Canon PowerShot S120 Manual (en)

In order to give the image desirable gradations and luminance, 
automatic adjustments are applied for an appropriate dynamic range.
Click the [Auto] button.
In order to give the image desirable gradations and luminance, 
the dynamic range is automatically adjusted.
Use [Linear] when you adjust images using image editing software that 
has advanced editing functions. Note that when [Linear] is 
checkmarked, the image becomes dim.
[Linear] - A function for advanced adjustment
[Contrast], [Highlight], [Shadow], and [Auto Lighting Optimizer] 
 will not function when [Linear] is checkmarked.
Adjusting Gradation and Luminance Automatically
White points, midpoint, and black points in the [Gamma 
adjustment] panel as well as [Highlight] and [Shadow] in the 
[Advanced] panel are adjusted automatically.
Settings that are reset and those that are not with this adjustment 
are as follows.
Settings that are reset
・Auto Lighting Optimizer 
・[Contrast] in the [Advanced] panel
Settings that are not reset
・Brightness adjustment
・White balance adjustment
White points and black points are automatically adjusted.
An image may look unnatural after automatic adjustment. In such a 
case, reduce the degree of shadow and highlight adjustment.