Working with the Levels Controls

You use the Levels adjustment controls when you want to manually set the tonal values of the shadows, midtones, and highlights in an image. By default, Aperture sets 0 as pure black and 1 as pure white. Constraining the white and black point values evenly redistributes the tonal range of the pixels between black and white. Reapportioning the luminance values increases the tonal range and contrast in the image. However, if you clip too far into either the black or white tonal values, tonal values that were originally near pure black and pure white are now changed to pure black and pure white, resulting in lost image detail.

In addition to adjusting the tonal values of an image for overall shadows, midtones, and highlights, you can also color correct an image by adjusting the levels of the red, green, and blue channels independently. Aperture provides a way to adjust levels for each color channel, allowing you to control the color tonality in an image.

You can also brush the Levels adjustment on selected parts of an image. For more information, see Making Brushed Adjustments.

About Working with Auto Levels Controls

You use the Auto Levels buttons in the Levels adjustment to analyze the image and adjust the levels based on that analysis. This is a good way to make a quick correction to an image before fine-tuning the Levels adjustment with the manual adjustment controls.

There are two ways to perform an Auto Levels adjustment. For more information, see the following sections:

You can also adjust the tolerance of the white and black clipping points of the Auto Levels adjustment. For more information, see Setting the Auto Levels Options.

Using the Auto Levels Combined Button to Adjust Combined Luminance

When you want to quickly adjust the levels of an image based on total luminance values—red, green, and blue channels combined—you choose Luminance from the Channel pop-up menu in the Levels adjustment and then click the Auto Levels Combined button. Red, green, and blue channels are adjusted by the same amount based on the total luminance. You use the Auto Levels Combined button for luminance when you want to correct the contrast of the image without modifying the color cast of the image.

Figure. Image before and after an Auto Levels Combined adjustment.
To automatically adjust the levels of an image based on total luminance
  1. Select an image.

  2. In the Levels area of the Adjustments inspector or the Adjustments pane of the Inspector HUD, choose Luminance from the Channel pop-up menu, then click the Auto Levels Combined button.

    Figure. Auto Levels Combined button in the Adjustments inspector.

The levels of the image are automatically adjusted. To fine-tune the Levels adjustment, see Adjusting the Luminance Levels in an Image.

Using the Auto Levels Separate Button to Adjust RGB Channels Individually

When you want to automatically adjust the levels of an image based on individual evaluations of the red, green, and blue channels, you use the Auto Levels Separate button in the Levels adjustment. Red, green, and blue channels are adjusted based on the evaluation of each channel. You use the Auto Levels Separate button when you want to correct the color cast in the image in addition to correcting the contrast.

Figure. Image before and after an Auto Levels Separate adjustment.
To automatically adjust the levels based on an evaluation of each color channel
  1. Select an image.

  2. In the Levels area of the Adjustments inspector or the Adjustments pane of the Inspector HUD, choose RGB from the Channel pop-up menu, then click the Auto Levels Separate button.

    Figure. Auto Levels Separate button in the Adjustments inspector.

The levels of the image are automatically adjusted for each color channel. To fine-tune the levels adjustment, see Adjusting the Luminance Levels in an Image.

Setting the Auto Levels Options

Aperture provides the option to set both the black and white clipping point parameters for the Auto Levels adjustments. Modifying the black and white clipping points from their default value of 0% expands the tonal range of the image. One reason for expanding the tonal range of an image is to preserve shadow and highlight detail when the image is printed.

Note: You may have to perform a few print tests to determine the output characteristics of your printer before settling on final values for black and white clipping points.

To adjust the black clipping point
  1. Choose Aperture > Preferences, then click Advanced.

    Figure. Auto Adjust Black Clip controls in the Advanced pane of the Aperture Preferences window.
  2. In the Advanced pane, do one of the following:

    • By default, the Auto Adjust Black Clip slider is set to the left side of the slider control. Drag the slider to the right to increase the tolerance of Auto Levels adjustments to colors beyond the analyzed black point in the image, and drag the slider back to the left to reduce the tolerance.

    • By default, the Auto Adjust Black Clip value slider is set to 0.10%. Double-click the number in the value slider, then enter a value from 0% to 10% and press Return.

    • Click the left or right arrow in the value slider to change the tolerance of Auto Levels adjustments to colors beyond the analyzed black point in the image, or drag in the value field.

      Clicking the left arrow reduces the tolerance, and clicking the right arrow increases it.

  3. Once the black clipping point is adjusted, click either the Auto Levels Combined or the Auto Levels Separate button in the Levels area of the Adjustments inspector or the Adjustments pane of the Inspector HUD to view the effect on the image.

To adjust the white clipping point
  1. Choose Aperture > Preferences, then click Advanced.

    Figure. Auto Adjust White Clip controls in the Advanced pane of the Aperture Preferences window.
  2. In the Advanced pane, do one of the following:

    • By default, the Auto Adjust White Clip slider is set to the left side of the slider control. Drag the slider to the right to increase the tolerance of Auto Levels adjustments to colors beyond the analyzed white point in the image, and drag the slider back to the left to reduce the tolerance.

    • By default, the Auto Adjust White Clip value slider is set to 0.10%. Double-click the number in the value slider, then enter a value from 0% to 10% and press Return.

    • Click the left or right arrow in the value slider to change the tolerance of Auto Levels adjustments to colors beyond the analyzed white point in the image, or drag in the value field.

      The left arrow reduces the tolerance, and the right arrow increases it.

  3. Once the white clipping point is adjusted, click either the Auto Levels Combined or the Auto Levels Separate button in the Levels area of the Adjustments inspector or the Adjustments pane of the Inspector HUD to view the effect on the image.

Adjusting the Luminance Levels in an Image

When you want to adjust the overall tonality of an image, you use the Levels adjustment controls based on a histogram that shows luminance. When the histogram is set to show luminance, it displays the cumulative brightness values for all three color channels for each pixel. You use the histogram’s luminance view when you want to adjust an image’s tonal values without affecting the color cast.

Figure. Image before and after a Levels adjustment.
To adjust levels in an image based on luminance
  1. Select an image.

  2. By default, the Levels histogram is off. Turn it on by selecting the Levels checkbox in the Levels area of the Adjustments inspector or the Adjustments pane of the Inspector HUD.

    The Levels histogram appears.

    Figure. Controls in the Levels area of the Adjustments inspector.
  3. Choose Luminance from the Channel pop-up menu.

  4. Do one of the following:

    • Drag the Black Levels and White Levels sliders to where they touch the outside of the histogram graph, constraining the image to its new black and white points.

      Figure. Black Levels and White Levels sliders placed at the edges of the luminance histogram graph in the Levels area of the Adjustments inspector.
    • Select the numbers in the Black (B) and White (W) fields, then enter a value from 0.00 to 1.00. By default, the black point is set to 0.00, and the white point is set to 1.00.

The tonal values for shadows and highlights in the image are updated, resulting in more defined blacks and highlight values as well as increased overall contrast.

You can also use the Gray Levels slider if you need to adjust the brightness values of the midtones while limiting the brightening effect in the black and white points. The brighter areas of shadows and the darker areas of highlights are affected, but the effect tapers off as it nears the dark blacks and bright whites.

Figure. Image before and after a Gray Levels adjustment.
To adjust the midtone levels in an image
Do one of the following:
  • Drag the Gray Levels slider until the brightness values in the image’s midtones are correct.

    Figure. Gray Levels slider moved to the dark side of the midtones of the luminance histogram graph.
  • By default, the gray point is set to 0.50. Select the number in the Gray (G) field, then enter a value from 0.02 to 0.98 to correct the brightness values in the image’s midtones.

Showing Quarter-Tone Levels Controls

When you need additional control over tonal values between the midtones and shadows as well as the midtones and highlights, you use the Quarter-Tone Levels controls in the Levels area of the Adjustments inspector or the Adjustments pane of the Inspector HUD. For example, if you need to add contrast to the midtone values independently of the black and white point settings, you use the Quarter-Tone Levels sliders. Unlike the Black Levels and White Levels sliders, Quarter-Tone Levels sliders are independent and affect only the tonal range of the pixels they represent: either shadows to midtones or midtones to highlights.

To show the Quarter-Tone Levels controls
  • Click the Quarter-Tone Controls button.

    Figure. Quarter-Tone Levels controls in the Levels area of the Adjustments inspector.

Adjusting the Brightness of an Image

You can also use the Levels adjustment controls to adjust the brightness of an image. When you adjust the brightness of the image, the Black Levels, Gray Levels, and White Levels sliders modify the tonality of the image based on the new brightness value.

Adjusting Brightness Uniformly with the Brightness Levels Slider

You use the center Brightness Levels slider at the top of the Levels histogram to uniformly adjust the brightness of the image.

To uniformly adjust the brightness of the image
  • In the Levels area of the Adjustments inspector or the Adjustments pane of the Inspector HUD, drag the Brightness Levels slider until the brightness of the entire image is correct.

    Figure. Brightness Levels slider at the top of the histogram in the Levels area of the Adjustments inspector.

Selectively Adjusting Brightness with the Shadow Brightness Levels and Highlight Brightness Levels Sliders

You can selectively adjust the brightness of the shadows and highlights in an image using the Shadow Brightness Levels and Highlight Brightness Levels sliders.

To selectively adjust the brightness of the shadows and highlights in an image
  • Drag the Shadow Brightness Levels and Highlight Brightness Levels sliders until the brightness values of the shadows and highlights in the image are correct.

    Figure. Shadow Brightness Levels and Highlight Brightness Levels sliders at the top of the histogram in the Levels area of the Adjustments inspector.

Using Levels for Color Correction

A powerful way to color correct an image is to adjust the levels of each color channel in the image. Adjusting the levels of each color channel is useful for removing color casts in an image. It’s important to understand that you’re adjusting the colors of the image within the RGB spectrum; therefore, you reduce yellow when you increase blue, you reduce magenta when you increase green, and so on.

To adjust the levels of a single color channel in an image
  1. In the Levels area of the Adjustments inspector or the Adjustments pane of the Inspector HUD, choose a color channel from the Channel pop-up menu.

    Figure. Red channel histogram in the Levels area of the Adjustments inspector.
  2. Drag the Black Levels, Gray Levels, and White Levels sliders (or enter values in the corresponding fields) to create the effect you want.

Adjusting the Levels of the Red Color Channel

You adjust the levels of the red color channel when you want to remove red and cyan color casts in the image.

Figure. Image before and after a Red Levels adjustment.
Red adjustment
Effect
Move Black Levels slider right
Adds cyan to the shadows, removing red.
Move Gray Levels slider left
Adds red to the midtones, removing cyan.
Move Gray Levels slider right
Adds cyan to the midtones, removing red.
Move White Levels slider left
Adds red to the highlights, removing cyan.

Adjusting the Levels of the Green Color Channel

You adjust the levels of the green color channel when you want to remove green and magenta color casts in the image.

Figure. Image before and after a Green Levels adjustment.
Green adjustment
Effect
Move Black Levels slider right
Adds magenta to the shadows, removing green.
Move Gray Levels slider left
Adds green to the midtones, removing magenta.
Move Gray Levels slider right
Adds magenta to the midtones, removing green.
Move White Levels slider left
Adds green to the highlights, removing magenta.

Adjusting the Levels of the Blue Color Channel

You adjust the levels of the blue color channel when you want to remove blue and yellow color casts in the image.

Figure. Image before and after a Blue Levels adjustment.
Blue adjustment
Effect
Move Black Levels slider right
Adds yellow to the shadows, removing blue.
Move Gray Levels slider left
Adds blue to the midtones, removing yellow.
Move Gray Levels slider right
Adds yellow to the midtones, removing blue.
Move White Levels slider left
Adds blue to the highlights, removing yellow.