Using Curves In Photoshop-- Part II

Article and Photography by Ron Bigelow

www.ronbigelow.com

Photoshop CS or Photoshop CS2 Used in this Tutorial

Shadow Values, Highlight Values, and Contrast: Info Palette Approach

Probably, the most common use of Curves is to alter the contrast in an image. The most direct way to do this is to make manual adjustments in the RGB channel. As with Levels, this can be done in two ways: the eyeball approach and the Info palette approach. The more exact of these methods is the Info palette approach. This method also allows for better tracking of how one adjustment affects the tonal values set by previous adjustments. On the other hand, the eyeball approach is quicker and easier.

This article will start with the Info palette approach since it far better illustrates what is happening as different adjustments are made.

Figure 1: Histogram

The first step to making contrast adjustments with Curves, utilizing the Info palette approach, is to set the shadow and highlight values. However, two problems immediately occur. The first problem is that Curves does not provide a histogram. This situation can easily be remedied by the use of the histogram tool. Histogram is launched by choosing Window/Histogram. The Histogram palette will look something like the one shown in Figure 1 (the histogram may look slightly different depending on how it is configured). If the histogram looks different from that shown in Figure 1, the histogram can be reconfigured with the Histogram options by clicking on the pop-up in the upper right hand corner of the palette and choosing Compact View.

Figure 2: Info Palette

The second problem is that it is necessary to know where the darkest and lightest points, that hold detail, are located in an image. Unfortunately, the human eye is simply not a good judge of such things. To resolve this issue, the Info palette is used. The Info palette shows the colors of any chosen point. The Info palette is launched by choosing Window/Info. The options for the Info palette are set by clicking on the triangle in the upper right hand corner of the palette and choosing Palette Options (see Figure 2). The Palette Options dialog box appears as shown in Figure 3. Both the First Color Readout Mode and the Second Color Readout Mode should be set to Actual Color. Clicking the OK button closes the dialog box.

Figure 3: Palette Options Dialog Box
Figure 4: Color Sampler

When the lightest and darkest areas that contain detail have been identified, they will be targeted. For this, the Color Sampler will be used. The Color Sampler is selected from the Tools palette as shown in Figure 4. The Color Sampler will allow color samplers to be placed on the image. The color values at the color samplers can then be read in the Info palette. Before placing the color samplers, it is necessary to set the sample size of the Color Sampler. This is done by clicking on the sample size pop-up in the upper left hand corner of the screen. For this purpose, it is best to select the 3 by 3 Average (see Figure 5).

 

Figure 5: Sample Size Pop-Up
Figure 6: Image before Curves Adjustment
It is now time to begin working the image. Figure 6 shows the image that was first shown in Part I of this series. This image is in need of some contrast work. Curves is the perfect tool for this task.

The first task is to set the shadow and highlight values. For this, it is necessary to place one color sampler at the point in the image that will be used to set the shadow values and a second color sampler at the point that will be used to set the highlight values. This is one of the few tasks were Curves is at a disadvantage compared to Levels. With Levels, holding down the Alt key, and moving the Black and White Input Sliders inward will indicate the darkest and lightest parts of the image. This will help in locating the points that will be used to set the shadow and highlight values. Unfortunately, Curves has no such capability. To get around this problem with Curves, two options exist for setting the shadow and highlight values: either a Threshold layer can be utilized (the method that I recommend) and the method described in the Threshold article used to set the color samplers (for more information, see Threshold), or the Color Sampler can be moved around the image while the Info palette is observed. This approach is performed as follows. As the Color Sampler is moved around, the Info palette indicates the tonal values (for the three color channels) of the location directly below the Color Sampler. These numbers will help indicate the best places for the two color samplers to be placed. For this article, the second option will be used.

Figure 7: Image with Sampler Added at Shadows

To select the point that will be used to set the shadow values, the Color Sampler is selected and is now moved around the dark parts of the image. For the setting of the shadow values, it is desired to find a position that has very low tonal values but still holds detail (an area has detail if the values in the Info pallet change as the Color Sampler is moved around). It is also a good idea for the position to be fairly neutral (i.e., the three tonal values are approximately equal) unless one is trying to remove a color cast. A glance at the image shows some very dark areas in the trees on the right side of the image. A position in these trees with values of 16, 19, and 20 was found. The Color Sampler is clicked at this point. A color sampler is added to the image (see Figure 7).

The value of the color sampler is shown on the Info palette (see Figure 8).

Figure 8: Info palette after Shadow Sampler was Added
Figure 9: Image with Sampler Added at Highlights

To select the point that will be used to set the highlight values, the Color Sampler is moved around the light parts of the image. For the setting of the highlight values, it is desired to find a position that has very high tonal values but still holds detail. It is also a good idea for the position to be fairly neutral unless one is trying to remove a color cast. The image shows some light areas in the rapids at the far end of the river. A position in the rapids with values of 190,194, and 212 was found (this image did not have any neutral highlights). The Color Sampler is clicked at this point. A color sampler is added to the image (see Figure 9).

The value of the color sampler is shown on the Info palette (see Figure 10).

Figure 10: Info palette after Highlight Sampler was Added
Figure 11: New Layer Dialog Box
Now that the preparation has been completed, the work with Curves can begin. Curves is launched by choosing Layer/New Adjustment Layer/Curves. The New Layer dialog box appears as shown in Figure 11. The Use Previous Layer to Create Clipping Mask is left unchecked. The Color is set to None, the Mode to Normal, and the Opacity to 100%. Clicking the OK button closes the dialog box and brings up the Curves dialog box as show in Figure 12.
Figure 12: Curves Dialog Box
Figure 13: Curves with the Shadow Values Set

It is now time to set the shadow and highlight values. For the shadow values, the color values should be set between about 2 and 8. The lower the value, the darker the shadows will be. To set the shadow values, the lower left end of the curve is dragged to the right until the desired values are indicated for the first color sampler on the Info palette. Then, the histogram is checked to see what the effect of the edit has had on the tonal distribution of the image.

With the lower end of the curve moved to the position shown in Figure 13, the Info palette shows that the original shadow values of 16, 19, and 20 are now values of 3, 5, and 5 (see Figure 14). The histogram (see Figure 15) shows that the left end of the histogram has shifted to the left. Thus, the shadows have been darkened.

Figure 14: Info Palette with the Shadow Values Set

 

 

 

 

 

Figure 15: Histogram with the Shadow Values Set

 

Figure 16: Curves with the Highlight Values Set

The highlight values are set next. For the highlight values, the color values should be set between about 245 and 254. The higher the value, the lighter the highlights will be. To set the highlight values, the upper right end of the curve is dragged to the left until the desired values are indicated for the second color sampler on the Info palette. Then, the histogram is checked to see what the effect of the edit has had on the tonal distribution of the image.

With the upper end of the curve moved to the position shown in Figure 16, the Info palette shows that the original highlight values of 190, 194, and 212 are now values of 224, 228, and 246 (see Figure 17). The histogram (see Figure 18) shows that the right end of the histogram has moved toward the right. Thus, the highlights have been lightened.

Figure 17: Info Palette with the Highlight Values Set

 

 

 

 

 

Figure 18: Histogram with the Highlight Values Set

 

 

Figures 19 and 20 show the original image and the image after the shadow and highlight values were set.

Figure 19: Original Image
Figure 20: Image After the Shadow and Highlight Values were Set
Figure 21: Curves with the Final Settings

The next step is to adjust the contrast. This is done by adding points along the curve to manipulate the contrast. These points can be moved around until the desired look is achieved. Figure 21 shows the adjustments for this image. To start off, it is desired to add some midtone contrast. This is done by adding points 1, 2, and 3. Point 1 anchors the curve. Point 2 darkens the quarter-tones. Point 3 lightens the three-quarter tones. The steepness of the curve between points 2 and 3 has been increased, resulting in the needed midtone contrast increase. To insure that the darkest tones don't get too dark, point 5 is added to pull the curve up in the shadows just a little. Point 4 is added to ensure that point 5 only lightens the darker shadows and doesn't lighten the lighter shadows. Lastly, Point 6 was added because point 3 had caused the sand on the riverbank to become too light. By pulling down the curve, point 6 darkened the sand.

Figure 22 shows the final histogram. In this case, this histogram doesn't look much different than the one shown in Figure 18. This is because the adjustments to the curve, with this image, are rather mild. With other images, the histogram may be changed to a greater degree.

Figure 22: Final Histogram

Figure 23 shows the Info palette after all of the Curves adjustments have been made. It can be seen that the Curves adjustments slightly changed the values for the shadow and highlight values. These points could be readjusted with the end points on the curve. However, for this image, the changes are slight, so no further adjustments will be made. Clicking the OK button closes Curves.

Figures 24 and 25 show the original image and the image after all the adjustments were finalized.

Figure 23: Info Palette with the Final Settings

 

Figure 24: Original Image
Figure 25: Image after all Adjustments were Made

Shadow Values, Highlight Values, and Contrast: Eyeball Approach

The eyeball method to setting the shadow values, highlight values, and contrast is a faster approach.

Figure 26: Curves with the Shadow Values Set

To set the shadow values, the lower left end of the curve is dragged to the right while the histogram and image are checked to see the effect of the edit. For most images, the left end of the histogram should just start to touch the left side of the histogram display area (this is correct for images that have dark shadows; for images without dark shadows, the histogram should not be moved so far to the left).

With the lower end of the curve moved to the position shown in Figure 26, the histogram (see Figure 27) shows that the left side of the histogram has shifted to the left. Thus, the shadows have been darkened.

Figure 27: Histogram with the Shadow Values Set

 

Figure 28: Curves with the Highlight Values Set

To set the highlight values, the upper right end of the curve is dragged to the left while the histogram and image are checked to see the effect of the edit. For most images, the right end of the histogram should just start to touch the right side of the histogram display area (this is correct for images that have highlights; for images without highlights, the right side of the histogram should not be moved so far to the right).

With the upper end of the curve moved to the position shown in Figure 28, the histogram (see Figure 29) shows that the right side of the histogram has shifted to the right. Thus, the highlights have been lightened.

 

Figure 29: Histogram with the Highlight Values Set
Figure 30: Curves with the Final Settings

The contrast adjustments are made in the same manner as with the Info palette approach shown above. Figure 30 shows the final Curves adjustments and Figure 31 shows the final histogram.

Figure 31: Final Histogram

 

Figures 32 and 33 show the original image and the image after all the adjustments were finalized.

 

Figure 32: Original Image
Figure 33: Image after all Adjustments were Made

 

Some might question why anyone would go to the extra trouble to use the info palette approach when the eyeball approach is so much easier. However, it is necessary to understand that with the eyeball approach, one runs the risk of degrading the image quality. In the eyeball approach, the Info palette is not used. Therefore, it is not possible to know exactly what is happening in the shadows and highlights. As a result, it is very possible that the endpoints of the curve will get moved inward too much. This will clip the shadows and highlights and will cause a loss of detail in those areas -- thus, degrading the image quality.

Manual RGB Channel Approaches -- Considerations

When the manual RGB channel approaches are used, the shadow and highlight values will not always be neutral. In other words, the red, green, and blue values of the shadows values may not be equal. This is also true for the highlight values. In some cases, this may be an issue. On the other hand, setting the shadow and highlight values with one of the manual RGB channel approaches is not likely to cause unusual color shifts in an image, which can occur with other methods that force the shadow and highlight values to be neutral.