Photoshop Levels
Output Levels section

 

Compress And Shift Tonal Range

We saw with Input Levels that we can remap tone to make our shadow areas darker, our highlights lighter and increase contrast.  But is there a way to lighten shadows, darken highlights and decrease contrast?  Yes.  That is one of the uses of the Output Levels section of the Levels dialog box.

The digital tonal range is defined in the Output Levels section.  The Output Levels section not only defines the digital tonal range, but it allows us to shorten it.  By shortening the tonal range, we are forcing image pixels to look more alike, thus decreasing contrast. 

The Output Levels sliders do not reset solid black and solid white to new values in the digital tonal range.  Instead, they remove solid black and white by changing where the digital tonal range begins and ends.  For example, the full tonal range of 0 - 255 can be changed to 10 - 240 so no image pixel will be darker than tone 10 nor lighter than tone 240.  With Input Levels, we stretch the image's tonal range to fit more of the full digital tonal range.  With Output Levels, we force the image's tonal range to fit into a newly defined and smaller, digital tonal range.

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Black Output Level Slider

When we shorten the digital tonal range, Adobe Photoshop automatically compresses the image's tonal range to fit.  By compressing the image's tonal range, we are forcing pixel tones to be closer together.  This causes a reduction in contrast.

In Figure 2, we changed the value of the black Output Level to 64, which is the tone of the left color chip.  When we do this we are not remapping the dark gray chip to black.  That is what we did using the Input Levels control.  We are remapping the image's tonal range so no pixel is darker than tone 64.

With the Output Levels control we are truncating the digital tonal range.  Photoshop will then automatically compress and shift the image's tonal range to fit the shortened digital tonal range.  Because the image's tonal range is being compressed, contrast will decrease and detail could be lost.  In this example, the image's tonal range was shifted to the right (towards white).  Thus, the overall image became lighter.  By mousing over Figure 2, we can see the before and after images.

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White Output Level Slider

In Figure 3, we changed the value of the white Output Level to 192,which is the tone of the right color chip.  Again, we are not remapping our image's light gray chip to white.  We are truncating the digital tonal range so no tone is lighter than tone 192.

Photoshop automatically compressed and shifted our image's tonal range to fit the narrower digital tonal range.  Because it is being compressed, contrast will decrease and detail could be lost.  Because it was shifted to the left (towards black), the image's tones became darker.  Mouse over Figure 3 to see the before and after effects.

 

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Decrease Contrast

Now we are going to use the Output Levels control to reduce the image's overall contrast by moving both the black and white sliders.  For demonstration purposes, we will move the black slider to 96 (which is halfway between the left, dark gray color chip and the middle color chip) and move the white slider to 160 (which is halfway between the right, light gray color chip and the middle color chip).

As can be seen in Figure 4, contrast has been substantially reduced.  We can mouse over Figure 4 to see the before and after effects.  Note that even though the black slider was moved well past 64 and the white slider was moved well beyond 192, the dark gray chip did not become black and the light gray chip did not become white.  With the Output Levels control, we are truncating the digital tonal range and forcing the image tonal range to fit.  In fact, with the adjustment shown in Figure 4, no image could have solid black or solid white because we are telling Photoshop the darkest an image tone can be is 96 and the lightest an image tone can be is 160.

Since we moved the black and white sliders the same amount, we did not shift the tonal range, but forced it to compress around mid tone.

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Input Levels vs. Output Levels

Still a little unsure of the difference between Input Levels and Output Levels?  Both cause a remapping of the image's tonal range.  Input Levels is used to reset the black, mid tone and white points of the image's tonal range to fit more of the digital tonal range.  Output Levels resets the beginning and end points of the digital tonal range to something smaller than 0 - 255.   When Output Levels is used to redefine the digital tonal range, Photoshop will remap the image's tonal range so that no image tone will exist beyond the Output Levels settings.

Lets take a look at two more examples.  Table 1 below uses a gradient to demonstrate the Input Levels.

Table 1. Input Levels

The gradient to the right goes from dark gray on the left to light gray on the right.  The dark gray is tone 77 and the light gray is tone 204.  Therefore, the gradient's tone goes from 77 to 204.	Take a dark gray-light gray gradient...
An Input Levels adjustment was applied to the gradient.  The Input Levels black slider was moved inward until its value became 77.  We are telling Photoshop to remap tone 77 to tone 0.  In other words, 77 is now 0. The white Input Levels slider was moved inward until its value became 204.  We are telling Photoshop to remap tone 204 to 255.  In other words, 204 is now 255. We are stretching the image's tonal range so that it fills the entire 0 - 255 digital tonal range.	... apply an Input Levels adjustment...
Specifically, we told Photoshop to stretch the image's tonal range until tone 77 became tone 0 and tone 204 became tone 255.  As we know, tone 0 is solid black and tone 255 is solid white.  The result of this adjustment can be seen to the right.	... to create a black-white gradient...
The resulting histogram can be seen on the right.  As we can see, the original tonal range, which went from 77 to 204, now stretches from 0 to 255.	... to fill the digital tonal range.

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In Table 2 we will use another gradient to demonstrate Output Levels.

Table 2.  Output Levels

The gradient to the right goes from solid black on the left to solid white on the right.  Therefore, the gradient's tone goes from 0 to 255.	Take a black-white gradient...
The Output Levels black slider was moved inward until its value became 77.  We are telling Photoshop that tone 77 is the darkest tone allowed.  Tones 0 - 76 are no longer allowed.  All image pixels whose tone was 0, now have a tone of 77. The white Output Levels slider was moved inward until its value became 204.  We are telling Photoshop that tone 204 is the lightest tone allowed.  Tones 205 - 255 are no longer allowed.  All image pixels whose tone was 255, now have a tone of 204. We are taking the image's tonal range of 0 to 255 and compressing it so that it fits a tonal range of 77 to 204.  No tone can be darker than 77 and no tone can be lighter than 204.	... apply an Output Levels adjustment...
The result of this adjustment can be seen to the right.  No tone in the adjusted gradient is darker than 77 and no tone is lighter than 204.	... to compress to a narrower tonal range...
The resulting histogram can be seen on the right.  As we can see, the original image's tonal range has been compressed to fit the new digital tonal range.	... thus removing solid blacks and whites.