Adobe says, of the Soft Light Layer Blending mode, “Soft Light darkens or lightens the colors, depending on the blend color. The effect is similar to shining a diffused spotlight on the image. If the blend color (light source) is lighter than 50% gray, the image is lightened as if it were dodged. If the blend color is darker than 50% gray, the image is darkened as if it were burned in. Painting with pure black or white produces a distinctly darker or lighter area, but does not result in pure black or white.”
I’ve put together some example images for your consumption to illustrate how I work.
I will convert a copy of the image to monochrome. The conversion will attempt to capitalise on the tonal qualities of each colour channel within the areas of the image. In this image, I would take primarily red channel areas to emphasise the face. Blue to reduce emphasis on the background.
The image above simply takes the RED channel (use curves to reduce the other channels to zero), desaturated. Then simply set this monochrome layer’s blend mode to Soft Light. On the left is the original image. On the right, soft light blended layers – top is a more subtle 40% opacity, whilst the bottom right is 100% to show the full force of the soft light layer mode on the red channel.
Below you will see the same effect, but for the GREEN and then BLUE channels.
And below, a crafted monochrome conversion specifically to use as a contrast boosting Soft Light Layer. It is subtle, but the original raw was quite contrasty already.