The Zone System was developed by Ansel Adams as a way to understand and
measure brightness. Understanding and quantifying brightness is just as important in
digital photography as it is in film photography.
The Zone System has eleven zones. Ranging from Zone 0 (solid black) to
Zone X (solid white). On the Adobe Photoshop controls Levels and
Curves, brightness is quantified into
values ranging from 0 to 255, in increments of 1. Therefore,
Photoshop allows us to distinguish 256 tones. Tone value 0 is solid black.
Tone value 255 is solid white.
If we apply a percent brightness to each Zone, then Zone X is 100% bright and
Zone 0 is 0% bright with the other Zones falling in 10% increments. Using
these percentages, we can match the 256 digital tones to the eleven Zones as
follows. Even though this is an approximate match, it serves the purpose
of assisting those familiar with the Zone System with the transition to the
digital tonal range.
Figure 1. The Zone
Why 256 discrete tones and not eleven like the Zone System?
First, the Zone System does not state that light only has eleven possible tones.
It recognizes there is a continuous gradation of tones from solid black to solid
white. What the Zone System does is divide the tonal range into eleven
segments to make working with tones more manageable and understandable.
Trying to work with thousands of individual tones would be a daunting task.
So why 256? A computer stores information in discrete units called bits.
A bit can only have 1 of 2 possible values. We can think of these values
as either on/off or yes/no. A computer (for our purposes here) uses bits
in groups of 8. To arrive at all possible values that 8 bits can store, it
is 2 to the power of 8, or 28 = 256. For those of us who can
capture (either using a digital camera or using a scanner) true 16 bit images,
then the total number of possible tonal values grows to 216, or
65,536 possible tones. Fortunately, even though Photoshop CS supports 16
bit images when working with its tone controls, such as Levels and Curves, it
lets us work with the 0 to 255 range.