Blend If is very much an overlooked and even mysterious feature to almost any Photoshop user. If you ever tried looking this feature up in manuals and books, you may not have been able to find it. In fact even searching Photoshop Help will not yield a title with Blend If in it (though the feature is referenced by function in ‘Specify a blending mode for a layer or group’ and ‘Specify a tonal range for blending layers‘). While the tool may not be a very popular target for tutorials and documentation, it is an enormously powerful tool that has been part of layers since the very beginning.

The Advanced Blending and Blend If sections of the Layer
Styles screen offer additional layer advantages, not often explored by very powerful.

What Blend If can do is help you target changes and corrections based on the color or tonal content of a layer. In a way it is like an auto-mask, in that it will mask a layer without you having to actually create a mask or a selection—and these masks can be highly complicated without much work. It will target content of a layer based on a set of sliders (the Blend If sliders on the Layer Styles dialog), and those slider positions. Before we go any further, let’s take a look at the basic functionality and how you control it before we really try to look at what it can do.

1. Open a new image 720 x 720 pixels with a white background, as shown below.
   
   

   
   The suggested sample image should use the settings shown here.

2. Create a new layer and call it Blend If Test.
3. Press D to set the default colors (black and white).
4. Choose the Gradient tool and be sure the Options are set to Linear Gradient, Normal mode, 100% opacity, and uncheck Reverse, Dither and Transparency.
5. Click on the lower left of the image and drag the cursor to the upper right, then release the mouse. The image should fill in a gradient from black to white from the lower left to the upper right.
6. Take a snapshot of the image by clicking the snapshot button at the bottom of the History palette (Windows > History). Leave the name as the default (Snapshot 1). This will make it easy to return to the state of the image before blending is applied and without having to open the Layer Styles dialog to reset.
7. Double-click the Blend If Test layer in the layers palette (anywhere but on the thumbnail or over the name). This will open the Layer Styles dialog.
8. Click on the black This Layer slider and drag it to the center of the slider range at 128 as shown below.

As you drag the cursor, the image on screen
should appear to swipe from the lower left corner half way across
the image. Notice that the layer in the layers palette retains its content.

The numbers on the Blend If sliders are measured in levels 0–255. This corresponds to black (0) to white (255) in a grayscale gradient. The change in position of the slider limits the range of what is visible in the layer (in this case the gradient) so it blends with what is below based on those slider positions. Everything to the left of the black slider and everything to the right of the white slider becomes transparent. If you shut off the visibility toggle for the Background layer you can see the transparency. Continuing from the exercise, try the following slider positions to get a better feel for the way it works:

• Move the black This Layer slider back to 0 and then move the white This Layer slider to 128.

  
  The gradient between gray and white will become transparent
  to see through to the background in the upper right of the image.

• Move the black This Layer slider to 192 and then move the white This Layer slider to 63

  
  The area between the white slider and the black slider become transparent
  to see through to the background in the middle of the gradient range.

Applying layers with Blend If can occasionally be confounding when using the This Layer slider. Any changes applied directly to the layer where the Blend If sliders are set may result in unexpected changes in the image. To test this out, make a Levels adjustment to the Blend If Test layer (Image > Adjustments > Levels). When the dialog opens, swing the center gray slider left and right and watch how the image behaves. Close the Levels dialog without committing the change. Now do the same thing with an adjustment layer by choosing Levels from the Create new Fill or Adjustment Layer menu on the layers palette. This is one more clear case for using adjustment layers instead of direct application of change.

The same concepts hold true for using the Underlying Layer sliders. The main difference is that the content of the current layer will blend based on the content of the layers below, rather than the content of the layer where you apply the blend—layer transparency still effects the current layer. To see the results of using Underlying Layers, do the following:

1. Click Snapshot 1 in the History palette to reset the image and Blend If sliders.
2. Double-click the Background layer and rename it to White Layer.
3. Change the order of the layers in the layer stack by pressing Command + ] / Ctrl + ].
4. Double-click the White Layer in the layers palette to open the Layer Styles dialog.
5. Click on the black Underlying Layer slider and drag it to the center of the slider range at 128.

   
   The white layer becomes transparent over the brighter area
   of the lower layer so you can see through it to the lighter half of the gradient.

6. Move the black Underlying Layer slider to 192 and then move the white Underlying Layer slider to 63.

   
   The white layer becomes transparent over the mid-tone area of the
   lower layer so you can see through it to the mid-tone ‘half’ of the gradient.

These examples are hard-edged application of Blend If in its simplest form. Several other features of Blend If allow partial blending and blending based on color ranges rather than just tone. Partial transparency (the real ‘blending’ form of Blend If ) can be created by splitting the sliders. Color targeting can be done by choosing ranges for the Red, Green and Blue sliders found under the Blend If drop list. Lets look at how to split sliders to have all the basic functions in tow:

Blend If can be adjusted per channel so that
blending can be targeted to specific color ranges.

1. Click Snapshot 1 in the History palette to reset the Blend If for the layers and the layer order.
2. Open the Layer Styles for the Blend If Test layer by doubleclicking the layer.
3. Move the black This Layer slider so it is at 128.
4. Hold down the Option/Alt key [Mac/PC] and click on the left of the black slider and then drag it to 0. The slider will divide into two parts.

   
   Holding the Option/Alt key allows you to split the slider,
   be sure to click on the side of the slider that is on the side you will be moving toward.

Splitting the slider will blend from 0% to 100% between the split halves. Splitting the sliders allows you to make a softer transition in blends, similar to blurring a mask or feathering a selection. The idea is that you gain control over how blends dissipate, rather than using them as an on/off switch for a particular range. As on/off switches the edges might end up hard and blocky, but by splitting the sliders you can offer better opportunity to control the blend.

So, what would you use Blend If for? Really, often for situations that seem otherwise hopelessly complex. For example, say you have taken a shot of a leafless tree in silhouette against a blue sky and you think it might look better with some other sky, some interesting clouds, or against a sunset, etc. It might seem to be a daunting task to make a selection between all those branches. You might try dabbling with the magic wand, but your results will be pretty sketchy. Blend If offers the opportunity to make the replacement without having to make a potentially unnerving and complex mask or selection. You can use measurements from your image to determine a range you want to replace, and then apply appropriate Blend If settings you make directly from the image.

That makes it sound like a miracle cure to use Blend If… and there are occasions where it will produce some amazing results with little effort. On the other hand, getting to do what you want may require combining it with masking or other techniques to achieve a result—like any other tool it is best to think of it as a companion to other functions rather than the lone ranger or some other hired gun.