Luminance Masking - Page 8
When looking at a mask, keep in mind that anything that is black will be hidden and white will be shown. A grey tone will allow part of the effect to be shown. Although our mask prevented much of the Multiply blending mode layer from affecting the shadows, the midtones are still being affected to a certain degree. The beauty of using a Layer Mask to control this effect, it that it can be altered and customized to suit your taste exactly.
Click on the Layer Mask in the Layers palette to ensure it is active, then use Curves:
Click This curve ensured that the full effect of the duplicate layer is shown in the highlights (by making them brighter) and further prevented it from affecting the midtones and shadows. A Gaussian Blur with a low radius will complete the mask. The higher the resolution of the file, the higher a value that can be used for the radius.