The history brush and palette are some of Photoshop's most powerful and least understood tools. The history palette can be found as a tab located next inside the actions palette window. If it is not present, it can be added (Window → History). In its simplest form, the history palette acts as a record of the image adjustment process. Each time an adjustment is made, it is documented on the history palette. By referring to the adjustment history, I am able to review the steps taken to produce a particular result. This can be very helpful when experimenting with new techniques.

While documentation is useful, the history palette offers additional controls to the digital photographer. Chief among these is the ability to undo mistakes. As image adjustments are made, the history palette goes beyond simply recording the name of the adjustment. Photoshop actually saves a pre-adjusted version of the image each time a change is made. These saved images are called history states and are displayed as individual steps in the history palette. When a mistake is made, the image can be returned to a previous state by clicking on the desired state. Moving backwards on the history palette enables me to perform multiple undoes.

There is a downside to the history palette. It uses memory. Let me rephrase that, the uses HUGH, RIDICULOUS AMOUNTS OF MEMORY! After all, to save each previous state, Photoshop must save the image and any layers associated with it. Working on a large image file can quickly take up any available RAM on the computer system. Being able to "move back in time" is convenient, but it comes at the cost of quickly using up all the available memory. The solution is to limit the number of history states that Photoshop will maintain (Edit → Preferences → General). My experience is that limiting the number of history states to 20 is a good compromise. Photoshop will record the first 20 steps as an image is adjusted. Once the 20 steps are filled, Photoshop continues to record history states, but each time a new state is recorded, the oldest state is lost. This provides me with a comfort zone in terms of how far I can go back (20 steps) while limiting the amount of memory involved. Of course, this number could be increased or decreased depending on how much memory my current system contains.

Limiting the number of previous states can sometimes present problems during image adjustment. For example, when retouching images, I often use the clone stamp and the healing brush to remove blemishes. Multiple small corrections produce better results then a single large correction, so it is not uncommon for me to perform 30-40 corrections in a single image. Once my adjustments are made, all 20 states on the history palette are the same, "clone stamp" or "healing brush." To return to a state before I began touching up, I would need to go back 40 steps. Fortunately, Photoshop provides a workaround in the form of the history snapshot.

Closer examination of the history palette reveals two types of history layers. At the top of the palette is a layer that contains a small thumbnail of the image (a history snapshot). Below this are a number of layers that are created as adjustments are performed (history states). No matter how many history states are created, the snapshots will be preserved as long as the file is open. Even moving beyond the 20 history states setting will not remove a snapshot from the palette. By default, the original image (no changes) is created as a snapshot allowing backing up to the original image at any time. We can create additional snapshots at any time by opening the history menu (the small arrow in the upper right corner of the history palette) and selecting "New Snapshot". Once selected, a prompt for the snapshot's name appears. Once the name is selected, the snapshot is added to the history palette immediately below any pre-existing snapshots (it is possible to have multiple snapshots for a single image).

Snapshots are useful when reaching a split in the road as far as image processing goes. Maybe I want to test out a new series of adjustments, but I am not certain if the results will be to my liking. No problem. Before beginning the adjustment process, I create a snapshot of the image. Once the snapshot is created, I can adjust the image at will, knowing that I can always return to that snapshot, no matter how many altered states I create.

Two features of the history palette need to be understood before using it. First, when backing up to a previous history state (returning from state 3 to state 1), it is possible to undo the undo (return back to state 3) provided no changes are made while at the earlier state. Once I change the earlier state (state 1), all subsequent states (states 2 and 3) are lost. Second, all history states and snapshots are lost when the image is closed.

So far I have discussed how the history palette can act as a sophisticated undo command; however, this doesn't differ significantly from Photoshop's standard undo command (File → Undo). The real advantage of the history palette is its ability to perform selective undoes. To be able to undo image adjustments from part of the image, while leaving the change on select portions is where the true power of the history tool lies. To apply history selectively, we need to use the history brush.

The history brush can be located immediately under the brush tool on the tool bar. Most users are familiar with the brush tool which paints with color. The history brush is unique in that it paints with alternate history states. To understand how the history brush works, let's look at an example. Let's take an image with dust problems and use the history brush to remove dust from the image.

I begin with an image containing a fair amount of dust scattered throughout the frame. To begin with, I want to apply a blur to the total image in order to remove the dust. This is accomplished by applying a Gaussian blur filter (Filter → Blur → Gaussian). The pixel radius of the blur is then adjusted until the largest piece of dust disappears. Once the proper level is set, click "Ok" to apply the Gaussian blur. The entire image will now be blurred to some degree, including the dust (which has disappeared) and the subject. Unless my goal is to produce an abstract image, a blurred subject is probably not the result I want. Now I bring in the history brush. First, I select the history state immediately before the Gaussian blur was applied (moving back a single step). This removes the blur from the entire image, returning sharpness to both the subject and the dust. Next, I need to select the history state from which the history brush will be painting. On the left side of the history palette, are a series of boxes, one next to each history state and snapshot. Inside on of the boxes is a small brush icon, similar to the history brush icon located on the toolbar. This icon indicates which history state the history brush will use as its source while painting. In this case, we want to "paint" from the blurred image (the state where we have removed the dust). Clicking on the small box to the right of the blurred history state will move the history brush icon to that state. Finally, I paint with the history brush over the dust spots in the image to remove them. What I am doing is painting portions of the blurred layer onto my sharp layer, selectively applying the blur to parts of my image. Since the Gaussian blur on the blurred layer removed the dust, painting from this layer will remove the dust from the sharp layer.

Some examples of how I have used the history brush are:

1. To remove oversharpening from an image. Some images need a lot of sharpening in some areas and little sharpening in others. I first sharpen the image, focusing on the areas that need a lot of sharpening. Once sharpening is complete, I look for areas that appear oversharpen. To selectively reduce sharpening, I use the history brush (set to a lower opacity) and paint from the history state immediately before the sharpening was applied.

2. To apply a special effect to only part of the image. In some cases, I like to depict motion be adding a blur to a running subject. To do this I apply a motion blur (Filter → Blur → Motion Blur) to the total image. To return some detail to the subject's face, I use the history brush (again, using a low brush opacity) and paint from the history state immediately before the blur was applied.

The uses for the history brush are limited mainly by imagination. Play with it and see what possibilities await.

Chris Gamel has been an active wildlife photographer for the past 14 years. The combination of his biological research and photographic interests has taken him around the world. In 2001, Chris discovered the joys of digital photography and he hasn't looked back. Chris lives with his wife and daughter in Salt Lake City where he teaches courses at Weber State University and Salt Lake Community College. To see more of Chris's photography, visit his web site at www.Animal-Antics.net.

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