These do not change the content of an image. Here we are improving poor exposure, removing noise or enhancing colours of an image which your camera may have failed to pick up.
Subtle changes are the best, especially non destructive ones.
1. Let's make the bluebells "bluer"
2. Levels and Curves
3. Multiple Exposures
Changing the hue means changing the actual colours of an image or part of an image, whilst the saturation refers to the intensity of the colour
Adjusting tones to brighten, darken, add contrast and shift colours. Which to use?
Sometimes when we take a photograph we or our camera makes a poor job of controlling the exposure. Combine multiple exposures is the trick here
This is probably the most often used layer as you can make all kinds of adjustments to an image such as add gradients, brightnness, colour changes, intensity, filters etc etc.