Zone System

A specific method for measuring and evaluating brightness

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 tonal 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 System

Zone	Min Digital Value	Mid Digital Value	Max Digital Value	% Brightness	Visible Detail	Gradient
0	0	6	12	0%	No
I	13	25	38	10%	No
II	39	51	63	20%	Barely
III	64	76	89	30%	Yes
IV	90	102	114	40%	Yes
V	115	127	140	50%	Yes
VI	141	153	165	60%	Yes
VII	166	179	191	70%	Yes
VIII	192	204	216	80%	Barely
IX	217	230	242	90%	No
X	243	249	255	100%	No

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.