Adobe Photoshop CS5 for Photographers - Martin Evening [147]
Figure 5.25 The Curves dialog represents the relationship between the input and output levels plotted as a graph. In this example, the shadow end of the curve has been dragged inwards (just as you would in Levels), in order to set the optimum shadow clipping point. You can control both the lightness and the contrast of the image by clicking on the curve line to add curve points and adjust the shape of the curve.
Let's now look at the Curves control options. At the top of the controls section you have a channel selection menu. This defaults to RGB or CMYK, meaning that all channels are affected equally by the adjustments you make. You use this menu to select specific color channels, which can be useful for carrying out color corrections (see Figure 5.26). The Auto button in Curves is the same as the one found in Levels. When you click on this button, it applies an auto adjustment based on how the Auto settings are configured (see Auto image adjustments on pages 344–345). The Input levels sliders work the same way as the ones in Levels and you can drag these with the key held down to access the threshold view mode that was described on the previous two pages. The Output and Input boxes provide numeric feedback for the current selected curve point so that you can adjust the curve points more precisely. You may sometimes see the histogram warning button light up. This warns you that the histogram displayed in the grid area is not an accurate reflection of what the true histogram should be and if you click on this, it forces an update of the histogram view. The Curves dialog uses the Point curve editor mode by default, but there is also a Pencil button for switching to a draw curve mode, which allows you to draw a curve shape directly. While in ‘draw curve’ mode the Smooth curve shape button is made active and clicking on this allows you to smooth a drawn curve shape. Overall, you are unlikely to need to use the draw curve mode when editing photographic images.
Figure 5.26 The default setting for Levels or Curves applies the user's corrections to the composite of the color channels. If you mouse down on the Channel menu, you can choose to edit individual color channels. This is how you can use Curves to make color adjustments.
If you double-click the eyedroppers, you can edit the desired black point, gray point and white point colors and then use these eyedroppers to click in the image to set the appropriate black, gray and white point values. There was a time in the early days of Photoshop where the eyedropper controls were important, but there are two reasons why this is less the case now. Firstly, you can accurately set the black and white clipping points in Camera Raw (as described on pages 180–185) and secondly, the color management system in Photoshop automatically maps the black point for you when you convert an image to CMYK, or uses the Photoshop print dialog to make a color managed print output. However, the gray eyedropper can still prove useful for auto color balancing a photo (as described on page 344).
On-image Curves editing
If you click on the target adjustment tool button to activate it and move the cursor over the document window, you will notice a hollow circle that hovers along the curve line. This indicates where any part of the image appears on the curve and if you simply click on the image a new curve point is added to