Using Curves In Photoshop-- Part I

Article and Photography by Ron Bigelow

www.ronbigelow.com

Photoshop CS or Photoshop CS2 Used in this Tutorial

When it comes to tonal and color adjustments, most photographers start out their learning with Levels. The advantage of Levels is that it is easy to learn and use. However, eventually, most photographers progress to the point where they want more control over their image editing than Levels can provide. In that case, the next step is Curves. Curves can do most everything that Levels can, but it provides photographers with much more power in the area of tonal and color control. While Levels is limited to simple adjustments of the midtone contrast through the use of the Gamma Input Slider, Curves allows for minute tonal adjustments along the entire tonal range of an image.

For those that have already read my series on Levels, (Levels -- Part I), much will seem similar. This is as it should be. Curves performs the same basic functions as Levels. The difference is in the power and flexibility available with Curves. Thus, the tasks, and the methods used to perform them, are very similar for the two tools. The difference will be in the details.

Capabilities

Curves is used for five tasks:

  1. Setting the shadow and highlight values.
  2. Setting the tones and the contrast.
  3. Performing color correction.
  4. Setting custom white balance.
  5. Preparing for printing.

When it comes to setting the shadow values, highlight values, and contrast, there are six ways that Curves can be used:

  1. Manual: RGB Channel.
  2. Manual: Eyedroppers.
  3. Manual: Individual Color Channels.
  4. Automatic: Enhance Monochromatic Contrast.
  5. Automatic: Enhance per Channel Contrast.
  6. Automatic: Find Dark and Light Colors.

Background

In order to avoid repetition of material on histograms and terminology, it is assumed that the reader has already read at least Part I of the Levels series (Levels -- Part I). Like Levels, Curves is a tool that is used to control contrast and to adjust colors. With that in mind, it is not surprising that the use of Curves is tied to the histogram. However, unlike Levels, Curves does not include a histogram in the dialog box. Rather, it is necessary to launch the histogram by choosing Windows/Histogram.

Figure 1: Histogram

Figure 1 shows a histogram of an image that could use some Curves help. The area on the far left end of the histogram has few, if any, pixels. Since this end of the histogram displays the darker pixels, the lack of pixels at this end shows that there are few true blacks or very dark pixels in this image. The area on the right end of the histogram also has few, if any, pixels. Since this end of the histogram displays the lighter pixels, the lack of pixels at this end shows that there are few true whites or very light pixels in this image. This means that the image lacks contrast. In addition, the majority of the pixels are shifted toward the left side of the histogram. Thus, the image is a bit on the dark side. In some cases, that might be the way the image should look. However, that is not the case for this image.

Figure 2: Image before Curves Adjustment
Figure 2 shows the image from which the histogram in Figure 1 came. The image clearly lacks contrast and could be lightened up a bit: a perfect job for Curves.

The Curves Dialog Box

Figure 3: Curves Dialog Box

Figure 3 shows the Curves dialog box. At the top of the dialog box is the Channel pop-up. This allows the photographer to choose whether to edit the RGB channel or one of the three individual color channels (i.e., red, green, or blue).

In the center of the dialog box is a graph. This graph can be thought of as a tonal map. Along the bottom is the Input axis. This axis indicates the input values of the tones (from 0 to 255) in an image before any changes are made with Curves. As can be seen in the bar directly below the graph, the tones run from black on the left to white on the right. Along the left is the Output axis. This axis indicates the Output values of the tones (from 0 to 255) in an image after changes have been made with Curves. As can be seen in the bar directly to the left of the graph, the tones run from black on the bottom to white on the top.

Through the middle of the graph is a diagonal line. This is the curve. The lower left of the curve represents the shadows, the middle represents the midtones, and the upper right represents the highlights. Manipulating the shape of the curve changes the tonal values in the image (more about that in a bit)

In the lower left hand corner of the dialog box are the Input and Output labels. For now, no values are listed. Once a point on the curve is created, the Input and Output values for the tone will be listed in the associated boxes that will appear.

To the right of the Input and Output labels is the point and freehand tools. For now, Curves is in the point mode. This mode allows the curve to be modified by clicking on the graph. Each click will add a point to the graph (up to fourteen points can be added). The curve is then modified by dragging the points around the graph. The freehand tool allows a curve to be directly drawn on the graph. The shape of this curve will define the relationship between the Input and Output tonal values.

In the lower right corner of the dialog box are three Eyedroppers. The left Eyedropper is the Black Eyedropper and is used to set the black point or shadow values (depending on how it is configured; we will use it to set the shadow values). The right Eyedropper is the White Eyedropper and is used to set the white point or highlight values (depending on how it is configured; we will use it to set the highlight values). The middle Eyedropper is the Gray Eyedropper and is used to set neutral gray in the image.

On the right side of the dialog box is a set of buttons. These buttons allow for the saving and loading of Curves configurations as well as the use and settings of Auto Curves.

Figure 4: Curves Dialog Box with Arrowheads Clicked
One of the nice things about Curves is that it is customizable to a certain degree. Most photographers tend to think of tones in terms of the 0 -- 255 tonal value range with the dark tones on the left and the light tones on the right of the Input bar and the dark tones on the bottom and the light tones on the top of the Output bar. However, for those photographers that want to be different, Curves can be modified by clicking on the arrowheads in the Input bar. This will switch the tonal values in the bars (see Figure 4). Now, instead of going from dark to light, the tones will go from light to dark.
Figure 5: Curves Dialog Box with a 10% Grid
Another modification that can be made to Curves is to change the grid size. Clicking inside the graph while holding down the Alt key will change the grid from a 25% grid to a 10% grid (see Figure 5).

Creating a Curve

Figure 6: Curves Dialog Box with a Modified Curve

The most common way to modify a curve is to click on the curve. This will create a point on the curve for each click. These points can then be moved around to modify the curve. Figure 6 shows Curves with two such points added. The upper point is currently selected. The Input and Output tonal values can now be read below. For this point, the original value of the tone was 155. The modification of the curve has changed the value to 151. In other words, this tone has become slightly darker.

If so desired, the values of any point can be modified by directly typing into the Input or Output boxes.

Figure 7: Ctrl Clicking on the image
A somewhat less common method of adding points to a curve is to click directly on the image. Clicking on the image will indicate where the tone, of the point that was clicked, exists on the curve. Clicking the image while holding down the Ctrl key will add a point to the curve. The point is added to the curve at the exact tonal value of the point that was clicked in the image (e.g., if the point where the image is clicked has a tonal value of 63, a point, with an Input value of 63, is added to the curve). This is seen in Figures 7 and 8. Figure 7 shows the image with the curser, turned Eyedropper, over the image. Once the image was Ctrl clicked, a point was added to the curve as shown in Figure 8. In Figure 8, both the Input and Output values are the same indicating that the curve has not yet been modified.
Figure 8: Curves with Point Added by Ctrl Clicking the Image
This approach is particularly good when a photographer wants to edit specific tones in an image. The photographer can identify the tones in the image and Ctrl click the image at those tones to add the associated points to the image. Those points can then be moved around the graph to edit the chosen tones (when used this way, the photographer would need to make sure that the Input values of the points did not change as the points were moved around).
Figure 9: Curves with Curve Created by Pencil Tool
Yet another option is to draw a curve using the Pencil tool. Once the Pencil tool has been selected, clicking on the graph with the tool and dragging the tool across the graph will produce a curve. This method is used relatively infrequently. However, it can be used in special cases where unusual shaped curves are required that are difficult or impossible to create with the other methods.

Examples of Curves

Both Levels and Curves allow for contrast adjustments. The problem with Levels is that the adjustments are rather limited. While this makes the adjustments quick and easy, it does not allow for fine contrast control. On the other hand, Curves provides the ability to make fine adjustments to contrast. Since up to fourteen points can be added to a curve, and moved on the graph to alter contrast, the control over contrast that is gained by using Curves is rather substantial.

In order to utilize Curves to edit images, it is necessary to understand how some basic curves impact the contrast of an image. To better understand these curves, three things need to be kept in mind:

  1. Pulling a curve upward lightens the tones.
  2. Pulling a curve downward darkens the tones.
  3. The steepness of a curve is related to the contrast of the image to which it is applied. The steeper the curve, the greater the contrast.

The following curves are fairly simple. Sometimes, they are used directly to edit an image. At other times, more complicated curves are used. However, even when more complicated curves are used, they are generally combinations of these more basic curves.

Figure 10: Curve to Lighten

Lighten: Figure 10 shows a curve that lightens the entire image. As can be seen at the bottom of the dialog box, the point on the curve had an original, tonal value (Input) of 94. After the editing, the value is 126 (Output). Thus, this tone, as well as all others on the curve, has been lightened.

However, an even better understanding of the effects of the curve is discovered by looking at the histograms of the image both before and after the application of curve. These histograms are shown in Figures 11 and 12. As can be seen when comparing these two histograms, the main impact of the curve is to shift the histogram to the right. This lightens the image. However, this is not the only thing that is going on. The gaps on the left side of the histogram demonstrate that the shadow tones are being expanded. This increases the contrast of the shadows but also can cause posterization (banding). The spikes on the right side of the histogram demonstrate that the highlight tones are being compressed. This decreases the contrast of the highlights. In addition, tones are lost due to quantization error (a loss of data due to rounding errors when new tones are calculated).

Whenever tones are expanded or compressed, a certain amount of image degradation occurs. Thus, this curve illustrates a key point -- the use of Curves introduces a certain degree of image degradation.

Figure 11: Histogram before Curves
Figure 12: Histogram after Curves
Figure 13: Curve to Darken

Darken: Figure 13 shows a curve that darkens the entire image. This is shown by the point on the curve. The original, tonal value was 129. After editing, the value is 80.

Figures 14 and 15 show the effects of this curve. The main impact of this curve is to shift the histogram to the left. This darkens the image. Further examination reveals gaps on the right side of the histogram that demonstrate that the highlight tones are being expanded. This increases the contrast of the highlights. However, it is unlikely to lead to posterization (as was the case in the shadows with the previous curve) since the highlights have a large number of tones (tones are not evenly distributed across the dynamic range of an image: highlights have lots of tones, shadows have few). The spikes on the left side of the histogram demonstrate that the shadow tones are being compressed. This is particularly bad -- tones are again lost due to quantization error. However, this is a much worse situation than when the highlights were compressed, with the previous curve, as the shadows have few tones to begin with. The loss of even a few due to quantization error will degrade the shadow image quality. In addition, the contrast of the shadows is decreased.

Figure 14: Histogram before Curves
Figure 15: Histogram after Curves
Figure 16: Curve to Increase Midtone Contrast

Midtone Contrast: Figure 16 shows a curve that increases the midtone contrast of the image. This is commonly referred to as an S curve and is very popular. This curve is created by placing one point at the middle of the curve in order to anchor it. Then a second point is placed on the lower end of the curve and pulled down (or a point could have been placed on the upper end of the curve and pulled up). When first viewing an image that has had an S curve applied, it appears that the curve has increased the overall contrast of the image. As will be seen in a moment, that is a bit of an illusion. Looking at Figure 16 shows that the steepness of the curve, in the midtone region, has been increased. Since steepness correlates to increased contrast. The contrast of the midtones has been increased.

Comparing Figures 17 and 18 confirms that the curve has expanded the midtones -- thus, increasing the contrast. However, the contrast has not been increased everywhere. If the midtones were expanded, something else had to be compressed to make room for the expanded midtones. Figure 18 shows spikes in the shadow region of the histogram. This clearly indicates that the shadows were compressed. This reduced the image quality in the shadows. Furthermore, this curve has made the shadows even darker -- not usually a desirable goal. While less noticeable, the curve has also compressed the highlights, reduced the highlight contrast, and made the highlights even lighter. So, while this curve may appear to enhance the overall contrast of the image, it really enhances the contrast of the midtones at the expense of the shadows and highlights.
Figure 17: Histogram before Curves
Figure 18: Histogram after Curves
Figure 19: Curve to Increase Contrast

Advanced Contrast: Figure 19 shows a somewhat more advanced curve that increases the contrast of the image. This curve is basically an S curve that has been modified with a few more points. Of particular importance are the two points located toward the ends of the curve. The lowest point on the curve increases the tones in the darkest shadows and increases the contrast in this tonal region. This will improve the quality of the shadows and add some shadow detail. The highest point on the curve decreases the tones in the highlights and increases the contrast in this tonal region. This will improve the quality of the highlights and add some additional highlight contrast.

However, nothing is free when it comes to Curves. While this curve improved the quality of the shadows and highlights, it compressed some of the tones between the shadows and midtones as well as between the highlights and midtones. However, in most cases, it is superior to the S curve for improving the overall contrast of an image.

Figure 20: Histogram before Curves
Figure 21: Histogram after Curves
Figure 22: Curve to Set Shadow Values
Shadow Values: The shadow values can be set with a curve like the one shown in Figure 22. The histogram in Figure 23 shows that the left side of the histogram contains few pixels. As a result, the image will have no true blacks. Rather, the darkest tones will be a dark gray. This will result in an image with low contrast. By applying the point shown in Figure 22, the tone with a value of twenty-two is mapped to a tone with a value of zero (black). The image now has true blacks (as seen in the fact the gap on the left side of the histogram in Figure 24 has been eliminated). Furthermore, the slope of the curve has been increased. This will result in an increased contrast in the image. This curve performs the same function as the Black Input slider in Levels.
Figure 23: Histogram before Curves
Figure 24: Histogram after Curves
Figure 25: Curve to Set Highlight Values
Highlight Values: The highlight values can be set with a curve like the one shown in Figure 25. The histogram in Figure 26 shows that the right side of the histogram contains few pixels. As a result, the image will have no true whites. Rather, the lightest tones will be a light gray. This will result in an image with low contrast. By applying the point shown in Figure 25, the tone with a value of 210 is mapped to a tone with a value of 255 (white). The image now has true whites (as seen in the fact that most of the gap on the right side of the histogram in Figure 27 has been eliminated). Furthermore, the slope of the curve has been increased. This will result in an increased contrast in the image. This curve performs the same function as the White Input slider in Levels.
Figure 26: Histogram before Curves
Figure 27: Histogram after Curves
Figure 28: Curve to Target an Image for a Specific Printer: Shadows

Targeting for Printing (Shadow Values): In those cases where it is desired to target an image for printing to a specific printer, the shadow values can be prepared with a curve like the one shown in Figure 28 (a better alternative is to use an ICC profile). By applying the point shown in this Figure, the tone with a value of zero is mapped to a tone with a value of 29. While this has targeted the shadow values for the printer, the slope of the curve is now less, resulting in a loss of contrast. This also reduces the number of tones in the image.

This curve performs the same function as the Black Output slider in Levels.

Figure 29: Histogram before Curves
Figure 30: Histogram after Curves
Figure 31: Curve to Target an Image for a Specific Printer: Highlights

Targeting for Printing (Highlight Values): When targeting an image for a specific printer, the highlight values can be prepared with a curve like the one shown in Figure 31. By applying the point shown in this Figure, the tone with a value of 255 is mapped to a tone with a value of 226. While this has targeted the highlight values for the printer, the slope of the curve is now less, resulting in a loss of contrast. This also reduces the number of tones in the image.

This curve performs the same function as the white Output slider in Levels.

Figure 32: Histogram before Curves
Figure 33: Histogram after Curves

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Curves -- Part II