use the second drive once it has filled the first and will delay that dreaded Scratch Disks Full message.

But, let's say that you've set your History States preference at a reasonable number (maybe 50), you've set it up to use all your drives and you still get that message. Well, that's when you might want to consider choosing Edit>Purge>History. That clears out the History palette, effectively causing Photoshop to forget the steps you used to get to the state you document is in (just like closing and re-opening the file). Once you've eliminated those past history states, Photoshop will once again start keeping track of the steps you perform from that point on. If you purge your history while you have multiple documents open, then Photoshop will clear the History for all open documents. But if you hold Option (Mac), or Alt (Win), then it will only clear the History for the document that you're currently working on.

Now that you have an idea of what's going on behind the scenes, let's start using these History States/Undos. I find that most people are used to typing Command-Z (Mac), or Ctrl-Z (Win) to undo the last thing they did (let's face it, we're just used to screwing up). If you type that key command a second time, then Photoshop will simply reapply (redo) whatever it is that you just undid. If you'd rather go back multiple steps, then add the Option key (Mac), or Alt key (Win) to the standard undo key command. Pressing that multiple times will allow you do get multiple undos! Then, if you undo too many steps, try Shift-Command-Z (Mac), or Shift-Ctrl-Z (Win) to reapply (redo) a step that you just undid. I know it's a tongue twister, but I've no doubt that you're crystal clear on this.

Or, if you hate keyboard commands you can just open the History Palette (Window>History), which will list the steps you've performed to get your image to its current state. Steps that appear in gray are ones that you undid. The highlighted step is the current state of your image. You can just click on different History States to effectively undo or reapply multiple steps.

The top-most step in the History palette represents what your image looked like when you first opened it. It's the only one that has a preview image (known as a snapshot). Click on that step anytime you want to get back to your original image (it's about the same as choosing File>Revert).

I think this should be enough to get you off and running. As you might imagine, there is a great deal more to the History palette, but I'll keep some of those ideas for future tips.

-Ben Willmore