In my Photoshop workshop on Saturday for the Photowalking Utah group, I explained how to use the multiply and screen blending modes to adjust brightness in an image, but I didn’t have time to go into details about the underlying principles. Here’s the bigger picture story.
Let’s begin with two separate layers. Each image has 11 bands of pure grey tones, stepping from 0 to 100% brightness in 10% increments (yes, that’s eleven bands). One layer has horizontal bands, the other vertical. If you see some weird distortions on the margins of each band, my resident color vision scientist, R, explains that these are common visual illusions called Mach bands.
When Photoshop applies the multiply blending instruction, what it really does is some mathematics, multiplication in fact. It takes the brightness of the base layer and multiplies with the brightness of the blend layer (base*blend). Some of us will recall from our math schooling that when we multiply together two fractions of less than 1, the result is always a smaller number than either of the two multipliers. I made a table of a few of the possible combinations, and the results do in fact match the brightness levels measured
|(A) Base||(B) Blend||A*B|
For the neon sign, a multiply blend at 100% opacity had too strong of an effect, so I reduced it to 50%
Screen, the counterpart to multiply
Mathematically, the screen blending mode is the inverse of the multiply mode, so we can use it to lighten images in a similar fashion. The actual equation is (1 – (1-base)*(1-blend)=result.
As in the multiply example, the solutions in the table for the screen blending mode are replicated in the color samples of the two layers blended together.
|(A) Base||(B) Blend||A*B|
Using multiply and screen in a PS workflow
Putting it all together, short version A few keyboard shortcuts makes this method of adjusting exposure really fast:
- Control (Command for Mac) J duplicates the layer
- Alt/Opt Shift M sets the blending mode to multiply (select a non-painting tool first, like Move or Crop, or you will set the blending mode for the tool, not the layer)
- Use Alt/Opt Shift S to set the blending mode to screen
- The keys 0-9 set the opacity, in 10% increments.
The method isn’t perfect, but it’s fast, and often good enough
Putting it all together, extended version, OR, isn’t [insert favorite method] a better way? The grace, power and downfall of Photoshop is that there are often dozens of ways to accomplish the same task. (See my list of B&W options from CS2, which I need to update for CS3’s ADDITIONAL options.) Yes, a curve can elegantly do the exact same thing. In fact, I used the tables above to generate the following pair of curves:
If Photoshop is using the same mathematical equation to generate the result, does it matter how Photoshop is directed to apply that math, via curve or blend? I think not. The far-too-common arguments over Photoshop lore and dogma (such as “always use an S-curve”) can make their proponents look ridiculous. A curve is not inherently better than a blend, if the underlying math is the same. I would choose the method that fits best into my workflow of the moment. Another tool in the toolbox is always welcome.
Into Practice You can download a file I made with the greyscale bars to test out my measurements. Remember to set your Info palette options to the HSB mode to read out brightness values using the Color Sampler Tool. If this post inspires, join our Flickr pool and contribute your examples of using multiply and screen blending to fix or improve an image that’s been languishing in your archives. Describe what you did (masking, opacity, etc.) and why. I’m looking forward to seeing your pictures.