"What actually happened to my image?" - if you ask yourself the above question from time to time after some tool or filter has changed your image the right or the wrong way, then this is the place to come to.
In this section you will find - among other things - tools for testing other tools that cause global tonal value and local contrast changes, that means tools from the first and second category on the right. Measuring and visualizing color changes produced by tools from the third group is more complicated, separate tools may follow later.
With the help of adequate test images many camera characteristics can be determined with an accuracy sufficient for most purposes. Verify and compare the sharpness and resolution of your camera and lenses, determine dynamic range and shooting characteristics of your digital camera.
Three Test images for verifying gradation, color, resolution and profil quality of desktop printers. More...
Tools of this category change tonal values independently from the original pixels' luminance and chrominance values. For example, all tools of the first group in the Photoshop menu Image/Alter (Brightness/Contrast, Levels, Curves, Color Balance, etc.)
To this group belong tools that primarily alter the edges of an image, i.e. sharpening and blur tools.
This group covers color correction tools like Hue/Saturation and Selective Color, but also the Channel Mixer (second group in the Photoshop menu). It doesn't cover Color Balance, because this tool is working globally (like Brightness/Contrast) without taking care of the original color.