Adjust the levels of an image

You can use the Levels adjustment controls 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.

To simplify your workflow, you can use the Auto Levels buttons in the Levels adjustment to automatically 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 automatic levels adjustment. When you want to correct an image’s contrast without modifying its color cast, you use the Auto Levels Combined button to adjust levels based on the total luminance of the combined red, green, and blue color channels.

Figure. Image before and after an Auto Levels Combined adjustment.

When you want to correct the image’s color cast in addition to its contrast, you use the Auto Levels Separate button to adjust levels based on an evaluation of each channel.

Figure. Image before and after an Auto Levels Separate adjustment.

You can also adjust the tolerance of the black and white clipping points of the Auto Levels adjustment.

In addition to adjusting the tonal values of an image for overall shadows, midtones, and highlights, you can 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 specific areas of an image. For more information, see Apply brushed adjustments.

Adjust the levels of an image automatically

Select a photo.
In the Adjustments inspector or the Adjustments pane of the Inspector HUD, choose Levels from the Add Adjustment pop-up menu.
Do one of the following:
- To adjust levels based on the total luminance of the red, green, and blue channels combined: In the Levels area of the Adjustments inspector, choose Luminance from the Channel pop-up menu, and click the Auto Levels Combined button.
- To adjust levels based on an evaluation of each color channel: Choose RGB from the Channel pop-up menu, and click the Auto Levels Separate button.

To fine-tune the automatic levels adjustment, see “Manually adjust levels in an image based on luminance,” below.

Adjust black and white clipping points for Auto Levels adjustments

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.

Choose Aperture > Preferences, and click Advanced.
In the Advanced pane, specify the following settings:
- To adjust the black clipping point: Use the “Auto adjust Black Clip” parameter controls.
  Dragging the slider to the right increases the tolerance of Auto Levels adjustments to colors beyond the analyzed black point in the image.
- To adjust the white clipping point: Use the “Auto adjust White Clip” parameter controls.
  Dragging the slider to the right increases the tolerance of Auto Levels adjustments to colors beyond the analyzed white point in the image.
After the black and white clipping points are adjusted, click either the Auto Levels Combined or 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.

Manually adjust levels in an image based on luminance

To manually adjust the overall tonality of an image without affecting its color cast, 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.

Figure. Image before and after a Levels adjustment.

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 dark blacks and bright whites.

Figure. Image before and after a Gray Levels adjustment.

Select a photo.
In the Adjustments inspector or the Adjustments pane of the Inspector HUD, choose Levels from the Add Adjustment pop-up menu.
To display the Levels histogram, select the Levels checkbox in the Levels area of the Adjustments inspector or the Adjustments pane of the Inspector HUD.
Choose Luminance from the Channel pop-up menu.
Do either or both of the following:
- To constrain the image to its new black and white points: Drag the Black Levels and White Levels sliders to where they touch the outside of the histogram graph.
  You can also select the numbers in the Black (B) and White (W) fields, and 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.
- To adjust the image’s brightness values: Drag the Gray Levels slider until the image’s midtones are correct.
  You can also select the number in the Gray (G) field, and enter a value from 0.02 to 0.98 to correct the brightness values in the image’s midtones. By default, the gray point is set to 0.50.

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.

Show the Quarter-Tone Levels controls

When you need additional control over tonal values between the midtones and shadows as well as the midtones and highlights (for example, to add contrast to the midtone values independently of the black and white point settings), you use the Quarter-Tone Levels controls. Unlike the Black Levels and White Levels sliders, the Quarter-Tone Levels sliders affect only the tonal range of the pixels they represent: either shadows to midtones or midtones to highlights.

In the Levels area of the Adjustments inspector or the Adjustments pane of the Inspector HUD, click the Quarter-Tone Controls button.

Adjust the brightness of an image

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

To adjust the brightness of the image uniformly: 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.
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.

Use levels for color correction

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

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.

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.

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.

If the Levels controls aren’t shown in the Adjustments inspector or the Adjustments pane of the Inspector HUD, choose Levels from the Add Adjustment pop-up menu.
To adjust the levels of the red color channel, choose Red from the Channel pop-up menu, and do any of the following:
- To add cyan to the shadows, removing red: Drag the Black Levels slider to the right.
- To add red to the midtones, removing cyan: Drag the Gray Levels slider to the left.
- To add cyan to the midtones, removing red: Drag the Gray Levels slider to the right.
- To add red to the highlights, removing cyan: Drag the White Levels slider to the left.
To adjust the levels of the green color channel, choose Green from the Channel pop-up menu, and do any of the following:
- To add magenta to the shadows, removing green: Drag the Black Levels slider to the right.
- To add green to the midtones, removing magenta: Drag the Gray Levels slider to the left.
- To add magenta to the midtones, removing green: Drag the Gray Levels slider to the right.
- To add green to the highlights, removing magenta: Drag the White Levels slider to the left.
To adjust the levels of the blue color channel, choose Blue from the Channel pop-up menu, and do any of the following:
- To add yellow to the shadows, removing blue: Drag the Black Levels slider to the right.
- To add blue to the midtones, removing yellow: Drag the Gray Levels slider to the left.
- To add yellow to the midtones, removing blue: Drag the Gray Levels slider to the right.
- To add blue to the highlights, removing yellow: Drag the White Levels slider to the left.