An image processing apparatus capable of obtaining a gamma characteristic (non-linear conversion characteristic) giving consideration to a difference between light and dark parts in a subject region is provided along with a method for controlling the same. A value indicating the difference between light and dark parts in the subject region is calculated based on image data. Then, by interpolating multiple basic gamma characteristics using the value indicating the difference between light and dark parts, a gamma characteristic depending on the difference between light and dark parts is generated and applied to the image data.