For advanced photographers, it's important to understand a variety of image-related technologies and concepts. Learning about such concepts will enable advanced photographers to fine tune their photographs, achieving just the right balance in color and contrast. One such image-related concept is called sensor linearity.

DSLR camera image sensors are linear devices, whereas human vision is non-linear.

This means that camera sensors often need to adjust for human viewing. A simple way to imagine this is to think of sensor linearity being represented by a straight line, with human vision being represented by a line which curves. 

The human eye is at its least sensitive in bright conditions, and so the brightest part of a scene will be the hardest for us to see. However, camera image sensors use most of the 255 available tones on the brightest stops, which leaves very few tones for the darkest stops. That means the human eye is more sensitive -- in darker scenes -- than a DSLR camera image sensor.

To compensate for this, DSLRs apply a tonal curve to this linear raw data, to make it more pleasing to the human eye when viewed on a monitor or as a print. In reality, RAW conversion programs or in-camera compression tends to apply a vaguely S-shaped curve to the data in order to compress the larger dynamic range in a way that is visually pleasing.