Threshold Adjustment Layer in Photoshop

Article and Photography by Ron Bigelow

www.ronbigelow.com

Photoshop CS2 Used in this Tutorial

Often, one of the first things that is done when editing an image is to set the shadow and highlight values. This is typically done using either Levels or Curves. One of the most exact methods of doing this is to use the Info palette in conjunction with the Color Sampler to identify locations that will be used to set the shadow and highlight values. Color samplers are set at these locations (for more information see, Levels -- Part I and Curves -- Part II). Then, the Eyedroppers in Levels or Curves are clicked at the color samplers to set the shadow and highlight values. The most challenging part is selecting the locations where the color samplers will be placed. For this, one must be able to differentiate between very small differences in tone. Frankly, the human eye just isn't accurate enough for this task.

When Levels is used, this is not a problem. With Levels open, holding down the Alt key and clicking on the Black Input Slider will turn the image white. As the slider is moved to the right, parts of the image will start to change color. The parts of the image that change color are being clipped. The first parts of the image to change color contain the darkest tones in the image. After evaluation with the Info palette, the shadow color sampler will be set somewhere in these dark areas (see Figures 1 and 2).

Figure 1: Levels with the Black Input Slider Moved to the Right
Figure 2: Image with the Black Input Slider Moved to the Right While Holding Down the Alt Key
Similarly, holding down the Alt key and clicking on the White Input Slider will turn the image black. As the slider is moved to the left, parts of the image will start to change color. The parts of the image that change color are being clipped. The first parts of the image to change color contain the lightest tones in the image. After evaluation with the Info palette, the highlight color sampler will be set somewhere in these light areas (see Figures 3 and 4).
Figure 3: Levels with the White Input Slider Moved to the Left
Figure 4: Image with the White Input Slider Moved to the Left While Holding Down the Alt Key

Once the shadow and highlight color samplers have been placed, the shadow and highlight values can be set as detailed in the Levels -- Part I article.

On the other hand, prior to CS3, Curves lacked any such capability to identify the darkest and lightest points in an image that can then be used to set the shadow and highlight color samplers. When one considers that Curves is a far more versatile and powerful tool than Levels, at first, this may seem a bit disappointing. Fortunately, the Threshold adjustment layer resolves the dilemma for those that have older versions of Photoshop. Even those that have newer versions of Photoshop may like to have an alternative method to identify the darkest and lightest points in an image. The rest of this article will focus on using the Threshold Adjustment layer to place the shadow and highlight color samplers.

The Image

Figure 5: Unedited Fall Color Image

To illustrate the procedure, the image in Figure 5 will be used. This image has not yet had any editing performed. Clearly, the image is flat and needs a boost in contrast. The first step to is to set the shadow and highlight values. For this, the shadow and highlight color samplers will need to be placed in the image.

The Procedure

Figure 6: Info Palette

Since the human eye is not accurate enough for this task, the Info palette will be used (choose Window/Info). The Info palette shows the colors of any chosen point. The options for the Info palette are set by clicking on the pop-up menu in the upper right hand corner of the palette and choosing Palette Options (see Figure 6). The Palette Options dialog box appears as shown in Figure 7. Both the First Color Readout Mode and the Second Color Readout Mode should be set to Actual Color. Clicking the OK button closes the dialog box.

Figure 7: Palette Options Dialog Box
Figure 8: Color Sampler

Next, the Color Sampler is selected from the Tools palette as shown in Figure 8. The Color Sampler will allow color samplers to be placed on the image. The color values at the color samplers can then be read in the Info palette. Before placing the color samplers, it is necessary to set the sample size of the Color Sampler. This is done by clicking on the sample size pop-up in the upper left hand corner of the screen. For this purpose, it is best to select the 3 by 3 Average (see Figure 9).

Figure 9: Sample Size Pop-Up

 

Figure 10: Layers Palette
At this point, the Layers palette is as shown in Figure 10.
Figure 11: Launching Threshold
Threshold is launched by clicking on the Create a new fill or adjustment layer icon at the bottom of the Layers palette (see Figure 11). The Threshold dialogue box is shown in Figure 12.
Figure 12: Threshold Dialogue Box
The Threshold dialogue box has only one adjustment: the Threshold Level (the Threshold Level can be adjusted by entering a value into the box or moving the Threshold slider). Any tone that is darker than the Threshold Level (i.e., a lower number) is turned to pure black. Any tone that is lighter than the Threshold Level (i.e., a higher number) is turned to pure white. This is shown in Figure 13, which shows the image after opening Threshold.
Figure 13: Image after Opening Threshold

Positioning the Shadow Color Sampler

Figure 14: Threshold Used to Identify Dark Tones

Locating the position where the shadow color sampler should be placed is fairly easy. The Threshold slider is moved all the way to the left. Then, the slider is moved slowly to the right. As the slider is moved to the right, parts of the image will start to turn black. The first parts of the image to turn black contain the darkest tones in the image. These are the areas that will be used to place the shadow color sampler. Figure 14 shows the Threshold Dialogue Box at the point where some of the image has just started to turn black. Figure 15 shows the image.

Figure 15: The Image with Threshold Used to Identify Dark Tones
Figure 16: Info Palette

At this point, the photographer can proceed in either of two ways. If it is desired to place the shadow color sampler while the Threshold dialogue box is still open, the Color Sampler is moved around the black areas of the image (it is best to set the monitor to 100% to better view the image). As the Color Sampler is moved around, the Info palette indicates the tonal values (for the three color channels) of the location directly below the Color Sampler (see Figure 16). For placing the shadow color sampler, it is desired to find a position that has very low tonal values but still holds detail. Once the location is identified, the Color Sampler is clicked at this point while holding down the Shift key. A color sampler is added to the image (see Figure 17). This method has the advantage that it is very easy to see the areas that have been identified with black by Threshold. On the other hand, the actual image is not visible.

 

Figure 17: Shadow Color Sampler Placed While Threshold is Open
Figure 18: Layers Palette with Threshold Layer Turned Off
If it is desired to place the shadow color sampler while the image is visible, Threshold is closed and the eye icon next to the Threshold layer is turned off (see Figure 18). The image now appears as it normally does (see Figure 19).
Figure 19: The Image after the Threshold Layer was Turned Off
The Color Sampler is now moved around the areas of the image that had been shown in black when Threshold was open. With this approach, it is a bit more difficult to exactly locate these areas. To assist with this task, the eye icon on the Threshold layer can be turned on and off (see Figures 20 and 21).
Figure 20: Threshold Layer Off
Figure 21: Threshold Layer On
As the Color Sampler is moved around, the Info palette indicates the tonal values of the location directly below the Color Sampler. Once the location is identified, the Color Sampler is clicked at this point. A color sampler is added to the image (see Figure 22). The Layers Palette appears as in Figure 23.
Figure 22: Shadow Color Sampler Placed While Threshold is Closed
Figure 23: Layers Palette

Positioning the Highlight Color Sampler

Figure 24: Threshold Used to Identify Light Tones

Locating the position where the highlight color sampler should be placed is just as easy. The Threshold slider is moved all the way to the right. Then, the slider is moved slowly to the left. As the slider is moved to the left, parts of the image will start to turn white. The first parts of the image to turn white contain the lightest tones in the image. These are the areas that will be used to place the highlight color sampler. Figure 24 shows the Threshold Dialogue Box at the point where some of the image has just started to turn white. Figure 25 shows the image.

Figure 25: The Image with Threshold Used to Identify Light Tones
Figure 26: Highlight Color Sampler Placed While Threshold is Open
Again, at this point, the photographer can proceed in either of two ways. If it is desired to place the highlight color sampler while the Threshold dialogue box is still open, the Color Sampler is moved around the white areas of the image while the tonal values are read on the Info palette. For placing the highlight color sampler, it is desired to find a position that has very high tonal values but still holds detail. Once the location is identified, the Color Sampler is clicked at this point while holding down the Shift key. A color sampler is added to the image (see Figure 26).
If it is desired to place the highlight color sampler while the image is visible, Threshold is closed and the eye icon next to the Threshold layer is turn off. The image, again, appears as it normally does (see Figure 27).
Figure 27: The Image after the Threshold Layer was Turned Off
The Color Sampler is now moved around the areas of the image that had been shown in white when Threshold was open. To assist with this task, the eye icon on the Threshold layer can be turned on and off (see Figures 28 and 29).
Figure 28: Threshold Layer Off
Figure 29: Threshold Layer On
As the Color Sampler is moved around, the Info palette indicates the tonal values of the location directly below the Color Sampler. Once the location is identified, the Color Sampler is clicked at this point. A color sampler is added to the image (see Figure 30).
Figure 30: Highlight Color Sampler Placed While Threshold is Closed

Setting the Highlight and Shadow Values

Once the color samplers have been placed on the image, the shadow and highlight values can be set in either Levels or Curves. The procedure for this is explained in Levels -- Part I and Curves -- Part II.

Figure 31 shows the original image. Figure 32 shows the image after the shadow and highlight values were set. Figure 33 shows the image after the editing was completed.

Figure 31: Unedited Image
Figure 32: Image with Shadows and Highlights Set
Figure 33: Final Image

Summary

The use of Threshold increases the ease with which the shadow and highlight values can be set in Curves.

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