The Blend If option determines how the tonal regions of a layer and the underlying layer blend together. The Blend If option is accessed by choosing Layer/Layer Style/Blending Options. The Layer Style dialog box appears as shown in Figure 1. The Blend If controls are found in the Advanced Blending section of the dialog box.
The Blend If pop-up can be found at the top of the Blend If section of the dialog box. This allows the photographer to choose whether Blend If adjustments will be based on the RGB channel (i.e., Gray) or one of the three, individual, color channels. Generally, the Gray option is chosen. The This Layer bar determines which pixels from the selected layer will blend. The Underlying Layer bar determines which pixels from the underlying layer will blend.
However, this creates a bit of a problem. There is now a rather abrupt transition between the pixels in the Active Layer that will blend and those that will not. This will, likely, result in rather harsh transitions between these tonal regions. Luckily, a simple solution exists. Holding down the Alt key (Option Key for a Mac) and clicking on either the Black or White sliders will split the slider into two sliders. As seen in Figure 3, there are now four sliders. For both the Black and White sliders, the inner halves of the sliders determine where the pixels in the Active Layer start to be eliminated from the blending and the outer halves of the sliders indicate where the blending has stopped completely. Figure 3 provides an example. In this image, at the shadow end of the bar, the Active Layer pixels start to be eliminated from the blending at a tonal value of thirty-four and are completely eliminated at a tonal value of twelve. Similarly, at the highlight end of the bar, the Active Layer pixels start to be eliminated from the blending at a tonal value of 221 and are completely eliminated at a tonal value of 241.
The abrupt transition between the pixels in the Active Layer that are blended and those that are not has been eliminated. At the same time, a greater amount of control over the blending has been achieved.
Probably, the best way to understand the Blend If option is to see a couple of examples of its use. Figure 6 shows an image that utilized a Levels adjustment to increase the contrast of the image. The Layers Palette for this image is shown in Figure 7.
With the Levels layer selected, Layer Style is launched, and the Blend If option accessed. Since it is desired to prevent the Levels adjustment from affecting the darker pixels, the Black This Layer slider is split and moved to the right as shown in Figure 9. This Blend If adjustment starts to phase out the effect of the Levels adjustment at pixels with a tonal value of twenty-one and has completely eliminated the effect at a tonal value of 5.
Figures 10 and 11 show the crops before and after the Blend If adjustments. As can clearly be seen, the Blend If adjustment had brought out some of the shadow detail.
In the first example, the editing was done using the This Layer adjustments. In the following example, the editing is done using the Underlying Layer adjustments.
Figure 12 shows a pattern that might be used for some graphics work. However, the pattern is black and white. It is desired to have a black and red pattern using the red shown in Figure 13. Now, a mask could be created that would blend the two images. However, a much simpler solution can be created by combining the pattern and the color through the use of the Blend If option.
One of the most valuable uses for the Blend If option is in sharpening. We all want sharp images. However, in the process of sharpening images, we do not want to sharpen the noise. The Blend If option can help resolve this issue.
With respect to sharpening and noise, the problem is the shadow areas. The shadow areas contain the most noise and the least amount of detail. Therefore, less sharpening should be performed in the shadows compared to the rest of the image. The Blend If option is perfect for this situation. To resolve the problem, a sharpening layer can be created and the Blend If option used to reduce, or completely eliminate, the sharpening in the shadows. For an explanation of this technique see the topic: Sharpening and Fine Tuning in my series on sharpening.
Sometimes, good things come in small packages. This is the case with the Blend If option. It is very simple to learn and simple to use. So much so that, sometimes, photographers tend to ignore it, preferring its more sophisticated looking cousins (e.g., Curves and masks). However, sometimes the most elegant solution is the most simple. For those cases, the Blend If option may just fit-the-bill.