When you want to quickly adjust the tonal curve of an image based on total luminance values—red, green, and blue channels combined—you use the Auto Curves Combined button. Red, green, and blue channels are adjusted by the same amount based on the total luminance. The Auto Curves Combined button corrects the contrast of the image without modifying its color cast.

When you want to automatically adjust the tonal curve of an image based on individual evaluations of the red, green, and blue channels, you use the Auto Curves Separate button. Red, green, and blue channels are adjusted based on an evaluation of each channel. The Auto Curves Separate button corrects the color cast in the image in addition to the contrast.

1. Select a photo.

2. If the Curves controls aren’t shown in the Adjustments inspector or the Adjustments pane of the Inspector HUD, choose Curves from the Add Adjustment pop-up menu.

3. In the Curves area, do one of the following:

   • To adjust the tonal curve of an image based on total luminance: Click the Auto Curves Combined button.

     

   • To adjust the tonal curve of an image based on an evaluation of each color channel: Click the Auto Curves Separate button.

     

The tonal curve of the image is adjusted. To fine-tune the tonal curve adjustment, see the following instructions.

You use the tonal curve in the Curves adjustment controls to adjust the overall tonality of an image. There are two types of tonal curve adjustments: RGB and Luminance. When you want to specify a tonal curve adjustment that modifies both the contrast and the tint in the image, you use the RGB tonal curve. When you want to adjust the overall tonality of an image without affecting its color cast, you use the luminance tonal curve.

As you perform a tonal curve adjustment, a histogram appears behind the tonal curve and is updated as you manipulate the tonal curve. You can specify the tonal range of the histogram to focus the graph on the part of the tonal curve you plan to work with. For example, if you plan to manipulate the contrast in the shadow areas of the image, you can limit the tonal range of the tonal curve and histogram to display pure black to 50 percent gray only. You can also extend the range of the histogram and tonal curve beyond pure white to capture highlight detail that would be clipped otherwise.

1. Select a photo.

2. If the Curves controls aren’t shown in the Adjustments inspector or the Adjustments pane of the Inspector HUD, choose Curves from the Add Adjustment pop-up menu.

   

3. Do one of the following:

   • To modify both the contrast and the tint in the image: Choose RGB from the Channel pop-up menu.

     Choosing the RGB tonal curve allows you to manipulate the tonal curve of the red, green, and blue color channels combined.

   • To manipulate the luminance of the combined red, green, and blue channels without affecting the tint: Choose Luminance from the Grayscale section of the Curves Action pop-up menu .

4. Choose a tonal range display option from the Range pop-up menu:

   • To set the histogram and tonal curve to display pure black to pure white: Choose Normal.

   • To expand the histogram view to display pure black to two times pure white: Choose Extended.

     Because some cameras are capable of capturing image information beyond pure white, using the Extended range option provides controls for bringing that information into the viewable image.

5. Choose the type of tonal curve adjustment to apply to the image from the Type section of the Curves Action pop-up menu.

   Choosing Linear—the default option—applies the Curves adjustment to the image evenly from pure black to pure white and beyond. Choosing Gamma-Corrected applies the Curves adjustment logarithmically to match the way the human eye perceives light, adding additional weight to the shadows.

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

   

7. To set a point along the curve, do one of the following:

   • Click the Add Point button and use the eyedropper tool to sample a portion of the image.

     

     Note: You can change the Loupe’s color value sample size by choosing an option from the Adjustment Action pop-up menu . For more information, see Sample color values and display camera information.

   • Click a place on the curve where you want to add a point.

     Aperture places a point on the curve in the exact tonal range of the sampled area of the image.

8. To adjust the shape of the curve, do one of the following:

   • Drag the point on the curve until that tonal area of the image looks correct.

     For example, dragging a point down in the shadow area of the curve darkens the shadows in the image, and dragging a point up in the shadow area lightens the shadows in the image.

   • Click a point on the curve and enter new In point and Out point values in the In and Out fields.

     The In point values represent the horizontal axis of the graph, and the Out point values represent the vertical axis of the graph.

   Points on the curve that are not selected remain anchored, allowing you to independently adjust each tonal area.

   

9. Repeat steps 7 and 8 for each tonal area in the image that you want to correct.

The tonal values in the image are adjusted, giving the image the appropriate amount of contrast.

You use the Black Point, Gray Point, and White Point eyedropper tools in the Curves adjustment controls when you want to have Aperture selectively modify the tonal values of the shadows, midtones, and highlights in an image automatically. Using an eyedropper tool, you sample the pixels of the tonal value in the image you intend to correct, and Aperture places a point on the tonal curve and automatically adjusts the image so that the pixels that fall in the same tonal range as the sampled pixels are set to either black, gray, or white. For example, you use the Black Point eyedropper tool to select the specific area of the image that should be black. Aperture samples the pixels within the selection, places a point on the curve, and then adjusts the curve to make the selected pixels—and all other pixels in the image of the same tonal value or darker—black.

In some difficult cases, you can use the Black Point, Gray Point, and White Point eyedropper tools in combination with points that you manually place on the tonal curve to neutralize a tint or create the specific contrast you want in the image.

1. Select a photo.

2. If the Curves controls aren’t shown in the Adjustments inspector or the Adjustments pane of the Inspector HUD, choose Curves from the Add Adjustment pop-up menu.

3. Do any of the following:

   • To set the black point in an image: Select the Black Point eyedropper tool to activate the Loupe, position the eyedropper over the darkest pixels in the image, and click.

     

     

   • To set the gray point in an image: Select the Gray Point eyedropper tool to activate the Loupe, position the eyedropper over a midrange color that is as close as possible to medium gray, and click.

     

     

   • To set the white point in an image: Select the White Point eyedropper tool to activate the Loupe, position the eyedropper over the lightest pixels in the image, and click.

     

     

   By default, the Loupe is set to magnify the image to 100 percent (full size). If necessary, you can increase the magnification of the Loupe by choosing an increased magnification level from the Loupe pop-up menu. For more information, see Loupe overview.

One of the most powerful ways to remove a color cast from an image or accentuate a desirable color cast is to adjust the tonal curves of the red, green, and blue color channels independently. 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 tonal curve of the red color channel when you want to remove red and cyan color casts in the image.

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

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

1. If the Curves controls aren’t shown in the Adjustments inspector or the Adjustments pane of the Inspector HUD, choose Curves from the Add Adjustment pop-up menu.

   

2. To adjust the tonal curve of the red color channel, choose Red from the Channel pop-up menu, place points on the curve, and adjust the points as necessary to remove or accentuate red and cyan color casts.

   For information about placing points on the curve, see “Manually adjust the tonal curve of an image,” above.

   You can adjust the red tonal curve in the following ways:

   • To add cyan to the shadows, removing red: Move the tonal curve down in the shadows.

   • To add red to the shadows, removing cyan: Move the tonal curve up in the shadows.

   • To add cyan to the midtones, removing red: Move the tonal curve down in the midtones.

   • To add red to the midtones, removing cyan: Move the tonal curve up in the midtones.

   • To add cyan to the highlights, removing red: Move the tonal curve down in the highlights.

   • To add red to the highlights, removing cyan: Move the tonal curve up in the highlights.

3. To adjust the tonal curve of the green color channel, choose Green from the Channel pop-up menu, place points on the curve, and adjust the points as necessary to remove or accentuate green and magenta color casts.

   You can adjust the green tonal curve in the following ways:

   • To add magenta to the shadows, removing green: Move the tonal curve down in the shadows.

   • To add green to the shadows, removing magenta: Move the tonal curve up in the shadows.

   • To add magenta to the midtones, removing green: Move the tonal curve down in the midtones.

   • To add green to the midtones, removing magenta: Move the tonal curve up in the midtones.

   • To add magenta to the highlights, removing green: Move the tonal curve down in the highlights.

   • To add green to the highlights, removing magenta: Move the tonal curve up in the highlights.

4. To adjust the tonal curve of the blue color channel, choose Blue from the Channel pop-up menu, place points on the curve, and adjust the points as necessary to remove or accentuate blue and yellow color casts.

   You can adjust the blue tonal curve in the following ways:

   • To add yellow to the shadows, removing blue: Move the tonal curve down in the shadows.

   • To add blue to the shadows, removing yellow: Move the tonal curve up in the shadows.

   • To add yellow to the midtones, removing blue: Move the tonal curve down in the midtones.

   • To add blue to the midtones, removing yellow: Move the tonal curve up in the midtones.

   • To add yellow to the highlights, removing blue: Move the tonal curve down in the highlights.

   • To add blue to the highlights, removing yellow: Move the tonal curve up in the highlights.