Adobe Photoshop CS5 for Photographers - Martin Evening [156]
Auto image adjustments
The Image menu contains three auto image adjustment options that are designed to provide automated tone and color correction: Auto Tone, Auto Contrast and Auto Color, examples of which are shown in Figure 5.49.
Figure 5.49 This shows the three auto adjustment methods (with equivalent descriptions for how they are described in the Levels and Curves Auto Color Correction Options dialogs). Auto Tone optimizes the shadow and highlight points in all three color channels. This generally improves the contrast and color balance of the image. Auto Contrast applies an identical tone balancing adjustment across all three channels that improves the contrast, but without altering the color balance. The Auto Color option maps the darkest and lightest colors to a neutral color (if the swatch colors shown in Figure 5.48 have been altered, you may see a different result). Lastly, I have shown an example of an Auto Contrast adjustment followed by a PhotoKit Color RSA Neutralize effect. There is a demo version of PhotoKit Color available on the DVD that comes with this book.
The Auto Tone adjustment ( ) works by expanding the levels in each of the color channels individually. This per-channel levels contrast expansion method will always result in an image that has fuller tonal contrast, but it may also change the color balance of the image too. The Auto Tone adjustment can produce improved results, but sometimes not. If you want to improve the tonal contrast, but without affecting the color balance of the photograph then try using the Auto Contrast adjustment instead ( ). This carries out a similar type of auto image adjustment as Auto Tone, except it optimizes the contrast by applying an identical Levels adjustment to each of the color channels.
Lastly, there is the Auto Color adjustment ( ). This provides a combination of Auto Contrast to enhance the tonal contrast, combined with an auto color correction that maps the darkest colors to black and the lightest colors to white.
If you open the direct Levels or Curves dialogs you will see a button marked ‘Options…’ just below the Auto button. If you click on it this opens the Auto Color Correction Options shown in Figure 5.48. If you are applying a Levels or Curves adjustment layer, then you'll need to -click the Auto button in order to open the Auto Color Correction Options dialog. The algorithms listed here match the auto adjustments found in the Image menu: ‘Enhance Per Channel Contrast’ is the same as ‘Auto Tone’. ‘Enhance Monochromatic Contrast’ is equivalent to ‘Auto Contrast’ and ‘Find Dark & Light Colors’ is the same as ‘Auto Color’. Notice however there is also a ‘Snap Neutral Midtones’ option. When this is checked, a Gamma adjustment is applied in each of the color channels which aims to correct the neutral midtones as well as the light and dark colors. A clipping value can also be set for the highlights and shadows and this determines by what percentage the endpoints are automatically clipped.
Figure 5.48 The Auto image adjustments can also be accessed when you click on the Options… button in the direct Levels or Curves dialogs, or -click the Auto button (circled) in the adjustment layer dialog. If ‘Snap Neutral Midtones’ is selected, Auto Color will neutralize these too. You can also customize the clipping values to determine how much the highlights and shadow tones are clipped by the image adjustment.
Color corrections using Levels or Curves
Of all the Photoshop color correction methods described so far, Auto Color probably provides the best automatic one-step tone and color correction method. However, if you prefer to carry out your color corrections manually, I would suggest you forget Variations or Color Balance and explore