We will use Layers to do the same editing that we did in the Selective Enhancement Page.  By using layers, we can easily change some prior editing, while not changing other prior editing.  This is extremely useful on images where the editing is a little complicated because it is very easy to screw something up a little at an early stage, and this screw-up can get magnified on subsequent enhancements.  Without layers, you can not un-do prior editing after a file has been saved, closed, and then re-opened.  Without layers, you can un-do consecutive edits during an editing session, but you can not un-do just a single edit done in the past.  The moral is that Layers are extremely important when you are doing more than a couple simple edits on an image.

VIP:  If you Save your file as a JPG file, all of the layers will be lost for future edits.  However, the effects of the layers will be applied.  The preferred approach is to duplicate your original photo and Save it in (.PSD) Photoshop file format.  You will be able to save all of the layers and the will not be any loss of detail caused by JPG compression.

In the following example, all of the steps are the same as in the Selective Enhancement Page, except where designated by "NEW."  There are also VIP new topics at the end about re-editing the prior adjustment layers.

This image has 3 different regions that are significantly different, namely the sky, the white rocks, and the reddish region in the middle.  If you perform a global histogram adjustment on this image, the image will be enhanced only a minimal amount.  This following steps will carefully select each of the 3 regions, and then enhance each of these regions independently.  We will use the Magic Wand Tool for selecting the sky, the Magnetic Lasso Tool for selecting the white rocks, and then join those two together and use the Inverse to select the red central region.
 
 

Fig. 1.  Before

Fig. 2.  After all of the changes below

Select the Sky

• Click on the Magic Wand Tool (2nd column and 2nd from the top)
• Click most anywhere in the sky ... See Selective Enhancements Page.
• Save this selection by Select > Save Selection, then give it a name, such as "Sky."  (We will load this selection later on.)

Enhance the Sky

• Select > Feather  and then input a number.  I used 3 on this 2MP image and this is the number of pixels that acts as a transition to the boundary.  I did not want a large transition on the sky.  Without feathering, there could be a noticeable edge artifact, like a tan line.
• NEW:  Layer > New Adjustment Layer > Levels.
• NEW:  In the dialog box, you can simply accept the default parameters and click on OK.  However, changing the Name to something more descriptive than "Levels 1" is a good idea because we will have more levels to adjust.  Give the Name field something like "Levels Sky."  The blend Mode and Opacity options do not need to be changed for this example.  After you click OK on this initial dialog box, another dialog box appears for the Levels adjustment.
• In this Levels Adjustment dialog box, you adjust the histogram range to enhance the image by sliding controls affecting the highlights, midtones and shadows. (See Figure 3 below)
• Move the rightmost triangle under the histogram to the left to adjust the highlights to about 227.
• Do NOT move the left slider triangle to adjust the dark area because we do not want any dark areas in the sky.
• Move the middle slider triangle to adjust the midtones.  I moved it to the right to about .58.
• Click OK.  The resulting adjustment is shown in Figure 4 (which also shows the Magnetic Wand selection discussed next).
• If you do not like the results, click on the "Step Back (edit undo)" button.  In fact, you generally want to bounce back and forth with Step Back and Step Forward to compare the change with what you had before.  In addition, you should zoom in and compare the changes.

Fig. 3  Histogram of the original Sky

Fig. 4  After Sky adjustment and the Magnetic Lasso selection of the White Rocks.

You can now Select > Deselect so that the Sky region is no longer selected.  However, when we start making the next selection, this deselecting will automatically take place.

Select the white rocky region in the lower left using the Magnetic Lasso Tool.

• Click on the Magnetic Lasso Tool  ... See Selective Enhancements Page.
• See Figure 4 to see the selected region.
• Save this selection by Select > Save Selection, then give it a name, such as "White Rocks."  (We will load this selection later on.)

• Select > Feather  and then input a number.  I used 10 on this 2MP image and this is the number of pixels that acts as a transition to the boundary.  Without feathering, there could be a noticeable edge artifact, like a tan line.
• NEW:  Layer > New Adjustment Layer > Levels.
• NEW:  In the dialog box, you can simply accept the default parameters and click on OK.  However, changing the Name to something more descriptive than "Levels 1" is a good idea because we will have more levels to adjust.  Give the Name field something like "Levels White Rocks."  The blend Mode and Opacity options do not need to be changed for this example.  After you click OK on this initial dialog box, another dialog box appears for the Levels adjustment.
• In this Levels Adjustment this dialog box, you adjust the histogram range to enhance the image by sliding controls affecting the highlights, midtones and shadows. (See Figure 5 below)
• The rightmost triangle under the histogram is OK already, you do not want to make it brighter.
• Move the left slider triangle a little to the right to adjust the dark area.  I moved it to 34.
• Move the middle slider triangle to adjust the midtones, if you want to.
• Click OK.  The resulting adjustment is shown in Figure 6 (which also shows the selection discussed next).
• If you do not like the results, click on the "Step Back (edit undo)" button.  In fact, you generally want to bounce back and forth with Step Back and Step Forward to compare the change with what you had before.  In addition, you should zoom in and compare the changes.

Fig. 5  Histogram of the original White Rocks.

Fig. 6  After White Rocks adjustment and the selection of the Red central region.

Select the Red central region shown in Figure 6

• Select > Deselect to close all previous selections and their feather properties.
• Select > Load Selection and then select the Sky option from the drop down menu. Click OK.
• Select > Load Selection, VIP check the option "Add to selection," and then select White Rocks from the drop down menu.  Click OK.
• At this point, you have selected both the Sky AND the White Rocks.
• Select > Inverse.  This inverts the selection so that everything except what is inside the previous selection is now selected.  The "inverse" is the "complement" of the previously selected set is the Red central region.  This is shown in Figure 6.
• Save this selection by Select > Save Selection, then give it a name, such as "Red Central."

• Select > Feather  and then input a number.  I used 10 on this 2MP image and this is the number of pixels that acts as a transition to the boundary.  Without feathering, there could be a noticeable edge artifact, like a tan line.
• NEW:  Layer > New Adjustment Layer > Levels.
• NEW:  In the dialog box, you can simply accept the default parameters and click on OK.  However, changing the Name to something more descriptive than "Levels 1" is a good idea because we will have more levels to adjust.  Give the Name field something like "Levels Red Central Region."  The blend Mode and Opacity options do not need to be changed for this example.  After you click OK on this initial dialog box, another dialog box appears for the Levels adjustment.
• In this Levels Adjustment this dialog box, you adjust the histogram range to enhance the image by sliding controls affecting the highlights, midtones and shadows. (See Figure 7 below)
• VIP!!  In the Channel box near the top, choose "Red" from the drop down menu.
• Move the rightmost triangle under the histogram to the left to adjust the highlights to about 217.
• I did not Move the left slider triangle to adjust the dark area.
• I did not Move the middle slider triangle to adjust the midtones.
• Click OK.  The resulting adjustment is shown in Figure 2 at the top of the page.  I also sharpened the image using the Unsharp filter.
• If you do not like the results, click on the "Step Back (edit undo)" button.  In fact, you generally want to bounce back and forth with Step Back and Step Forward to compare the change with what you had before.  In addition, you should zoom in and compare the changes.
• NEW:  Layer > New Adjustment Layer > Brightness/Contrast.
• NEW:  In the dialog box, you can simply accept the default parameters and click on OK.  However, changing the Name to something more descriptive than "Brightness/Contrast 1" is a good idea because we will have more levels to adjust.  Give the Name field something like "Contrast Red Central Region."  The blend Mode and Opacity options do not need to be changed for this example.  After you click OK on this initial dialog box, another dialog box appears for the Contrast adjustment.  Move the Contrast slider to the right to about +15.

Fig. 7  Histogram of the Red Channel of the original Red Central region.

Figure 8 shows the original Histogram levels for Figure 1 before any enhancement was performed.  Figure 9 shows the Histogram levels for Figure 2 after all of the preceding steps were performed.
 
 

Fig. 8  Histogram of the original image shown in Figure 1

Fig. 9  Histogram of the final image shown in Figure 2

NEW and VIP:
The power of using Layers is that we can now go back and selectively modify any of the previous edits that we did earlier.

In order to easily access different layers, click on the Layers Tab located in the upper right of the screen.  You should see something similar to Fig. 10 shown below.  If you do not see a large enough version of this, you should stretch out the boundaries of the Layers view.  The lowest layer is called the background, and then the next 3 layers are displayed from the bottom up in the order that they were created.  There are a zillions of things that you can do with these layers, but I will mention just a few basic ones.  The most important concept of the layers is that Adjustment Layers only store information about the type of the editing that is done and where it is applied.  For these examples, each layer stores the instructions that it is a Levels adjustment, what the sliders were moved to, and what selected area it was applied to. The Adjustment Layer does not store the resulting RGB pixels of the enhanced image.   The lowest layer called the Background layer contains all of the RGB pixels to which the higher up adjustment layers will be applied to.
 
 

Fig. 10  View of the Layers

Modifying previous edits:   On each of the top 4 layers, there are two larger rectangles (not the two little ones containing an eye).  The right rectangle is black and white, where the white region indicates the selection to where the adjustment was applied.  The rectangle on the left contains a figure that looks like a small histogram with a slider under it, which is a visual indicating that this was a Levels adjustment.  If you double click on this left rectangle, the levels dialog box appears showing the exact place where you moved the sliders.  For example, if you click on the left rectangle of the top Layer for the Red Central Region, you will see the Levels dialog box shown in Fig. 7.  You can then change the levels adjustment if you want to.  You can do this with any layer.

If you want to delete a layer, right click on the layer and then click on Delete.  You will be asked to confirm this deletion.  If I deleted any of the top 4 layers, it will be as if I never did that particular edit.  Be careful not to delete the background layer at the bottom.

Before deleting any layer, it is easy to see what the result will look like with that layer deleted.  The small rectangle on the far left of a layer contains a small image of an eye.  If you click on that eye, the effects of removing that layer will be displayed.  To see the effect of the adjustment layer again, click on that small far left rectangle again.  This will toggle the eye back on.

As noted at the very beginning, if you Save your file as a JPG file, all of the layers will be lost for future edits.  However, the effects of the layers will be applied.  The preferred approach is to duplicate your original photo and Save it in (.PSD) Photoshop file format.  You will be able to save all of the layers and the will not be any loss of detail caused by JPG compression.  If you need to convert it to a JPG because you are printing it at some store, then so a SaveAs to a JPG file type.  Keep the PSD file for future changes.