It seems that brightness, contrast, saturation and sharpness are the four simplest controls of image. On the surface, they are four mutually exclusive controls. However, as a matter of fact, they are related to each other and intertwined in a way to such a degree that the change of any one of them will lead to rather complicated effects in the image in terms of the other three controls. Only when users have mastered a rather good knowledge about how these four controls are related and how to make use of them in a harmonious way can users achieve the desired image effects in the end. It is wise for users to think twice about what they really want to accomplish before taking actions to change the brightness, contrast, saturation and sharpness, whether to increase or to reduce them.
Generally speaking, the overall effect of brightness, contrast, saturation and sharpness varies according to different contents in the photo. Take the increase of contrast as an example. With the increase of contrast, the shadow will be darker while the highlight will be brighter. However, if most details in the photo are very bright, for instance, an overexposed sunset, then we will end up with less contrast with the increase of its contrast. The reason is that there is no shadow in the photo at all, which means that the separation of shadows and highlights in an image containing only highlights will just compress the highlights. As a result, the image will get less contrasty. We can safely come to the conclusion that it is vital to have a good understanding about how these four simplest controls affect each other and how they work in a harmonious way. It is a bit of art to use brightness, contrast, saturation and sharpness to achieve a balance.