The top four sliders on the Extras tab are meant for adjusting the brightness of specific areas of the image. They let you increase or decrease the brightness of the bright and dark areas independently. They can be seen as an fine tuning mechanism for the Exposure feature of the Brightness tab.
Adjusting the highlights or shadows is partly a matter of taste, but you can use these sliders to emphasize certain image areas and reveal hidden details from the dark or bright areas. To avoid producing artifacts or hard edges in the image, each slider also slightly increases the other brightness area. Additionally the sliders are limited to a certain range, because larger or smaller slider values would damage the image by producing popart-like effects.
You can also use the Highlights and Shadows sliders to adjust the midtone contrast of the image. For doing that you have to set the Highlights slider to a negative value and the Shadows slider to a positive value.
The Highlights Slider
This slider lets you amplify or suppress the bright image areas. Don't increase the slider value too much unless you to want to risk overexposing certain image areas.
The H-Contrast Slider
The H-Contrast slider controls the contrast of the highlight adjustment. A larger value produces more contrasty adjustment. A low value may produce a too flat and saturated effect. If the Highlights slider is set to zero, the H-Contrast slider has no effect.
The Shadows Slider
Making darker areas in the image more visible can be achieved with the Shadows slider. But don't boost the shadows too much or they may get a somehow faded grayish look, because they were shifted into the midtones.
The S-Contrast Slider
The S-Contrast slider controls the contrast of the shadow adjustment. A larger value produces more contrasty adjustment. A low value may produce a too flat and saturated effect. If the Shadow slider is set to zero, the H-Contrast slider has no effect.
The Sat Fix check box from the Brightnes tab also influences the Highlights or Shadows slider. If the 'Sat Fix' check box is activated, ColorWasher will process the image while keeping saturation constant when adjusting contrast, exposure, highlights, midtones and shadows. This can improve some photos and avoid oversaturation, but sometimes if the adjustment is too strong, it decreases or increases saturation too much. In such a case it is better to deactivate the Sat Fix check box. An alternative to Sat Fix is the Saturation slider.
The 'Histo Fix' check box removes gaps that can be seen in the histogram view of a corrected image. This means that the number of colors in the image will increase. These gaps are only a sign that a photo was corrected, but otherwise you can forget about them. Please only use 'Histo Fix' if you don't plan to sharpen the photo, because sharpening automatically removes the histogram gaps. However, this feature produces a larger JPG file size.