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magic mid-tones tutorial



Mask Subtraction

While the Basic Mid-tones mask is symmetrical with respect to light and dark tones, mask subtraction opens up the possibility of changing the position of the mid-tones on the tonal spectrum. Subtraction is when the pixels from one selection are subtracted from the pixels of another selection. Photoshop keeps track of and recalculates the percentage that each pixel is selected during the subtraction operation, so, as with the original masks, adjustments filtered by the new selection are perfectly feathered and blend seamlessly into the image. The process for subtraction is as follows:

  1. Create masks of the desired selections on the Channels palette.
  2. Ctrl+click on the thumbnail for the first mask in order to load it as a selection (Mac: Command+click).
  3. Then Alt+Ctrl+click on the second mask to subtract it's pixels from those already selected. (Mac: Option+Command+click).

What results is a new selection that is mid-tones-like in that it's feathered at both ends but it doesn't have to lie in the middle of the tonal spectrum. A couple examples will help illustrate how subtraction works.

Figure 9 is an image where I felt that some of the light tones, mostly the whites in the rock, were too light. The histogram showed the brightest whites were just fine sitting somewhere around 230. For white rock in bright sun, that would be expected and would print OK. The goal was to darken the not-quite-so-bright whites, leaving the brightest values mostly unaffected in order to maintain good contrast in the highlights. The rollover shows the image after the adjustment.

Figure 9
Figure 9

In this case the Light Lights and Super Lights masks were the two masks I needed to use. When doing subtraction, it's very important to load the correct mask first, and that mask is the one that contains the greatest range of tonal values, which in this case was the Light Lights mask. Once loaded, the Super Lights was subtracted from it using the procedure listed above.

The mask for this adjustment (Light Lights minus Super Lights) is shown in Figure 10. While it's difficult to see here, in extreme magnification, the lightest pixels in the image are very dark which is expected after the subtraction of the Super Lights. This means these very lightest tones will hardly be affected by the adjustment, which is exactly what was intended. The rollover shows a selection of just the Light Lights (no subtraction of the Super Lights). This makes it a little clearer how the subtraction of the Super Lights mutes the adjustment on the lightest pixels. As the image's lighter pixels get darker in the mask, they will be affected less by the adjustment.

Figure 10
Figure 10

Figure 11 shows the Curves adjustment that was applied through the subtracted mask. Nothing too severe since the mask had properly targeted the necessary tones. Note that there is slight S-shape to the curve. Doing this through the targeted tones helps to insure that contrast is maintained or increased.

Figure 11
Figure 11

Figure 12 shows the selection graphically on the tonal spectrum. The top is the tonal spectrum. In the middle, the Light Lights selection was filled with yellow. In the bottom section, the Super Lights were subtracted from the Light Lights and the resulting selection filled with yellow. A couple of things are clearly visible here. One is that the brightest white values are nearly unaffected after subtraction; they're still very white with very little yellow. That's exactly what was desired with the subtraction of the Super Lights, so the selection is properly targeting the desired tones. The second is the dual feathering of the selection at both the light and dark ends after subtraction. Even though this mask is shifted to the light end of the tone spectrum, these feathered ends have a mid-tones-like quality and insure that the selection blends perfectly into the other tones in the image.

Figure 12
Figure 12

Figure 13 below had a different problem. The image is too light and needs more substantial dark tones, so the problem is how to adjust them. This was a film image and, after scanning and a few adjustments, some tones in the rock cracks were already pure black (0/0/0) and some were getting close to it (the bushes on the rock slope). While I wanted to darken the dark values, I also wanted the adjustment to spare the darkest tones in the image. The rollover shows the results.

Figure 13
Figure 13

Subtraction was again used to craft the appropriate mask. In this case it turned out that subtracting the Dark Darks from the Expanded Darks selected the appropriate tones to accomplish the goal. With this mask, the darker tones in the image were appropriately darkened but the darkest tones were mostly unaffected. Once again the mask helps illustrate what is happening here. Figure 14 shows the mask that resulted after subtraction. The rollover is the mask of just the Expanded Darks (no Dark Darks subtracted). It shows that the blacks in the image are nearly white in the Expanded Darks mask meaning that the darkest tones in the image would be fully affected by the adjustment. The goal, however, was to spare the darkest areas of the image from going darker with the adjustment, so this is a problem. By subtracting the Dark Darks from the Expanded Darks, the resulting mask (no rollover) shows that the dark areas of the image are now also dark in the mask and therefore more shielded from the darkening adjustment. This is exactly what was desired. The subtracted mask provided a useful way to target the dark tones while leaving the very darkest tones unaffected.

Figure 14
Figure 14

Figure 15 shows the Curves adjustment that was used with the mask. An S-curve was not needed in this case to maintain or improve contrast. Darkening through Dark-series masks insures an increase in contrast as will be explained in the next section.

Figure 15
Figure 15

Figure 16 below illustrates the subtraction using the gradient image. The middle section is the Expanded Darks filled with yellow. In the bottom section the Dark Darks are subtracted from the Expanded Darks and the resulting selection filled with yellow. This more graphically portrays the tones that were adjusted in the image and how subtraction produced a mid-tones-like selection with the characteristic dual-tapering sides. This time it's slightly shifted to the darker end of the tonal spectrum.

Figure 16
Figure 16

Summary: Since there are many luminosity masks, there are also many ways to combine them to target specific tones. Subtraction is a good way to create mid-tone-like, dual-tapering masks in different parts of the tonal spectrum. These masks work well for making adjustments that enhance or maintain image contrast.