Photoshop Levels
Input Levels Section

 

Stretching Image Tonal Range

The Input Levels section is used to lighten highlights, darken shadows, reset mid tone and bring out, or lose, detail.  To darken shadows, we move the black slider inward or type a number larger than 0 in the left most text box.  To lighten highlights, we move the white slider inward or type a number smaller than 255 in the right most text box.  To reset mid tone, move the gray slider, known as the gamma slider, to the left or right.  When these adjustments are made, Adobe Photoshop remaps the image's pixels from their current tone to a new tone.

When we stretch an image's tonal range, we are bringing out subtle differences in pixels, thus increasing apparent detail.  However, if we stretch it too much, we will begin to clip details in the highlights and shadows.

The Input Levels section remaps tones in a nonlinear manner.  This means the closer a pixel is to the change the more it will be affected and the further away it is from the change the less it will be affected.  Other Photoshop controls, such as Hue/Saturation, remap tone in a linear manner.  This means all pixels are affected by the same amount when we apply a change.  If we wish to apply the same change to all pixels, then a linear control, such as Hue/Saturation, would be appropriate.  However, most photographers find themselves using a non-linear control, such as Levels and Curves, more often.

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Default Input Levels Settings

To see how Input Levels works, lets use three color chips, each with a different tone, and apply different Input Levels adjustments to it.

We will find the three color chips in Figure 2.  They are to the left of the Levels dialog box.  The dark gray chip has a tone of 64.  The medium gray chip has a tone of 127.  The light gray chip has a tone of 192.  Look at the histogram in Figure 2.  It is a very simple, three vertical bar histogram.  The vertical bar between the black and gray sliders represents the dark gray chip.  The vertical bar over the gray slider is the medium gray chip.  The vertical bar between the gray and white sliders is the light gray chip.  Remember, the tones of these chips are 64, 127 and 192.

The blue arrows show the relationship between the Input Levels text boxes and the Input Levels sliders.  The Input Levels box on the left represents the black slider.  The Input Levels box in the middle represents the gray gamma slider.  The Input Levels box on the right represents the white slider.

The gray and white shaded areas show the relationship between the image's tonal range and the digital tonal range.  In Figure 2, the shaded areas show us the default relationship is 1:1.  This means both the Input and Output black points map to tone 0 and the Input and Output white points map to tone 255 and mid tone maps to mid tone.

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Black Input Level Slider

We use the Input Levels black slider to tell Photoshop where solid black begins in our image.  It defaults to 0.  If we change 0 to a higher value, then this value becomes the starting point of black and any image pixels whose tone falls between 0 and this new value will become solid black.

In Figure 3, we will make an adjustment using the black Input Level slider.  We will reposition the black slider to be underneath the vertical bar for the dark gray chip.  The left most Input Levels box that said 0 now says 64.  This causes the image's tonal range to stretch so that solid black is now anything between 0 and 64, inclusive.  This causes the dark gray chip to become solid black.

To compare this change to the original image, mouse over Figure 3.  The red arrows represent the controls that changed.  The gray and white shaded areas show the new relationship between image and digital tonal range.

When the black slider was moved, Photoshop automatically repositioned the mid tone, or gamma, slider so that it stays half way between the black and white sliders.  This caused the other two chips to become darker.  The middle gray chip became darker because it now falls between mid tone and black.  The light gray chip also became darker because it is now closer to mid tone.  But the change in these two chips is not as drastic as the dark gray chip becoming solid black.  This is because the Levels control is a non-linear adjustment.  This means the further away a pixel is from the change being made, the less impact the change will have on the pixel.

 

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White Input Level Slider

We use the Input Levels white slider to tell Photoshop where tone 255 begins in our image.  It defaults to 255.  If we change 255 to a lower value, then this value becomes the starting point of  tone 255 and any image pixels whose tone falls between the new value and 255 will become tone 255.

In this example, we will use the white Input Level slider by repositioning it to be right underneath the vertical bar for the light gray chip.  The right most Input Levels box that said 255 now says 192.  This causes the image's tonal range to stretch so that solid white is now anything between 192 and 255, inclusive.  This causes the light gray chip to become solid white.  To compare this change to the original image, mouse over Figure 4.

Once again, when the white slider was moved, Photoshop automatically repositioned the gamma slider.  This caused the other two chips to become lighter.  The middle gray chip became lighter because it now falls between mid tone and white.  The dark gray chip also became lighter because it is now closer to mid tone.

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Mid Tone, or Gamma, Slider Right

The gray slider in the Input Levels section allows us to adjust our image's mid tone values.  On the digital tonal scale, mid tone is 127 (128 is also valid).  However, the number we see in the middle text box of Input Levels is 1.0, not 127.  1.0 is a gamma value, not a tonal value.  Gamma controls the overall brightness of an image.

By moving the gamma slider, or by changing the gamma value to a number other than 1, we are telling Photoshop to reset mid tone in our image.

In Figure 5, we moved the gamma slider to the right to be underneath the vertical bar for the light gray chip.  This is the vertical bar on the right.  When we did this, we told Photoshop to reset mid tone from 127 to 192.  Therefore, any pixels whose tone was formerly between 127 and 192, which is normally lighter than mid tone, is now darker than mid tone.  To compare this change to the original image, mouse over Figure 5.

When the gamma slider is moved to the right, more image pixels will be placed between mid tone and black.  In addition, the remaining light toned pixels are shifted closer to mid tone.  Thus, more image pixel tones are becoming darker than originally.  Therefore, the overall image becomes darker.  So even though we are moving the mid tone slider to the white end, the image becomes darker.

As we can see by the shaded areas in Figure 5, we are compressing the image's now much larger darker-than-mid tone tonal range.  And we are stretching the image's now much smaller lighter-than-mid tone tonal range.  The compression of the darker pixels can cause a loss of detail in this area.  The stretching of the lighter pixels increases separation which can increase apparent detail.

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Gamma Slider Left

In Figure 6, we moved the gamma slider to the left to be underneath the vertical bar for the dark gray chip.  This is the vertical bar on the left.  When we did this, we told Photoshop to reset mid tone from 127 to 64.  Therefore, any pixels whose tone was formerly between 64 and 127, which is normally darker than mid tone, is now lighter than mid tone.  To compare this change to the original image, mouse over Figure 6.

When the gamma slider is moved to the left, more image pixels will be placed between mid tone and white.  In addition, the remaining dark toned pixels are shifted closer to mid tone.  Thus, more image pixel tones are becoming lighter than originally.  Therefore, the overall image becomes lighter.  So even though we are moving the mid tone slider to the black end, the image becomes lighter.

As we can see by the shaded areas in Figure 6, we are compressing the image's now much larger lighter-than-mid tone tonal range.  And we are stretching the image's now much smaller darker-than-mid tone tonal range.  The compression of the lighter pixels can cause a loss of detail in this area.  The stretching of the darker pixels increases separation which can increase apparent detail.

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Increase Contrast

One of the most popular Levels adjustment is to reposition the Input Levels black and white sliders to be just outside the histogram.  This adjustment maximizes contrast and separation in both the shadow and highlight areas, which increases apparent detail.  However, if any of the pixels that get remapped to solid black or solid white had any detail in them, this detail will be lost when we make them solid black and white.

Figure 7 shows such an adjustment.  The black slider was moved underneath the dark gray chip and the white slider was moved underneath the light gray chip.  This makes the darkest pixels black and the lightest pixels white.  Mouse over Figure 7 to see before and after images.

If we move a slider past any pixels on the histogram, we have moved their tone beyond the effective boundaries of the tonal range.  Therefore, the tone of these pixels will become solid black or solid white depending on which slider we are moving.