Tone

Brightness is the amount of light being reflected off, or being emitted by, an object
Tone is the quantification of Brightness

Understanding tone is critical to anyone who wishes to master exposure or print making.  Whether it be film or digital cameras, or chemical or digital darkrooms.
 

Tone ranges from pure white with no detail to pure black with no detail with an almost infinite number of shades in-between.  It is common to refer to these shades as shades of gray.  For example, middle tone gray, light gray, dark gray, etc.  This does not mean tone cannot be used with color images.  In fact, there is no difference between measuring tone with color images vs. black and white images.  All images have the Lightness characteristic.

Tone is controlled in the camera, both film and digital, by exposure and is limited by the type of film and sensor.  If part of the image we are photographing is dark toned and we want to keep that tone, then we have to expose the film or sensor so that part will be captured as dark in our camera.

Tone is controlled in the chemical darkroom by how the film is developed, the exposure in the enlarger, any dodging and burning, the type of paper and how the paper is developed.  When exposing the paper in the enlarger, a proper exposure has to be made.  Otherwise the tones in the negative will not be reproduced in the print as desired.
 

Figure 1 can seem strange to those who are used to seeing a typical tonal range in white-gray-black.  The traditional white-gray-black range is more representative because tone is irrespective of hue and saturation.  However, it is important for one to understand that tone management is just as critical when working in color.  Therefore, I use a colored range chart in order to demonstrate this.  We can see as tone transitions from Zone II to Zone 0, so much detail is becoming lost (in both film and digital) due to lack of light that the image begins to darken and eventually becomes solid black.  Likewise, as tone transitions from Zone VIII to Zone X, light becomes so intense that the film or sensor is unable to capture details and the image begins to lighten to the point of becoming solid white.

Tone in the digital darkroom is controlled by various controls in the image editing software and is limited by the abilities of the video card, monitor, printer, paper and ink.  The computer and image editing software are capable of recognizing more tones than can be realistically displayed on a monitor and most monitors can display more tones than even the best inkjet printers can reproduce.