Blend mode focus: Multiply
How it works and when to use it
Layers are arguably the most powerful feature of Photoshop and blend modes add to their versatility tremendously. When stacking layers in their default ‘Normal’ mode, they are essentially just collections of pixels laid on top of each other – they do not interact in any way and while this is sometimes desirable, for many effects to look credible and natural, we must force Photoshop to get each layer to ‘speak’ to the others.
The Multiply Blend Mode is so named because of the mathematical calculation behind its function – Photoshop literally multiplies the colour values of one layer with those in the layer below it in the stack. If you imagine the layers stacked in 3D and a light source below them, changing the blend mode of a layer to Multiply is the equivalent of duplicating the transparent image, so that you are now looking through two ‘sheets’ of photograph as opposed to one, creating a visibly darker image. While it is not essential to understand the math behind the algorithm, it is fairly simple – Layer 1 x Layer 2/255 where 255 represents pure white.
In our example here, looking at the blue channel, we have a colour value of 123 for the background colour. Each of the circles has a different value, ranging from 0 for the pure black spot, to 255 for the pure white spot. In the Normal Blend Mode, as in the first image, the layer group with the circles simply sits on top of the background and containing solid colour layers, blocks the pixels below it from view.
In the second image, the circle layer group has had the Multiply blend mode applied and we can see the effect of the equation in action. The black circle remains entirely unaffected, since a value of zero cannot produce any other number than another zero (0 x 123/255=0). Meanwhile, at the opposite end of the value scale, the white circle has apparently vanished. In reality it is still present, but since the new colour value is the same as the background, it is no longer visible (255 x 123/255 = 123).
All the intervening circles have adopted the expected effect when using Multiply – that of becoming darker than their original colour value. The bottom left circle for example has a original value of 118. When applying the Multiply formula (118 x 123/255) we get a value of 56.9 – closer to black than before. Due to the background colour being blue, this produces a darker blue colour, as the pixels from the background show through the layer above them. Predictably, the darker the default colour in ‘Normal’ mode, the darker the colour in Multiply.
In the real world
What does this mean in a real life context? Well in short, whenever a layer is switched to Multiply, unsurprisingly the overriding effect is that of darkening. Any colour that is blended with white in Multiply will remain unchanged, while those blended with lower colour values will be darkened i.e. brought closer to black. Blending with black will always produce black. In this image the background layer was duplicated and this top layer was assigned the Multiply Blend Mode. As we can see from the histogram, the tones are shifted to the left post-blending, since every value has been shifted closer to black. The effect is apparently greater on the darker tones, since these have greater density to start with. Meanwhile the brighter tones have the appearance of being darkened to a lesser degree. This creates the impression of added contrast, although the darkening effect is the main reason you’d use Multiply. Interestingly, there are some blown highlights in this scene, which equate to 255 white. Since this will always produce another 255 in the Multiply equation, these areas remain unaffected.
When to use Multiply
Multiply is often a better option for darkening an image than using either the Darken Blend Mode or simply the Brightness/Contrast command, since the added contrast produces a more natural effect. When applying colour to an image with a brush, repeated brush strokes create an increasingly darker colour, which is useful for building up an effect gradually.