Using Curves In Photoshop-- Part III

Article and Photography by Ron Bigelow

www.ronbigelow.com

Photoshop CS or Photoshop CS2 Used in this Tutorial

Shadow Values, Highlight Values, and Contrast: Eyedroppers

Figure 1: Eyedroppers on Curves Dialog Box

Another common way to use Curves for managing contrast is through the use of the Eyedroppers. The Eyedroppers are in the lower right hand corner of the Curves dialog box (see Figure 1). The left Eyedropper is the Black Eyedropper and is used to set the black point or shadow values. The right Eyedropper is the White Eyedropper and is used to set the white point or highlight values. The middle Eyedropper is the Gray Eyedropper and is used to set neutral gray in the image. In the Eyedropper approach, the Black and White Eyedroppers will be used to set the shadow and highlight values respectively.

Figure 2: Image with Three Color Samplers

One of the advantages of the Eyedropper approach is that the shadow and highlight values can be made neutral. The drawback is that forcing these values to be neutral can sometimes cause color shifts in the rest of the image. It is therefore necessary to track the color values at three tonal locations: the shadows, the highlights, and the midtones. This is easily done with the Color Sampler and the Info palette. These are used to put three color samplers in place. The first two color samplers are set down in the shadows and highlights as was done in the Info Palette Approach. The third color sampler is set down in an area of midtones. This area should be as neutral as possible.

Figure 2 shows an image with the three color samplers. One point that needs to be noted is that this image has a slight, blue color cast because it was shot in the shadows. This will be addressed during the editing. It should also be noted that there are no perfectly neutral highlights in this image. Figure 3 shows the histogram of the unedited image, and Figure 4 shows the info palette with the color values of the three color samplers.

Figure 3: Histogram of the Unedited Image

 

 

 

 

Figure 4: Info Palette with Color Values at the Three Color Samplers
Figure 5: Black Eyedropper on Curves Dialog Box
To set the shadow values, Curves is launched and the Black Eyedropper is double clicked (see Figure 5).
Figure 6: Color Picker Dialog Box for Selecting Target Shadow Color
The Color Picker dialog box appears as shown in Figure 6. This Color Picker is used to set the shadow values. These values will then be assigned to whatever point is clicked with the Black Eyedropper. The values are set by entering the values directly into the R, G, and B boxes. The values should be set between about 2 and 8. The lower the value, the darker the shadows will be. As long as the same value is entered into each of the three boxes (e.g., 5 in this case), the shadow values will be neutral. Clicking OK closes the Color Picker.
Figure 7: Black Eyedropper Clicked at Color Sampler #1
To set the shadow values, the Black Eyedropper is simply moved around the dark areas of the image (the curser turns into the Black Eyedropper as it is moved over the image). As the Black Eyedropper is moved around, the Info palette indicates the tonal values (for the three color channels) of the location directly below the Black Eyedropper. For the setting of the shadow values, it is desired to find a position that has very low tonal values but still holds detail. It is also a good idea for the position to be fairly neutral (i.e., the three tonal values are approximately equal). However, such a point was already located and identified with color sampler #1. Therefore, the Black Eyedropper is simply clicked at the location of color sampler #1 (see Figure 7). This will set the shadow values.
Figure 8: Histogram After Shadow Values Were Set
Figure 8 shows the Histogram after the shadow values have been set. Comparing this histogram to the one in Figure 3, it can be seen that the left side of the histogram has slid left just a small amount. This has made the shadows a bit darker.
Figure 9: Info Palette after Black Eyedropper Clicked at Color Sampler #1
The Info Palette (see figure 9) shows the result of this work. Color sampler #1 originally had values of 11, 12, and 11. The values have been changed to 5, 5, and 4 (a nice neutral shadow). One might be tempted to ask why the values are not 5, 5, and 5. After all, that is what was set in the Color Picker for the shadow values. The answer resides in the small difference in position between color sampler #1 and where the Black Eyedropper was set. The curser is simply not accurate enough to align them perfectly.
Figure 10: White Eyedropper on Curves Dialog Box
To set the highlight values, the White Eyedropper is double clicked (see Figure 10).
Figure 11: Color Picker Dialog Box for Selecting Target Highlight Color
The Color Picker dialog box appears as shown in Figure 11. This Color Picker is used to set the highlight values. These values will then be assigned to whatever point is clicked with the White Eyedropper. The values are set by entering the values directly into the R, G, and B boxes. The values should be set between about 245 and 254. The higher the value, the lighter the highlights will be. As long as the same value is entered into each of the three boxes (e.g., 250 in this case), the highlight values will be neutral. Clicking OK closes the Color Picker.
Figure 12: White Eyedropper Clicked at Color Sampler #2
To set the highlight values, the White Eyedropper is simply moved around the light areas of the image (the curser turns into the White Eyedropper as it is moved over the image). As the White Eyedropper is moved around, the Info palette indicates the tonal values of the location directly below the White Eyedropper. 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. However, it has already been determined that the location of color sampler #2 will be used to set the highlight values. Therefore, the White Eyedropper is simply clicked at the location of color sampler #2 (see Figure 12). This will set the highlight values.
Figure 13: Curves Dialog Box After Highlight Values Were Set
Figure 13 shows the Histogram after the highlight values have been set. Comparing this histogram to the one in Figure 3, it can be seen that the right side of the histogram is just touching the right side of the histogram display area. This has made the highlights lighter.
Figure 14: Info Palette after White Eyedropper Clicked at Color Sampler #2

The Info Palette (see figure 14) shows the result of this work. Color sampler #2 originally had values of 225, 229, and 236. The values have been changed to 254, 254, and 253 (a neutral highlight).

Now, the Info palette shows a couple of very interesting pieces of information. For color sampler #2, as just mentioned above, the color values before editing were 225, 229, and 236. Before editing, this highlight was not neutral. There is more blue than the other two colors. This is not surprising as this image was shot in the shadows (the light in shadows, generally, has a blue tint). Thus, the image, as shot, had a blue tint. The values for color sampler #2 after the editing are 254, 254, and 253. Thus, after editing, the highlight values are neutral. This has caused a color shift in the highlights. In particular, the amount of blue has been reduced compared to the other color values. Color sampler #3 shows that this color shift has not been restricted to the highlights, but it has also occurred in the midtones. Before editing, the color sampler #3 (midtone) color values were 159, 157, and 158. This was a fairly neutral point. After editing, color sampler #3 has color values of 179, 173, and 169. This point is no longer neutral. As with the highlight values, the amount of blue has been reduced.

This color shift is clearly seen in Figures 15 and 16, which show the original image and the image after the shadow and highlight values were set.

One of the advantages of the Eyedropper approach is that it gives neutral shadows and highlights (if the Color Picker is set up properly). However, Figures 15 and 16 show that forcing the shadows and highlights to be neutral can cause color shifts. When comparing the two images, it is clear that a color shift has occurred in the second image. Now, in this case, the color shift worked to the photographer's advantage. The color shift got rid of the blue tint in the original image and allowed the natural, warm tones of the old wood to come forth. This saved the photographer some work as an additional color correction step will no longer be needed.

However, in other images, color shifts could result that will be to the detriment of the photographer. Undesired color shifts could be introduced that would need to be removed by a color correction step.

Figure 15: Original Image
Figure 16: Image after the Shadow and Highlight Values were Set
Figure 17: Curves with the Final Settings

Some additional contrast will further improve the image. The contrast adjustments are made in the same manner as with the Info palette approach used in Part II of this series (see Shadow Values, Highlight Values, and Contrast: Info Palette Approach).

Figure 17 shows the final Curves adjustments and Figure 18 shows the final histogram.

Figure 18: Histogram with the Final Settings

 

Figure 19: Info Palette with the Final Settings

Figure 19 shows the Info palette after all of the adjustments have been made. It can be seen that the Curves adjustment slightly changed the values for the highlight values. This could be readjusted with the upper end point 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 20 and 21 show the original image and the image after all the adjustments were finalized.

Figure 20: Original Image
Figure 21: Image after all Adjustments were Made

Shadow Values, Highlight Values, and Contrast: Individual Color Channels

Figure 22: Image with Different Shaped Color Histograms

The previous techniques all used the RGB channel. Thus, all three colors were adjusted at the same time. This works well for most images. However, sometimes it is desirable to adjust each color channel separately. This is most often the case when the color histograms have significantly different shapes. In this situation, the individual color channel technique is an option. Figure 22 shows an unedited image that falls into this category. Now, the first thing that should be noticed is that this image is underexposed (this was due to wind that forced a higher shutter speed than the light level would normally require). Figure 23 shows a histogram of this image. The histogram clearly shows the underexposure. It also shows that the color channels have different shaped histograms that start and stop at different points. Normally, I would not attempt to recover an image that was underexposed this much as the image quality is usually not up to standard. However, I like this image so much, let's see what can be done with the Individual Color Channel approach.

Figure 23: Each Color Channel has a Different Shaped Histogram
As with the previous techniques, the first two tasks that need to be performed are to set the shadow values and the highlight values. However, with this technique, it is necessary to be able to watch what is happening with all three color channels at the same time. For this, the Histogram tool is again required. Once launched, the Histogram palette will probably look something like the one shown in Figure 24 (the histogram may look slightly different depending on how it is configured). The options for the Histogram are set by clicking on the pop-up in the upper right hand corner of the palette and choosing All Channels View. If the channels are not in color, clicking on the pop-up again and choosing Show Channels in Color will render each histogram in its appropriate color. Figure 25 shows the Histogram with all three color channels.
Figure 24: Histogram

 

 

 

 

Figure 25: Histogram with Color Channels

To start the editing, Curves is launched.

Figure 26: Curves Channel Pop-Up Accesses Color Channels

Next, the Channel pop-up is accessed (see Figure 26), which reveals the three color channels. For this example, the red channel is selected first. The red channel is now displayed (see Figure 27). All edits performed will affect only the red channel.

Figure 27: Red Channel in Curves

The shadow and highlight values can now be set similar to the way they were set in the eyeball technique. The only difference is that the edits only affect one color channel at a time.

To set the red 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).

To set the red 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).

Figure 28 shows the Curves Dialogue Box for the red channel with the adjustments for the shadow and highlight values. The upper end of the curve has been moved in rather drastically. This is required because of the underexposure condition. Properly exposed images will not require so extreme of an edit. Figure 29 shows the histogram for the red channel after the adjustments.

Figure 28: Curves' Red Channel with Shadow and Highlight Values Set
Figure 29: Red Histogram with the Red Channel Shadow and Highlight Values Set.
The green and blue channels are edited in the same manner. Figures 30 -- 33 show the green and blue channels in Curves after editing.
Figure 30: Curves' Green Channel with Shadow and Highlight Values Set
Figure 31: Green Histogram with the Green Channel Shadow and Highlight Values Set.
Figure 32: Curves' Blue Channel with Shadow and Highlight Values Set
Figure 33: Blue Histogram with the Blue Channel Shadow and Highlight Values Set.
Figure 34: Histogram with Color Channels after Editing
Figure 34 shows the Histogram after all the shadow and highlight values have been set.
Figure 35: RGB Channel of Curves for Setting the Contrast
The last step is to set the contrast. This is done by adjusting the RGB channel of Curves as shown Figure 35. It is important that this is done in the RGB channel. Making changes to the individual color channels will cause color shifts in the image. The contrast adjustments are made in the same manner as with the Info palette approach used in Part II of this series (see Shadow Values, Highlight Values, and Contrast: Info Palette Approach).
Figures 36 and 37 show the original image and the image after all the adjustments were finalized.
Figure 36: Original Image
Figure 37: Image after all Adjustments were Finalized
There is one caveat with this technique; it is very easy to end up with color casts when setting the shadow and highlight values by using the individual color channels. I recommend this approach only for individuals that are very experienced in image editing.

Articles

Curves -- Part II     Curves -- Part IV