Photoshop’s Hard Light definition is the longest one in the “help” file, and actually gives some helpful information!
Hard Light: Multiplies or screens the colors, depending on the blend color. The effect is similar to shining a harsh spotlight on the image. If the blend color (light source) is lighter than 50% gray, the image is lightened, as if it were screened. This is useful for adding highlights to an image. If the blend color is darker than 50% gray, the image is darkened, as if it were multiplied. This is useful for adding shadows to an image. Painting with pure black or white results in pure black or white.
The more of these definitions you read, the more they start to sound the same. The first line sounds almost exactly like the “Overlay” definition. Oh, except that there is a difference:
Overlay: Multiplies or screens the colors, depending on the base color.
So while Overlay, remember, preserves the “highlights and shadows” of the underlying layer(s), Hard Light does that to the layer set to “Hard Light.” Theoretically, if you have two layers set to either of these blend modes, then switch them and change to the other blend mode, the result should be the same. And they are!
Top layer is set to Overlay:
Layers are switched; new top layer is set to Hard Light:
So obviously it doesn’t make much of a difference which one you choose to use when you’re dealing with a two-layer document. But understanding the difference will help you when you have a multi-layer document and you want to apply blending modes to the layers.
Here’s a practical application of Hard Light, as suggested by Photoshop Help: make shadows and highlights.
I’ll start with a basic rounded button shape:
I create a selection that is the same shape and fill it with a gradient on a new layer. I make sure the gradient doesn’t have pure whites or blacks, but lighter and darker shades of grey:
By setting this new gradient top layer to Hard Light, the button looks like it has a metallic sheen:
You can apply this same effect to photos or more complex illustrations as well, by using a new, different layer set to Hard Light and painting in different shades of grey — and it preserves the original layer!
Here’s a link to another tutorial that uses Hard Light (plus lots of other blending modes and tricks!) to take a photo and make it look like a watercolor sketch.