Using Curves In Photoshop-- Part IV

Article and Photography by Ron Bigelow

www.ronbigelow.com

Photoshop CS or Photoshop CS2 Used in this Tutorial

Shadow Values, Highlight Values, and Midtone Contrast: Automatic

Figure 1: Options Button on Curves Dialog Box

All of the methods used so far have been manual. The following method is an auto routine. Once a few settings are input, the shadow and highlight values are automatically established.

The procedure starts off by launching Curves. The options for Auto Curves are set by clicking on the Options button in the lower right hand corner of the palette (see Figure 1).

Figure 2: Auto Color Correction Options Dialog Box

The Auto Color Correction Options dialog box appears as shown in Figure 2. The dialog box allows the choice of three algorithms:

Enhance Monochromatic Contrast: This option adjusts all of the color channels at the same time. This produces an effect similar to that of the manual Info pallet and eyeball approaches.
Enhance Per Channel Contrast: This option adjusts each channel separately. This produces an effect similar to that of the manual individual color channel approach. One issue with this approach is that it can produce color casts.
Find Dark and Light Colors: This approach identifies the average darkest and the average lightest pixels and makes adjustments based on those values.

The Snap Neutral Midtones identifies a near neutral color in the image and adjusts the gamma to make the color neutral. In most cases, it is best to leave this unchecked as this option can cause color shifts in an image. The Shadows/Clip box sets the percent of the shadow values that are clipped. This value should be very low (less than one percent). The Highlights/Clip box sets the percent of the highlight values that are clipped. This value should also be very low (less than one percent). Once these values are set, clicking the OK button closes the Auto Color Correction Options dialog box. Clicking the OK button on the Curves dialog box closes Curves.

Figure 3 shows an original image. Figures 4 -- 9 show the image (and the associated histogram) after adjustment with the three Auto Color Correction options.

Figure 3: Original Image

Figure 4: Histogram of Original Image

Figure 5: Image after Enhance Monochromatic Contrast

Figure 6: Histogram after Enhance Monochromatic Contrast

Figure 7: Image after Enhance Per Channel Contrast

Figure 8: Histogram after Enhance Per Channel Contrast

Figure 9: Image after Find Dark and Light Colors

Figure 10: Histogram after Find Dark and Light Colors

The last step is to adjust the contrast. The contrast adjustments are made in the same manner as the contrast adjustments with the Info palette approach used in Part II of this series (see Shadow Values, Highlight Values, and Contrast: Info Palette Approach).

Figures 11 -- 14 show the images with the contrast adjustment.

Figure 11: Original Image

Figure 12: Enhance Monochromatic Contrast Image with Contrast Adjustment

Figure 13: Enhance Per Channel Contrast Image with Contrast Adjustment

Figure 14: Find Dark and Light Colors Image with Contrast Adjustment

One thing that should be kept in mind is that these automatic methods will rarely give the level of quality as the other methods. They simply can not analyze and optimize an image as well as a person that is competent at image editing. Thus, the automatic methods should not be used for serious photography where image quality is a major objective. The main benefit of the automatic methods is speed -- but that speed comes at the cost of image quality.

Shadow Values, Highlight Values, and Contrast: Which Method is Best

The answer is: It depends.

The info palette method is one of the best for high quality images when perfectly neutral shadows and highlights are not required.
The eyeball method is a good option when speed is more important than maximum quality and perfectly neutral shadows and highlights are not required.
The eyedropper method is the best method when neutral shadows and highlights are desired.
The individual color channels method is best used for special cases where the color histograms have significantly different shapes. This method can easily cause color shifts in the image.
The automatic methods should be used only for casual photography where speed is the primary concern and a reduction in quality is acceptable.

Performing Color Correction

Figure 15: Image with a Serious Color Cast

The fact that Curves allows independent adjustment to the three color channels makes it a tool that can be used for color correction. Figure 15 shows an image with a serious color cast. This shot is of a morgue in an old ghost town. The casket to the left of the door is a child's casket. The cold blue color cast totally ruins the old, worn feeling that one gets when they look into the building. The tone of the image needs to be warmed up.

Figure 16: Blue Color Cast Removed by Adjusting the Blue Channel Curve

Figure 17: Original Image

Figure 18: Image after Adjustment in Blue Channel of Curves