In any version of Photoshop or Elements you have access to layer Blend Modes with the default being Normal. If we make a color or density correction to an image, the default Normal mode of the layer means we influence both the luminosity and the color of the image. That is a default we can override. There are quite a few layer Blend Modes available to us and each does something different to the image depending on the content of the layer and its relationship to the layers below it. We will start with using just the two modes that allow us to separate the image color and content.
Cycling through the Blend Modes in Photoshop is now automatic as you move your mouse down the list in the drop down box. As you pass over each blend option the effect is shown on the image. This makes it possible for you to easily see the effect immediately. In older versions with the Move Tool (V) active you can cycle through the Blend Modes using the Shift key and the + and - keys or the up and down arrow keys to move forward or backward through the modes.
There are six groups of Blend Modes including the Normal, Darkening, Lightening, Contrast, Comparative, and Component Color groups. The Normal group modes have no immediate effect on the image except what the layer content provides. The normal mode just applies the layer content without changes. The Dissolve mode does nothing at 100%, but as the layer opacity is reduced a noise pattern is introduced. In my estimation there are better, more controllable ways to introduce noise into an image if needed. Dissolve mode has little value to a photographer.
The Darkening group modes all serve to darken the image in various ways depending on the actual mode and layer content. Whites in Darkening modes simply disappear. Similarly the Lightening group lightens the image and blacks disappear which is useful in compositing. The Contrast group lightens values above middle gray and darkens values below. The Comparative group reveals differences between layers and is useful for aligning images in composites,. The Component Color group decides how a layer is applied to existing layers relative to hue, saturation, color or luminosity.
Blend modes interact layer with the layer below to modify the effect. There are 27 Blend Modes, but most are not especially valuable for photographers. There are four more articles on blend modes that describe in greater detail the blend modes most useful to photographers. This article is a very simple overview of all the blend modes available in Photoshop. There are no samples in this article as most effects are very dependent on the image content. Even once you get the general idea of how they work, experimenting with the blend modes remains the only real way to discern their effect on any image. Blending them in with opacity and fill percentages along with masking gives you a lot of options for image control.
Normal - no change at 100%. At reduced opacity the layer underneath becomes visible relative to the density of the top layer. No math is involved in the decisions except the layer opacity.
Dissolve - no change at 100%. No pixel blending math. At reduced opacity underlying pixels are revealed through a dither pattern which introduces noise. Not of much value to photographers.
White is invisible (produces no change).
Darken - result colors will be darker. White will be invisible. Compares the layers and returns the darker of the two based on the values of individual pixels. No blending is done.
Multiply - multiplies the lower layer luminosity value and blend color of the top layer resulting in darker tones. Similar to a two slide sandwich. White produces no change. Blacks remain black. Useful in many ways for darkening an image or part of one. With an adjustment layer the bottom layer tones are multiplied resulting in a darker overall image. Useful in creating vignettes and intensifying brighter areas in an image.
Color Burn * - Darker result than multiple adding contrast which results in more saturated midtones but reduced highlights. Similar to the burn tool results but can be used in a more controlled manner.
Linear Burn * - Darker than Multiply but less saturated than Color Burn - most contrast is added in darker values.
Darker Color - Compares individual color channels and the result is the darker color by channel. Differences are very minor compared to Darken mode only because the math is different. Darker Color uses the composite color of a pixel rather than the individual color channels.
Black is invisible (produces no change).
Lighten - result colors will be lighter. Black will be invisible. Compares the layer with below and returns the lighter value.
Screen - opposite of Multiply. Always a lighter value. Useful in brightening an image or a part of one with masking.
Color Dodge * - generally results in blown high values, and increased saturation in midtones. Similar to the dodge tool effect but with greater control through masking.
Linear Dodge * - a stronger result than Screen. Can be used to open shadows with low opacity.
Lighter Color - compares base and blend and returns the brighter of the two using the composite values rather than individual channel values. The opposite of Darker Color.
Contrast Modes darken if darker than 50% gray - lighten if lighter than 50% gray - 50% gray is ignored except by Hard Mix. The effect in an increase in contrast and the result depends on the math used by each mode.
Overlay - a combination of Multiply and Screen with reduced effect of each. The results is a strong increase in overall contrast. The base layer colors are used to create the effect. Other contrast modes use the blend layer values instead.
Soft Light - similar to overlay with less contrast. A very useful mode for compositing as well as managing image contrast.
Hard Light - intense increase in contrast resulting from using the blend layer values for calculation.
Vivid Light * - extreme version of Overlay - rarely useful at full opacity.
Linear Light * - uses Linear Dodge on lighter values and Linear Burn on darker pixels resulting in an even more intense contrast boost. Useful in the Frequency Separation retouching technique with a layer primarily composed of middle gray values such as those produced by the high pass filter.
Pin Light - tends to remove midtone values resulting in unusual patches of color depending on the values in the top layer. Little value to photographers.
Hard Mix - result can only be black, white or one of the six basic channel colors. Of little value.
Also referred to as inversion, subtraction or cancellation modes:
Difference * - white inverts color value. Black does nothing. Putting the top layer of two images with identical content is useful in checking for alignment.
Exclusion - similar to Difference except that 50% gray produces 50% gray. Lighter values tend to invert the colors. Darker values tend to gray down blacks.
Subtract - subtracts the top layer pixel value from the base layer. Black has no effect. Lighter tones darker the image overall by subtracting the brightness from the underlying pixels. White simply turns the image black.
Divide - strangely enough it is the opposite of Subtract with the effect being a lightening of the image. Black turns the image pure white. White has no effect. With a solid color layer representing a color cast the divide mode will remove the color cast of an image if the color is presented at 100% brightness. Usually a lowered layer opacity yields a more useful effect.
Hue - blends the top layer color into the image without modifying the base luminosity. Maintains the saturation level of the base layer at 100% opacity. Neutral colors turn the image into monochrome.
Saturation - basically retains the color (hue) and luminosity of the base layer while introducing the saturation values of the top layer. Therefore neutral grays of any luminosity convert the image to black and white as saturation is removed. There is no difference based on the brightness of the neutral value.
Color - maintains the base layer luminosity while introducing the blend color. Useful in modifying the color of a selected area while retaining the detail and luminosity, especially at lowered opacity. Neutral values turn the image into black and white.
Luminosity - maintains the base layer colors while modifying the luminosity values. Therefore the color of the top layer is ignored while the brightness is modified. Vignettes and similar modifications to an image in Luminosity mode will avoid color shifts.
Pass Through - this mode is applied automatically when you create a group from layers. That retains the expected behavior of the individual layers within the group relative to the underlying layers. Setting a group to Normal mode only allows the contained layers to affect the group and not the underlying image layers.
* These modes produce different results when the Fill opacity is reduced rather than the overall opacity is reduced.