Blend Modes

One of the many advantages of working in Photoshop is the ability to combine pixel layers using different algorithms, known as blend modes.

There are twenty-seven different blend modes to choose from, accessed from the Layers panel, which will vastly alter the way the pixel layer they are applied to blends with the layers beneath it. Each of these modes has a specific purpose, and they can also be used to achieve results not possible through normal means. Perhaps the easiest way to understand this concept is through the use of the Screen blend mode.

If you want to combine two images to simulate a double exposure you place each one on a separate layer in Photoshop, and set the blend mode of the uppermost layer to screen. The lighter values in the top layer will lighten those in the layer(s) beneath, the but darker values will not affect them in any way. This is precisely how double exposure works in the camera. During the second exposure, light entering the camera lightens areas already exposed on the film. However, areas of the second exposure which are black will not affect any area of the previous exposure. If you took two exposures of an object against a black background each in a different location within the frame and combined them in this way, you would have two identical images on the same frame. This is in fact how digital cameras which are capable of double exposure combine two exposures; the first is held in the image buffer, and the second is combined with it using the screen mode.

The modes are divided into groups of related function. From top to bottom, the first is the normal group. The Normal mode uses no special algorithms, but allows you to alter the opacity of the layer it’s applied to. The Dissolve mode creates stippled edges where there are areas of partial transparency, such us through the use of a layer mask.

The second group is the darken group. Modes in this group use pixel values in the applied layer to darken those of the underlying layers.

Next is the lighten group. It is the opposite of the darken group, mode for mode.

The fourth or “light” group affects values which are lighter or darker than 50% gray. Values in the applied layer which are 50% gray do not affect those beneath. However, values lighter than 50% gray lighten them, while values darker than 50% gray darken them.

The fifth group combines layers by first inverting them, then applying algorithms from the other groups, such as Linear Burn (Subtract), Color Dodge (Divide) or by calculating the difference between pixel values (Difference and Exclusion).

The last group combines layer based on luminosity or chromaticity. It allows you to simulate working in other color modes such as L*a*b*.

Advanced use of Blend Modes
While these modes affect how pixel layers are combined, they also affect how adjustment layers are applied. For example, If you want to lighten or darken an image without affecting its color, use the luminosity mode with adjustment layers such as Brightness/Contrast, Levels, Curves or Exposure. This is essential for use with CMYK images, which exist in a non-linear color space. Conversely, if you wish to make a color adjustment without affecting lightness, use the Color mode.

In addition to these basic applications, there are also advanced applications which will be explained in these upcoming tutorials:

• Advanced Exposure Adjustments
• Advanced Contrast Adjustments