This instructable demonstrates how to selectively enhance photos using "layer masks" and "blending modes" as found in "Adobe Photoshop Elements ^® or similar.

This technique can be used for

hand-coloring monochrome images
selective lightening
selective darkening
selective sharpening

In the above photo a subtle "glow" has been added to the right-hand side of the face and hair.

Step 1: Layer Masks

[ mantra: black conceals ... white reveals ]

Photoshop^® and Photoshop Elements^® include two sets of special layers called "fill layers" and "adjustment layers" which are recognized by a little square box to the right of the layer "icon". This box is known as a "layer mask".

"Layer masks" are so useful that Photoshop^®, and Photoshop Elements 9^® onwards, allow you to convert an ordinary layer into an "adjustment layer".

Applying "black" paint to a 'layer mask" allows you to selectively "conceal" portions of that layer . Applying "white" paint to a 'layer mask" allows you to selectively "reveal" portions of that layer . Hence the above mantra.

Let’s have a look at some examples … but before we do let's define some important keyboard combinations:

CTRL-I means press the "ctrl" key and the "i" key at the same time.
CTRL-J means press the "ctrl" key and the "j" key at the same time.
CTRL-Z means press the "ctrl" key and the "z" key at the same time

Step 2: Fill Layers

[ This technique can also be used for hand-coloring monochrome photos. ]

Watercolor artists often enhance a portrait by adding a "glow" to the image. Let’s emulate this effect with the help of a yellow "fill layer".

Method

Open a portrait.
Select "Layer|Fill Layer" from the pull-down menu and fill it with a nice "yellow".
Set the blending mode to "screen" and move the "opacity" slider to the left.
Now move the "Opacity" slider to the right until "yellow".starts to appear. This will be our "glow’.
There is a "mantra" in Photoshop: "white reveals", "black conceals".
Press CTRL-I. This toggles (inverts) the layer mask from "white" to "black" which causes the "yellow" to disappear. The "black" is "concealing" the yellow".
With the "foreground" color in your left-hand toolbar set to "white", choose a soft-edged brush with an opacity of say 20% and brush the hair and edges of the persons face. The face and hair will gradually acquire a glow. The "layers palette" for this editing techniques is shown in the second image.

This technique owes its success to the adjustment mask to the right of the "yellow" square. This mask was originally "white" until we inverted the color by typing Ctrl-I. If you look carefully at the "layer mask" you will see faint brush marks. Since our brush was loaded with "white" paint the brush marks are gradually revealing the "yellow".

Step 3: Selective Darkening / Lightening

A similar technique can be used to darken portions of an image.

Method

In this case we add a "Levels" adjustment layer, set the blending mode to "Multiply", and adjust the opacity until the area you wish to adjust is darkened by the correct amount.
With the adjustment layer still highlighted "invert" the adjustment box from white to black by pressing "Ctrl-I". This blocks the the "multiply" effect and the image returns to normal.
Now comes the amazing part. Set the "foreground" color on your toolbox to "white" and the "background" color to "black". Choose a large soft-edged brush and simply brush over the area you want darkened. The "Layers" palette for this edit is shown in the second image. So how does it work?

Have a close look at the black area of the "layer mask" and you will see "white" brush-marks in it. "White "allows the blending mode effect to pass through whereas "black" blocks the effect. If you set the brush opacity to say 20% then it will take several brush strokes to reach 100% "white" which means that you have heaps of control over your shading.

If you make a mistake simply set the foreground color to "black" and paint out your mistake or you can click the previous step in your "Undo History" palette.

Selective Lightening

As for "Selective Darkening" except you set the blending mode to "screen"

Step 4: Selective Sharpening

When it comes to portraits people don't like to see their wrinkles. The following methods allow you to sharpen their hair, eyes, nose, and mouth without sharpening their skin.

Method 1

Open an image in Photoshop^® (or Photoshop Elements 9^® onwards), create a duplicate copy by pressing "Ctrl-J", and sharpen the copy using one of the sharpening tools.
Add a "Layer Mask" to the top layer and fill it with "black" by pressing "Ctrl-I". This changes ("inverts") the color in the "layer mask" from "white" to "black" which "hides/conceals" the sharpening. Remember the mantra: "white reveals" "black conceals".
With the "layer Mask" box selected, and your "foreground color" set to "white", "brush" over the image area you wish to reveal. You can control the amount of sharpening that is revealed with each brush stroke by reducing your brush "opacity" to say 20%. The second and third images show the "Layers Palette" and "end result" when we selectively sharpen the left side of the portrait.

Method 2

[ Work-around for Photoshop Elements 8^® and earlier ]

Photoshop Elements 8^®, and earlier, doesn't allow you to add a "layer mask" to a normal layer, but don't despair … there is a work-around in the form of a "clipping mask".

All adjustment layers have a "layer mask" to the right of the layer icon. If we "clip" an adjustment layer to the "target" layer then the "layer mask" on the adjustment layer behaves as though the "target" layer has a "layer mask". Let's see a "clipping mask" in action …

Open an image in Photoshop Elements, create a duplicate layer by pressing the "Ctrl-J" keys, and sharpen it using one of the sharpening tools.
Insert a "levels" adjustment layer between the two layers but don't touch the "levels" sliders. We are now going to borrow the "layer mask" from the levels adjustment layer. To do this we highlight the top layer and select "Layer|Create Clipping Mask" from the pull-down menus. The top layer will move to the right and a small down-arrow will appear signalling that the two layers are now "clipped" or "grouped" together [1].
From here on the process is the same as for method 1.
High-light the "layers mask", press "Ctrl-I" to invert the "layer mask" color, then paint over the area to be sharpened using a brush with the "foreground" color set to "white". Only the right eye and right-side of the hair has been sharpened in this example. The final result is shown in the last image. The "layers palette" for this edit is shown in the second-to-last image.

[1] Alternatively you can press "Ctrl-G" (G="Group") or hold the "Alt" key down and click the line separating the two layers. Two inter-linked rings will appear then the top layer will move to the right as before. "Alt-click" the line again to reverse this action.