Motivation: Use Overlay Effect to control precisely contrast
Features:PaintShop Pro use of a B+W layer with Overlay blend to add exact contrast

One of the problems of applying the brightness+contrast effects is that you often have to spend a great deal of time masking the image to get precise the brightness+contrast effect you want. The Overlay Effect takes advantage of a long used darkroom trick in which a either a negative or a semi-transparent positive of an image is used to develop/filter the final image. This darkroom trick is easily emulated in any popular photo processing program that supports layers with blending.

In this case PaintShop Pro is used on an image of cacti taken at Allen Gardens (see PixofToronto). The overall technique is conceptually simple: 1-duplicate the original image, 2-convert the duplicate to a blackwhite layer, and then 3-blend the original using the overlay option, finally, 4-adjust to taste. If this sounds like a recipe that is why they called it cooking in the darkroom.

So start by Layers | Duplicate (or in the layers palette right click on the background layer and choose the Duplicate command). Next, to convert the new layer to B+W choose either Image | Grayscale or Adjust | Image | Colorize and reduce the saturation to zero. Use Colorize if you want to add a tinge of some hue - otherwise Grayscale is quick and easy.The screen shot above shows the now B+W duplicated layer.

We are almost done. make sure the duplicated layer is your active layer by clicking on it. Then for Blend mode switch from Normal to Overlay (see Screenshot at left). Lower the the opacity of the controlling B+W layer to suit you needs. Now the whole trick of the Overlay effect is that the duplicate layer acts as a precise mask controlling the contrast. By varying the opacity the contrast varies inversely - the 100% opacity = greatest contrast, 0% opacity = least contrast. But there is a big difference between the Overlay effect and simply using the Adjust | Brightness Contrast command.

The Overlay Effect narrows the contrast range but increases the gradations so you get finer control Thinks of the duplicated layer as acting as a best guess of what the brightness+contrast should be. If you don't like the range you are in adjust the brightness contrast of the duplicated layer. for obvious reasons this is often called the contrast control layer.
The cutaway screen shot at the left shows the two layers in action. On the left is the duplicated contrast layer with its right half cutaway. This allows us to see the underlying background layer. The right portion of the background will get no contrast adjustment=>hence a added trick option. In landscape shots where the sky is properly exposed but the foreground is consequently way underexposed - you may want to just cutaway the sky portion of contrast or duplicated layer. Use a mask with feathering (20-50 pixels)to do the cutaway. this guarantees that only the foreground will be effected - but again with precise control and fine gradations.

The image on the left is our final image. As you can see the contrast has been lifted to emphasize the zebra-like colors of the cacti as opposed to the rather flat original (the right part of the cutaway image above). It also emphasizes the thorny edge of the cacti - giving the yellow flowers an innocence in the midst of raspy danger.

So in this application of the Overlay effect we heightened the contrast. In the next example (see Overlay Effect II)we use the same basic technique to reduce the contrast which is particularly important on sunny days when shadows and direct sunlight often make a picture too contrasty.

Lets review now the advantages of the Overlay Effect. It allows more precise control of contrast often bracketing the contrast into the optimum range. Also the Overlay Effect provides finer control or gradations of contrast then using the the familiar Brightness+Contrast tools. In addition, Overlay Effect can often eliminate the need for several masking and then contrast control steps. Finally, when used in conjunction with the Orton effect (see here), Overlay Effect provides for stunning diffusion effects.