Friday, June 29, 2007
Strategies for selectively lightening and darkening an image
Instead of using an adjustment layer set to Screen or Multiply, you can paint on a normal layer, capable of containing pixels, set to Overlay to both lighten and darken on a single layer. Use black paint to darken and white paint to lighten. As the effect is quite strong, use low opacities when painting. You can selectively reduce the opacity of any area where the effect seems too strong by using the Eraser tool at a low opacity.
A side effect of this method is an increase in saturation in the affected areas. A quick cure for this is to duplicate the layer, change its mode to Saturation, and lower its opacity until the saturation of those layers has been reduced appropriately. Here, you're using the neutral values on this layer at their relative opacities to reduce saturation in the very same areas that were darkened or lightened.
The Blend mode Softlight offers a similar effect to Overlay, but is less intense. Its side effects aren't as uniform. Areas that are darkened increase in saturation, while areas that are lightened decrease in saturation. You can use a third method to eliminate the saturation shifts associated with Blend mode corrections. Turn off the dodging and burning layer (click on the eye icon in the Layers palette); at the top of the layer stack, create a new layer. Holding the Option and Shift keys, select Merge Visible from the Layers palette submenu (this will merge a copy of all visible pixels as affected by corrections into the new layer), and change the Blend mode of the layer to Saturation. Turn the Blend mode corrections back on.
You can plot Blend modes, lightening and darkening effects on the tonal scale as a curve. Multiply has a stronger effect on shadows than on highlights (Fig. A). Screen has a stronger effect on highlights than on shadows (Fig. B). Overlay has a uniform effect on both shadows and highlights, and little effect on midtones (Fig. C). Black paint on a layer set to Overlay has a stronger effect on shadows than on highlights (Fig. D). White paint on a layer set to Overlay has a stronger effect on highlights than on shadows (Fig. E). Reducing the opacity of either the layer or the paintbrush used modifies the plotted curve; essentially, as opacity is reduced, the curve is progressively drawn closer to a straight line curve that represents no change. BOTTOM RIGHT: Call up the Blend mode in the Layers palette, and this is what you see.
Build these curves as adjustment layers and you can achieve the same effects as corrections made with Blend modes. Or, for greater precision, you can build curves designed specifically for the tones in an image. In addition, you can change the Blend mode of the Curves adjustment layer to Luminosity and eliminate saturation shifts.