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 Levels. The options for Auto Levels are set by clicking on the Options button in the lower right hand corner of the palette (see Figure 1).
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 Levels dialog box closes Levels.
Figure 3 shows an original image. Figures 4 -- 9 show the image (and the associated Levels dialog boxes) after adjustment with the three Auto Color Correction options.
The last step is to adjust the midtone contrast by using the Gamma Input Slider. Moving the Gamma Input Slider to the right increases contrast. Moving the Gamma Input Slider to the left decreases contrast. A Gamma Input Slider setting of 0.85 adds some additional contrast to these images (see Figure 10).
Figures 11 -- 14 show the images with the Gamma Input Slider adjustment.
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.
The fact that Levels 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.
The correction is easily made in Levels by accessing the color channels and adjusting the Gamma Input Slider. Moving the Gamma Input Slider to the left increases the color of the channel. Moving the Gamma Input Slider to the right decreases the color of the channel and increases the complementary color. For the image in Figure 15, the blue channel is opened in Levels and the Gamma Input Slider is moved to 0.55 (as shown in Figure 16). Figures 17 and 18 show the original and final versions of the image.