The Global tab sheet lets you apply some global contrast effects. Other than the Dynamic, Adaptive and Local effects, which adjust the contrast in small neighborhood areas in the image, these global effects take the whole image into account when adjusting the contrast.
The Arrow-Down Button
Clicking on the arrow-down button will display a menu with various local presets. These local presets are useful settings for the controls of the current tab sheet. Each tab sheet has its own arrow-down menu with different presets. Only the "Reset" option is the same. It sets the controls of the tab sheet to their default values.
The Brighten slider lets
you increase or decrease the brightness of the image in EV steps from -3.0 to
+3.0. For many images you will only need adjustments between -1.0 and +1.0 though.
It doesn't work like the brightness or gamma sliders that are known from various
graphics applications. It doesn't cut off shadows or highlights and also doesn't
change the contrast.
S-Contrast, H-Contrast, Contrast
The S-Contrast slider lets you decrease and increase the contrast in the shadow areas whereas the H-Contrast slider does the same for the highlight areas. If you activate the check box at the left of these two sliders, the two sliders will be combined into one Contrast slider. This Contrast slider adjust the contrast over the whole tonal range. If the S-Contrast and H-Contrast sliders have the same values, they work identical to the Contrast slider.
The Stretch slider adjusts the black and white point of the image. Based on the slider value the appropriate black and white points are automatically selected and the tonal range of the image is stretched. Some image may need a very high Stretch value before you see any change, but most image will already show an effect at very low values. So be careful to not use a extreme value, otherwise you will blow out details in the image.
Please notice: The Stretch effect may look a bit different if only a part of the image is displayed in the preview. That is because it calculates internal correction values based on the displayed image data. So it can be best judged if you use the "Fit" zoom option.
The Equalize slider distributes the tonal values evenly throughout the image. If you apply it to a high-contrast image, it will lower the contrast and restore the midtones. If it is applied to a low-contrast image, it will increase the contrast. So it depends on the image what effect it produces. Usually it also brightens up the image a bit.
The Polarizes slider works similar to polarizing glass filters. Positive slider values make blue areas darker and yellow areas brighter whereas negative values have the contrary effect. You could also call it a color contrast effect, because it also slightly modifies the colors of the image. It may do a nice job on some images, but may not work so good on other images.