Definition - Tone

Tone: 

Refers to the lightness or darkness of something.

My own definition:

How dark or light a colour appears/the different shades of one colour.

Tone has been used in a particular way here; the picture has a dark tone around the edge and gets lighter as you move into the centre. This has been done to trigger feelings of mystery and curiosity in the viewer; the lighter tone is towards the end of this bridge/port, making the viewer wonder what could be so interesting at the end.

In this picture, tone has been used to give this picture a 3-dimensional feel. The lighter tones are only found on one side of the rocks and the darker tones on the other; this shows that the Sun is on the right and because the rocks are 3D, they're creating shadows.

The sky in this picture is a much lighter tone than the main focus, which is the Eiffel Tower. By doing this, the photographer gets the viewer to pay their full attention on the Eiffel Tower rather than the sky and the clouds.

This picture also shows the element of depth as well as tone; as you move your eyes along the picture, the tone eventually gets lighter. This forces the viewer to move their eyes to the ocean and creates an atmosphere of adventure for them.