Blending Modes for Photographers
Once you understand how a magic trick works, the technique behind it becomes a tool you can use yourself. The same could be said for the magic of blending modes in Photoshop. Most users learn a few recipes for optimizing images that include specific uses but never really how to use blending modes in general. Let's see if we can change that.
A layer in Photoshop combines with the one below it based on its blending mode. One layer "blends" into another in a specific way. The Photoshop user interface groups blending modes based on their effect, but in a way this serves to somewhat obscure their actual purpose. In Photoshop, all the blending modes that generally result in darker colors are grouped together just as all those that produce lighter colors are. But in learning about blending modes it is generally easier to take these in pairs, one from the lighten group and the corresponding one from the darken group, and so on. As such, I will be talking about them in a different order from how Photoshop lists them, but there is a method to the madness as I'm hoping you will agree after reading what follows. I'm also going to go into more detail on those blending modes most useful for photographic image adjustment. At least half of the modes in Photoshop serve mainly as ways of producing composite images and special effects and will be discussed only briefly.
The default blending mode is referred to as Normal. With this mode, the top layer doesn't so much blend with the one underneath, it obscures it. If the upper layer is completely opaque, you won't see the bottom layer at all. If not, you will be able to see through to what's underneath based on the degree of transparency of the top layer. This is consistent with what happens with objects in the real world, so understanding how this works in Photoshop is pretty straightforward.
The Dissolve blending mode works somewhat the same way even if the results don't appear so. The difference between is that for each point, Dissolve always picks the entire top pixel or the bottom pixel, not a combination of the two. The choices are made randomly, but are weighted based on the transparency of the upper layer. If the upper layer is completely opaque it will always be chosen. If it is completely transparent, the bottom layer will always show. For transparency values between these two extremes you will proportionally sometime see the top pixel and sometimes the one underneath. This mode has little use for photographic images since it the transitions between neighboring pixels will be abrupt rather than smooth.
Hue, Saturation and Luminance are all modified implementations of the Normal blending mode, but only a single aspect of the upper layer is considered with the other two ignored. Hue uses only the hue from the top with the resulting saturation and luminance coming solely from the contents of the bottom layer. Similarly, Saturation blends only the saturation of the upper layer with the hue and luminance of the upper layer ignored. Luminance works the same but considers only the luminance (brightness) of the upper layer.
The Color blending mode combines the action of the Hue and Saturation blending modes. Only the luminance of the upper layer is ignored in the blending.
Multiply and Screen blending modes form a pair with the former darkening the result and the latter lightening it. Technically, multiply does what it sounds like: pixel values from the top and bottom layers are multiplied together to determine the result. Screen does the reverse in that the inverse of each value is multiplied. Functionally, what this means is that Multiply darkens colors proportionately to how dark they are to begin with. Dark colors get darker, but even darker colors get much, much darker. Just think about your basic multiplication tables: 2 x 2 = 4, 3 x 3 = 9, 4 x 4 = 16, and so on. The series of products of multiplication goes up much faster than the series of numbers being multiplied together. Screen yields results that are increasingly lighter based on how light the source pixels work. Just think of the opposite of Multiply.
One common use for these two blending modes is to correct contrast problems. Duplicate a layer and blend it with itself with Multiply and you can increase whatever contrast it may have to make darker edges progressively more exaggerated. You can perform a similar feat to lessen contrast in an image with blocked up shadows by combining a duplicated image layer with itself using Screen. If the effect is too intense, you can minimize it by adjusting the opacity of the top layer.
Overexposure and underexposure can also be corrected to a degree by means of Multiply and Screen, respectively.
If the blended layers are not the same, the color of the result from Multiply and Screen combine the colors of both layers in the same way as their brightness. As such, they affect both contrast and saturation. This can be problematic if used. Just as darker areas get exponentially darker with Multiply, saturated colors get exponentially more saturated. Be careful.
Black multiplied with anything is black, and White screened with anything is white.
Overlay blending mode combines the effects of both Multiply and Screen. If the upper layer is lighter than medium toned, the underlying pixel is screened. If the upper layer is darker than medium, the underlying pixel is Multiplied. An image Overlaid with itself yields results similar to the application of a S-curve for contrast.
One excellent use for Overlay is to perform dodge and burn adjustments non-destructively. Create a layer filled with medium gray above an image layer and set the blending mode to Overlay. You can then paint on this gray layer with black to burn, or white to dodge.
Soft Light can be considered a more elegant implementation of Overlay in that it does the same thing, but the results tend to be less harsh. Frankly, I use Soft Light much more than Overlay for this reason.
Hard Light is also similar to Overlay but while Soft Light is a somewhat softer implementation, Hard Light lives up to what its name implies as a harder implementation. Unlike even the somewhat harsh effects of Overlay, only Hard Light retains the extremes of Multiply and Screen. Painting with pure black in Hard Light blending mode yields pure white. Painting with pure black yields pure black. We're talking extremely harsh – effects that have little use for photographic images, at least not with some other factor to tone down their effects. Vivid Light, Linear Light and Pin Light are further variation on the same.
Linear Dodge and Linear Burn, Color Dodge and Color Burn form pairs of blending modes that lighten (dodge) or darken (burn) the color of the bottom layer based on the color of the blended layer. The effect is achieved by adjusting the brightness in the case of the Linear modes or by means of contrast adjustment in the case of the Color modes. As such, the effect of the Linear modes is similar to adjusting the White point in Levels, while that of the Color modes is analogous to adjusting the Black point. Hard Mix basically combines Color Dodge and Color Burn based on whether the values are lighter (dodge) or darker (burn) than medium gray.
Difference, Exclusion, Subtract, and Divide round out the list of blending modes and are useful for technical image analysis and special effects.
Assuming at least the modes I covered in more detail make reasonable sense, the best way to fully understand what each blending mode does is to experiment with your own images and see for yourself. Blending modes really aren't that complicated once you gain some experience with them. And having this just might give you the edge when you're trying to repair what ails a problem image.