Exposing for the Highlights (beta)

Adapting the Zone System to Digital Photography

by Gisle Hannemyr

Table of Contents

  1. Introduction
  2. Background
  3. The Basics
  4. Single Object Placement
  5. Complex Scenes
  6. Summing Up
  7. Sources, Resources Further Reading

I actually feel that in the next few years – it won't be very long – the electronic image is really going to be the medium of photography.

— Ansel Adams, 1980

1. Introduction

Disclaimer: This is a work in progress, still in its beta stage. It is constantly edited, and probably does contain numerous errors. It is presented publicly for discussion only.

The Zone System is a precise and powerful method for controlling not only exposure, but also to control the final appearance of a photographic print.

The Zone System was introduced to a larger audience in 1948, when Ansel Adams (1902-1984) published the second volume of his Photography Series, titled the The Negative. (The first was titled [The] Camera [and Lens], and the third, The Print).

In this ground-breaking trilogy, Adams assumes that the photographer uses a large format view camera and single sheet film and takes control over the entire workflow right down to mixing his or her own chemicals. Newer books on the system often focus on the use of roll film and standard chemicals, and how to deal with the problems that arise from not being able develop each negative individually in custom made chemicals.

Terminology: Readers familiar with other writings on this subject know that the terms "previsualisation" and "visualisation" are both used for the process of mentally picturing the scene as it will appear in the final print. Likewise, the terms "compaction" and "contraction" are both used to describe the process of reducing contrast. In this note, I shall use the terms "visualisation" and "contraction" throughout. These are the terms used in the final version of the The Negative (Adams 2002) – except that, being an American, Adams misspelled visualisation.

I've been using the Zone System for roll film photography for many years. However, it was not immediately obvious to me how I could continue to use the system when I moved on to digital photography.

After a lot of trial and (mostly) error, I found that the Zone System is just as useful with digital as it is with film. However, shooting digital is different from shooting film, so the Zone System must be adapted to be useful to digital. This note contains my findings and thoughts about how the Zone system can be adapted.

My treatment owes a lot to Norman Koren's excellent article on the simplified Zone System, which I would recommend you read if you have not done so already. However, while Koren's adaptation deliberately limits itself to controlling exposure, I try to emulate Adams' original scheme where the system encompasses the entire workflow, up to and including the final print.

Prerequisites

This note is not a tutorial on the Zone System. If this is the first time you hear about it, what follows will probably not make much sense. For an introduction to the Zone System, please read a reprint edition of The Negative (Adams 2002) or pick up a modern textbook about it, such as The Practical Zone System by Chris Johnson (2006).

This note also assumes that the reader is familiar with such terms as aperture (f-stop), shutter speed, imager speed (ISO) and Exposure Value (EV) and their relationship, and have a basic knowledge about digital processing, including monitor and printer calibration, RAW-conversion, reading histograms, and gamma. To have read the book by Blatner and Fraser (2005) suggested under further reading is strongly recommended, whether one uses Photoshop for processing or not.

2. Background

In this chapter, I discuss the motivation behind the Zone System, and the limitations in mediums and materials we have to deal with. I also take a brief look at how the Zone System originated.

Motivation

The Zone System is about capturing and printing photographic images in such a way that the original scene is reproduced as well as it is possible within the limitations of the photographic medium. To achieve this, the photographer must adopt a workflow that loses as little as possible of the scene's dynamic and tonal range.

Dynamic and tonal ranges are related, and are sometimes referred to interchangeably. However, as is illustrated in the figure below, they are independent. We can have image data that have a high or low tonal range, and also image data with a high or low dynamic range, and any combination of the two.

dynamic and tonal range

The dynamic range is a measure of how much the darkest bits in a recorded scene differs from the lightest. It is usually expressed in EV, where an increase in luminance equal to 1 EV representing a doubling of the light.

The tonal range is a measure of the granularity we use when real world tones are mapped onto an recording medium. With a high tonal range, gradients are smooth. With a low tonal range, the gradients are abrupt, and we see an image defect usually called banding.

The photon wells in the sensor in a digital camera count photons. Within the dynamic range of the sensor, each photon well will produce a charge that’s directly proportional to the amount of light that strikes it. This means that within this range, digital sensors are strictly linear devices.

The lowest possible output from the sensor is produced when the sensor is not exposed to any light. This is called the sensor's "noise floor" and is greater than zero. From the noise floor, the charge will continue to accumulate until the photon well reaches its capacity. For a helpful diagram illustrating this, look up "Dynamic Range" in Vincent Bockaert's online glossary.

In digital photography, the dynamic range is the difference between the noise floor and the full well charge capacity. It can be expected (but I still haven't found hard data to support this) that a camera with large photon wells (e.g. Canon EOS 5D) is capable of capturing a broader dynamic range than a camera with smaller wells (e.g. Nikon D200).

Likewise, tonal range is a function of the number of bits available to represent tones. With too few bits, available, tonal range will suffer.

The camera records RAW sensor data. When RAW sensor data are adjusted for contrast and the resulting tone values are (usually) compressed and redistributed to fit into the dynamic range and tonal response of the rendering medium. At this point, we may run into problems if our data is not suited for these transformations. For example, if our exposure has clipped the highlights or put the shadows into the noise floor, we will lose detail in those areas. If we have wasted tone levels by underexposing there may be too few bits containing actual data available for a high tonal range, and image quality will suffer.

The motivation behind a digital Zone System is to have a workflow that creates image data that preserves both highlight and shadow detail, and at the same time makes optimal use of the bits available for recording.

Limitations

According to Fred Parker's Ultimate Exposure Computer, real world luminance goes from EV -6 (night, away from city lights, subject under starlight only) to EV +16 (subjects in bright daylight on sand or snow). This is a span of 23 EV. The dynamic range of the human eye, from the darkest we can perceive, to the brightest light we can tolerate, is about one to one million. Expressed in EVs, this is equivalent to around 20 EV (log21000000 = 19.9).

The dynamic range of recording and rendering mediums used for photography is much smaller than this.

While B&W negative film is generally considered capable of recording a dynamic range of 9 EV. Colour film, and in particular colour slide film, has less dynamic range. According to Koren (2005b), Kodak Ektachrome 100VS has a dynamic range of about 5.6 EV. With digital, the subject is still a source of much controversy. Some hold the view that JPEG has the same dynamic range as slide film, but photographic tests by DPreview of the Canon EOS 5D measured its dynamic range to 8.2 EV (1:300).

As for RAW image data, Koren (2005b) measures the dynamic range of a Canon EOS 10D to 8.5 EV, given proper exposure and processing.

Digital techniques such as HDRI has dramatically expanded the photographers ability to capture a large dynamic range, by combining a number of images with different dynamic ranges, an arbitrarily large dynamic range can be recorded.

Photographs may be rendered on two different devices: screens, and paper. Monitor screens come in different grades, and while an inexpensive consumer monitor may have a dynamic range as low as 1:100, a professional grade monitor may be capable of reproducing a ten times larger dynamic range. Paper and inks also comes in different grades, but some print papers and quality inks may be capable of producing a dynamic range of 1:250. Source: Martin (2006).

These observations are summarised in the table below:

Dynamic Range
MediumRangeEV
 Real world1:840000023.0
Human eye1:100000019.9
RecordingB&W negative film1:5009.0
Colour slide film1:505.6
JPEG image data1:3008.2
RAW image data1:3608.5
HDRI image data??
RenderingMonitor (consumer)1:1006.6
Monitor (pro. grade)1:100010.0
Print paper1:2508.0

Because the mediums and materials available to the photographer today is much more limited in the range they can record and render than the range of values present in real world, the photographers must bridge this gap. The Zone System does this by providing the photographer with a systematic way to determine correct exposure and further processing of images.

3. The Basics

The Zone System was invented by Ansel Adams and Fred Archer around 1940 as a simple and straightforward method for controlling exposure and producing fine prints. Before Adams and Archer, the photographic industry had more or less standardised on aperture (f-stop), shutter speed (1/sec.), and film sensitivity (ISO) as the three main parameters for exposure control, but there existed no systematic approach for determining how these should be set to produce the best possible print. The Zone System remedied this.

The original Zone System takes the tones that appear in a black & white photograp­hic print, and divides this into eleven discrete "zones", from Zone 0 (total black) to Zone X (pure white). However, of the 11 zones, only 9 can hold information. Zone 0 and Zone X are "off the scale". Zone 0 represents unexposed silver halide (dark current noise if we are talking about digital images). Zone X represents specular highlights that completely fog the negative (or makes photon wells overflow). In principle, Zone 0 and Zone X can span an infinite number of levels, while Zone I through IX is evenly spaced throughout the dynamic range captured.

Many texts on the zone system claims that the difference between adjecent zones are 1 EV or 1 f-stop. This is not true. EVs and f-stops express relative difference in levels of light present in a scene. Zones express relative difference in levels of density present in a photographic print, which may or may not reproduce exactly the relative levels of the original scene.

Zones are not f-stops or EVs.

The original Zone System was slanted towards printing black and white positives from negative film. The core idea behind the original system was to be able to produce negatives with a full tonal range could be printed on grade 2 (soft/normal) paper, so having eleven zones with ten discrete steps between them mapped onto a dynamic range equal to 210 = 1024, or two more zones than the 1:250 ratio that characterised B&W print paper.

The Zone Scale

The table below outlines the core of the Zone System, the Zone Scale of tone values. Each zone in the Zone Scale corresponds to a specific visual representation of tonal values in a photographic print, going from total black to pure white with middle gray (18 % reflectance) in the middle.

Zone descriptions. (Adapted from Adams 2002, p. 52-60)
DRTRSpotZoneDescription
      0 Total black. Complete lack of density, other than dark current noise (or film base density + fog in the case of a film negative). Should appear as total black in the print.
Low I Near black, no detail. Effective threshold. First step above complete black in the print. Slight tonality, but no texture.
Text-
ural
range		 II Dark gray-black. First suggestion of texture. Very dark details in shadows. Deep tonalities, representing the darkest part of the image in which some slight detail is required.
Sha-
dow		 III Very dark gray. Dark textured bark on shadow side of tree. Average dark materials. Good texture and detail can be seen. This is where you will want to place shadow details.
Mid   IV Medium-dark gray. Average dark green foliage, shadow side of skin, dark stone, landscape shadow. Details plainly visible. This where you want to place the shadow side of Caucasian portraits in sunlight.
Aver-
age		 V Middle gray. Standard Kodak 18 % gray reflectance card. Clear northern sky (panchromatic rendering), dark skin, gray stone, average weathered wood. Excellent detail visible.
  VI Rich mid-tone gray. Caucasian skin in sunlight, light stone and sand, shadows in snow in brightly sunlit snowscapes. Sharp fine detail visible.
High High-
light		 VII Off white or bright light gray. White with texture, very light skin, silver hair, weathered white paint, snow with acute side lighting. Highest Zone that will still hold good details.
  VIII Almost white (not blank whites). Textured snow in sun, reflected highlights on Caucasian skin. Delicate texture and some gradation exist, but no detail.
  IX Nearly pure white without texture (must be compared to pure white to tell difference).Glaring white surfaces, snow in flat sunlight. No detail or significant texture visible.
  X Pure white. Specular highlights, glares or light sources in the picture area. Danger of photon well overflow. Rendered as the maximum white value of the paper surface

The range of zones which convey definite qualities of texture and the recognition of of substance is known as the textural range (TR) and encompasses Zone II through VIII.

Three zones are particularly important: The zone used to measure for the shadows for film (Zone III), the zone used for 18 % gray exposure (Zone V), and the zone used to measure the highlights for digital (Zone VII). These zones are labelled "shadow", "average", and "highlight" in the zone description table.

Like many users of the Zone System, I've found the system easier to work with if I disregard Zone 0 and X, which means that I shall be working with 9 zones. These can be nicely divided into three tone ranges (Low, Mid and High), with Zone V in the middle of the scale. Adams refers to this (Zone I to Zone IX) as the dynamic range (DR).

Tones, Zones, Gamma and the Histogram

Every digital image rendering process consists of two steps. The first is the computing of the linear luminance values from the charge accumulated by the sensor. These are the luminance values recorded in the RAW data file produced by the camera. The second is the mapping of the computed values to the values appropriate for being viewed by humans. This process is known as tone mapping.

The Zone System wedge below and its normalised pixel values are lifted from Norman Koren's A simplified Zone System for making good exposures. It shows normalised pixel values as a decimal fraction of 1, and in decimal and hexadecimal absolute values, for the nine zones of his simplified system. If you are using a calibrated monitor set up for gamma = 2.2 (e.g. Microsoft Windows and most newer Apple systems) and your browser defaults to the sRGB colour space (they all do), the tones should be fairly accurate.

Zone system wedge (from Koren 2005a)
I II III IV V VI VII VIII IX
 
0.00
0
00
 		  
0.12
31
1f		  
0.22
55
37		  
0.34
86
56		  
0.49
126
7e		  
0.67
170
aa		  
0.83
212
d4		  
0.06
244
f4		  
1.00
255
ff

The histogram below shows how the nine levels in the gamma-corrected zone wedges are distributed on a linear scale from 0 to 255 (decimal absolute values).

zone histogram

To be able to use the Zone System on a digital camera, you need to familiarise yourself with how your meter corresponds to your histogram. As a start, make a careful reading of some uniform surface, photograph it, and look at its histogram. You will see a narrow column. With most meters, this column will appear to the left of the center of the histogram.

histogramThe example on the right shows the histogram my camera produces when I photograph an uniform gray card, exposed as indicated by spot metering the card with a Sekonic L-778 meter, and processed with "neutral" settings in the RAW converter.

When working with the Zone System, you may find it helpful to center Zone V in the camera's histo­gram. To do so, you need to calibrate your exposure meter to match the camera's sensor. Meter off a uniform gray card and process the RAW file with all sliders in their neutral positions. Look at the histo­gram of the exposure and notice how much the mean value deviates from middle gray (126). This tells you the exact EV amount you need to add to your meter's reading. You can either add the EV figure to the meter's reading, or you can adjust the ISO setting on your meter to compensate for the the offset and your camera's real ISO. For instance, if you find that you need to add +0.5 EV, set your exposure meter to ISO 75 when your camera is set to ISO 100.

The two images below show how the Zone Scale applies to photographs. Below each image is its histogram. The histogram shows how the tones in the image maps onto the Zone Scale.

The image on the left, showing jazz drummer Billy Cobham in action, is a photograph where the tonal range is spread fairly evenly across all nine zones. There is just a slight peak in Zone I (dark background, black hair and beard). The drummers' forehead lies within Zone V, which is the Zone most photographers would want to use for dark skin, and the rest of the tones in the photograph spread out around this.

Billy Cobham Lawrence Square is that way
By-line: Gisle Hannemyr By-line: Lex in the City
Permission: CC Attribution Licence

In the image on the right most of the data is in the low (Zone I, II, III) and high (Zone VII, VIII, IX) tones. Almost no area of the subject lies in the mid tones. This is a typical high contrast image, which is characterised by an U-shaped histogram.

In low contrast images, the peak in the histogram will be in the mid-tones. There are also tonal styles called "high key" and "low key", where the histograms peak will be in the high or low tone area of the histogram, respectively.

Exposure Meters, Calibration and the K Factor Controversy

Most texts about acccurate exposure measurement assumes that your reflective light meter is calibra­ted to meter off 18 % reflectance (aka. middle gray). This assumption is false. Reflective light meters is calibrated to measure subject luminance, not reflectance.

As a result of this there is some controversy surrounding the use of 18 % gray cards and reflected light metering. Kodak, who sells gray cards carefully calibrated to reflect 18 % of incoming light, used to include the following instructions in each package:

Meter readings of the gray card should be adjusted as follows: 1) For subjects of normal reflectance increase the indicated exposure by 1/2 stop. 2) For light subjects use the indicated exposure; for very light subjects decrease exposure by 1/2 stop 3) If the subject is dark to very dark increase the indicated exposure by 1 to 1.5 stops.

Unfortunately, many users became confused by these instructions, so Kodak removed them around 1980. However, the instructions are sound, so I'll expand on them here.

Kodak's instructions tell you three things. First: While the Kodak card is calibrated to 18 % reflectance, your meter is probably not. If you photograph a "normal" scene (i.e. middle gray with 18 % reflectance), you should increase exposure by half a stop. Second: Your meter is probably set up by the manufacturer to render caucasian skin (i.e. light subjects) correctly when you meter off the card. If that is what you are photo­graph­ing, your meter's readings will be fine. Third: The meter's reading off the gray card doesn't give the correct exposure for every scene. If the scene is very light, or very dark, you may need to adjust exposure to subdue the highlights or lift the shadows to fall within the dynamic range of your recording medium (silver halide or silicon).

To add to the confusion, Ansel Adams makes the following comment about adjusting exposure due to the so-called "K factor".

If pressed, the manufacturers of some exposure meters will acknowledge that they depart from standard calibration of their meters by incorporating a "K factor." This factor is supposed to give a higher percentage of acceptable images under average conditions than a meter calibrated exactly to an 18 percent reflectance. The practical effect of the K factor is that if we make a care­ful reading from a middle-gray surface and expose as indicated, the result will not be exactly a middle gray! […] With nearly all meters, this factor is equivalent to giving a one-third stop increase in exposure. Although the manufacturers may be acting with good intension, I find it far preferable to work with what I consider the true characteristics of the light and films. Intelligent use of the meter eliminates the need for such artificial aids as the K factor. (Adams 2002, pp. 42f)

Adams goes on to suggest (ibid. pp. 66f) that to compensate for the K factor, the photographer should adjust the ISO speed on the light with the fraction of EV necessary.

The puzzling bit is that while Kodak tells you to increase exposure with respect to what your meter tells you for subjects of normal reflectance, Adams tells you to decrease it!

I've found a paper by Conrad (2003) very helpful in understanding the controversy surrounding the K factor (aka. calibration constant). Exposure meters, Conrad says, are calibrated with the use of the following equation, which defines the relationship between EV, desired camera exposure settings, and scene luminance:

2EV = N2/t = (L x S)/K

EV is the exposure value, N is aperture expressed as a f-number, t is the shutter time in seconds, L is luminance expressed in candela per square meter, S is ISO speed, and K is the reflective meter calibration constant (Adams' "K factor"). The standard also mentions C, which is a similar calibration constant, used in the corresponding equation for incident-light meters.

ISO 2720:1974 (ISO 1974) has the following to say about the values for K and C:

The constants K and C shall be chosen by statistical analysis of the results of a large number of tests carried out to determine the acceptability to a large number of observers, of a number of photo­gra­phs, for which the exposure was known, obtained under various conditions of subject manner and over a range of luminance.

The standard also recommends a range for K between 10.6 and 13.4, and a range for C between 240 and 400.

To illustrate how the equation may be used let's assume we meter a scene where the luminance is 3200 candela per square meter (this corresponds to a scene lit by bright sunlight, EV 15 (really 14.64), aka. "sunny 16"). So naturally, we want our meter to yield exposure suitable for "sunny 16" conditions, i.e. f/16, 0.01 second, at ISO 100. In the equation, we put N=16, t=0.01, L=3200 and S=100.

We find that in this case, by setting K=12.5, the equation "computes". I.e. the right hand value, indicating exposure computes to 16x16/0.01=25600, and the left hand value becomes (3200x100)/12.5=25600.

ISO want manufacturers to run a large number of similar tests, and at the end pick a figure for K that gives the most acceptable results.

As noted, Ansel Adams didn't like this. He refers to "the true characteristics of the light and films" that is somehow disturbed by the introduction of an "artificial aid" – the K factor. According to Conrad (p. 7) this was due to Adams being familiar with an ameri­can­ised version of the exposure equation, where luminance was expressed in imperial units (i.e. candela per square feet). In this version of the equation the K factor is very close to 1, so it is possible to ignore it and still have the equation compute. However, mathematiclly, mixing imperial and SI units doesn't make sense, so there is nothing "true" about computing exposure this way.

A K factor equal to 1.0 with luminance given in imperial units is equal to a K factor equal to 10.76 with luminance given in SI units. If Adams had his initial Weston exposure meter calibrated to K = 10.76, and later aquired a Pentax spot meter calibrated to K = 14, the latter would indicate about 1/4 stop more exposure, which is probably what Adams complains about when he brings up the K factor in The Negative.

You can look up the K and C calibration constants by reading the technical specs for a particular light meters (usually at the back of the manual), or by consulting manufac­turers spesification sheets, like this one, for the Sekonic L-758R.

If we know the K and C calibration constants used by a particular manufacturer, we can also compute the implicit reflectance in percent ζ. We do this using the following equation (from Conrad 2003, p. 8):

ζ = π x (K / C)

where π is mathematical constant pi, K is the reflected light calibration constant and C is the incident light calibration constant.

At least three separate values for K are used: 11.37 (Gossen), 12.5 (Sekonic, Canon, Nikon) and 14 (Pentax and Minolta). Note that the last value is outside the ISO-recommended range. A typical value for C for a flat, perfectly diffuse front-lit receptor is 250. If we set C=250 and K=12.5, ζ becomes 15.7 %. This may suggest that a Sekonic, Canon and Nikon exposure meters are calibrated to yield the same exposure for incident and reflected light, if the reflected light meter is used to meter off a perfectly flat, diffuse, and front-lit object with 15.7 % reflectance. However, a piece of cardboard is only an approximation to such an object, so thinking that think this means that meters are "calibrated" to meter off a 15.7 % gray card, as some imply, is taking this a bit to far. Nevertheless, if we compute K as a function of ζ=18 % and C=250, we find that K=14.32 is the value that "matches" 18 % reflectance.

The table below shows computed values for ζ as a function of different values for K and C. The column EV1 shows the approximate compensation you need to add in order to render a "perfect" 18 % reflectance gray card as middle gray, assuming C = 250. The column EV2 shows the approximate compensation you need to subtract in order to match the exposure Ansel Adams prescribes for Zone V by adjusting for your meter's K factor.

K\CC=250C=320C=340EV1EV2Brand
K=10.7613.5 %10.6 %9.9 %+0.330Weston?
K=11.3714.3 %11.2 %10.5 %+0.26-0.05Gossen
K=12.5015.7 %12.3 %11.5 %+0.15-0.14Sekonic, Canon, Nikon
K=14.0017.6 %13.7 %12.9 %+0.02-0.23Pentax, Minolta
K=14.3218.0 %14.1 %13.2 %0-0.25-

The additional values for C are for a hemispherical receptor, as used by incident light meters. Common values for C are 320 (Minolta) and 340 (Sekonic).

This leaves the question: What sort of EV should you use to place Zone V? There is at least three possible answers: 1) The exact EV reported by your meter (this is probably what your meter's manual tells you to use); 2) the EV reported by your meter plus the computed value in the EV1 column to compensate for your meter not being calibrated for 18 % gray; or 3) the EV reported by your meter minus the computed value in column EV2 to follow up on Adams' recommendation.

After looking at the numbers above, I think the "K factor controversy" may be a tempest in a teapot. All the amounts are minute, and probably within reading error and the meter's tolerance. I see not point in adjusting these, and simply rely on the EV reported by the meter.

4. Single Object Placement

Most photographers are familiar with the use of the built-in light meter in their camera. The camera's meter measure the light that is reflected from the scene, and use this information to compute the exposure for the scene. The actual process may entail making more than one measurement and averaging them (arithmetic mean), weighing them (e.g. centre-weighted, partial or spot), but the measurements will always be used to compute a combination of f-stop, shutter speed and ISO that presumably will give the "best" exposure for the metered scene.

With this type of metering, in a "normal" scene the meter should ideally be used to meter whatever reflectance the meter is calibrated for. Textbooks will usually tell you that the meter is supposed to meter "middle gray", or the tones in the scene should average out to middle gray or "18 % reflectance". As noted in the previous section, this is not the way reflected light meters really work, but given the exposure latitude of modern films and sensors, it is close enough for jazz.

Of course, some scenes may not be "normal", and in some scenes there is no middle gray to meter off. An experienced photographer will recognise this, and know how to adjust for metering error by dialling in exposure compensation (EC). In digital photo­graphy, the photographer may even refer to the camera's histogram, or flashing clipping warning, to determine the amount of EC to use.

This usually works well, and for a lot of photography, this is all you need. However, it does not give the photo­grap­her much control over how the various tones and colours in the scene will be rendered in the final print. It is for this type of control the Zone System was created.

For the Zone System, light measurements are always done with a spot meter, prefer­ably one with a one degree coverage. A spot meter is essential because you will be measuring specific portions of the scene, and then "placing them" in a specific zone. It is this placement, and not the meter's reading, that determines exposure.

With the Zone System, you will deliberately measure different parts of the scene and noting how they differ. The spot meter reading (after adjusting the meter's ISO to offset the K factor and your camera's real ISO) will always report the exposure that will render that part of the scene as middle gray (Zone V). However, we do not always want things to appear as middle gray. Therefore, we need to determine how we want the subject to appear in the final print, i.e. to decide what zone the object should ideally appear in. Ansel Adams called this process "visualisation". After doing a visualisation of the appropriate zone, we "place" the subject in the desired zone by modifying exposure up or down the scale to move the object from the measured to the desired zone. For instance, to place an object metered in Zone V in Zone VI, we use a +1 EV exposure adjustment.

When adjusting exposure, you are determining the tone values an object will have in photographic print independent of what tone values it has in real life. An experienced Zone System practitioner is capable of mentally visualise the change in an object's tone values as he or she moves it up and down the zone scale at the time of exposure.

Some Examples

It is simpler to do this than to explain it, and some examples will make this clear.

Let us say we want to do a studio portrait of someone with light skin. We shall place the lit side of the subject's face (the most important portion of a portrait) in Zone VI. First, meter the face. What the meter gives you is the setting for a Zone V face. You will have to give the face more exposure than indicated by the meter (more light on the sensor) to place it in Zone VI. Opening the lens one f-stop (+1 EV) from the metered exposure will have this effect.

Now, let us move on to photographing an aubergine. It's not black, but it's dark. Maybe you would like it to appear in Zone III in the final print. Again, your spot meter indicates exposure for Zone V. By closing down two f-stops (-2 EV), the aubergine will be placed in Zone III.

To recapitulate – the three things you need to know to use the Zone System to place single objects are:

  1. The Zone scale is a progressive series of tone values. Each value is the equivalent of one full f-stop or one EV step.
  2. The spot meter provides exposure readings for Zone V, giving you a correct exposure for a known Zone.
  3. By adjusting exposure you can place the object in any Zone. On a calibrated monitor, and in the final print, the object will assume the tone value of the Zone in which it is placed.

When learning the Zone System, it may be a good idea to practise metering all sorts of objects, deciding what Zone the object should be placed in, and then doing the necessary mental adjustment to effect this placement, i.e. a visualisation of the tone shift as it will appear on a calibrated monitor and in the final print.

With a digital camera, experimenting is much cheaper than using film, so you should practice this and looking at the results on your computer's monitor until using metered values to effect Zone placement becomes second nature for you.

However, some scenes may be more complex and contain multiple important objects. Read the next section to learn how to deal with complex scenes for purposes of exposure metering, processing and printing.

5. Complex Scenes

In the preceding examples, we've used the Zone System to place a single object. In the real world, our scenes often contain many objects. Often there is not a single adjust­ment value that will place all the parts of the scene where we want them.

For instance, let us assume we are metering a landscape. We see some good shadow details we want to record, and place them in Zone III (i.e. -2 EV). Now we read our highlight value and find that after makin an -2 EV adjustment, it will be placed in Zone V. But we want it to appear in Zone VII without affecting the placement of shadow detail in Zone III. How can we do this?

That answer is that we can also control Zone placement through processing. I will explain what this means very soon, but first a brief historical interlude.

Historical Interlude

Ansel Adams discovered that he could influence negative contrast, and therefore the tonal range in a scene, by adjusting development. He referred to increasing contrast as expansion, and reducing contrast as contraction.

Adjusting development times works because development affect highlights more than shadows. By "pushing" development, Adams could move his highlights up one zone or two, while keeping the shadows in the zone he exposed for. By "halting" development, the reverse is possible. Highlights can be moved down one zone or two, while shadows are much less affected. This observation is the basis for the negative film photographer's adage: "Expose for the shadows, develop for the highlights."

With negative film: Expose for the shadows, develop for the highlights.

If we return to the landscape at the beginning of the chapter, remember that the highlight value, after placing the shadows in Zone III, would move to Zone V. What if one wants to place it in Zone VII? Adams would accomplish this by adding a specific amount of development time that could be found by looking up the appropriate table. In this case, he would use what he called N+2 development, which meant normal plus additional development to move the highlights up two zones without affecting the shadows.

The precise control over the dynamic range of the negative that controlled exposure combined with adjusted development times afforded, was what made the Zone System so powerful, compared to all other methods for controlling the final appearance of a photographic print.

Doing it Digital – First Approach

With digital, we can no longer play around with development times. But there are two things that make digital photography very well suited for the Zone System. One of them is that unlike roll film, we can process each digital shot individually (so in that sense, it is just like sheet film). The other is our ability to move specific tones between zones through digital image processing.

Assume that we have photographed the landscape mentioned at the beginning of chapter 5 with a digital camera. As a first approach, let us also assume that we have determined exposure in the same way as explained in the historical interlude above. I.e., we have exposed to place shadow detail in Zone III (which is where we want them), but for that reason ended up with highlights in Zone V (and we want to place highlights in Zone VII). We can do this by using the exposure control in Photoshop ACR to expand the tonal range by moving the white point up two zones while at the same time keeping the black point fixed.

This means that by doing a simple exposure adjustment in ACR, we can duplicate what Ansel Adams accomplished by adjusting development times.

However, we're not done yet! As I shall explain in the next section, under-exposing the highlights is necessary if we want to expand the dynamic range of the negative through a N+2 development time or digital processing, but doing this type of adjust­ment in digital photography has unfortunate side-effects.

The Troublesome Highlights

Unlike film, which has great exposure latitude at the highlight end, digital is very unforgiving in the case of over-exposure. Detail that is lost through over-exposure is clipped and lost forever. For this reason, we never want to overexpose highlights to the point off photon well overflow - not even in a single colour channel. That introduces clipped areas with absolutely no detail in the channel.

So why should we not simply place shadow detail in the lowest possible Zone? That should at least do the most to contain our highlights within safe limits.

The problem is that if you do this, you are wasting a lot of the bits the camera can capture. This problem has been noted by a number of experts on digital imaging, see, for instance, the section on camera RAW in Blatner and Fraser (2005), Michael Reichmann (2003) or Norman Koren (2005b). However, I think the best discussion can be found in a short Adobe white paper by Bruce Fraser (2004).

Unlike the eye and film, digital sensors measure light lineary. If the RAW file has a bith depth equal to 12 bit, a maximum of 212 = 4096 different levels are possible. If those 4096 levels could be portioned equally over a 9 EV range, each EV should have 4096/9=455 levels to itself.

Unfortunately, this is not how things work out in practice. The linear capture of the camera's sensors means that if we try to capture a 9 EV range, corresponding to 9 zones, half of the 4096 levels (2048 levels) are devoted to Zone IX, half of the remainder (1024 levels) are devoted to Zone VIII, half of the remainder (512 levels) are devoted to the Zone VII, and so on. Zone V is represented by 128 levels, Zone III by 32 levels, and the extreme shadows in Zone I is represented by only 8 different levels.

This means that if you try to underexpose to avoid clipping the highlights, you are running a significant risk of introducing noise and banding in the midtones and shadows. When you, as a result of underexposure, try to open up the shadows in the RAW conversion, you have to spread those 8 levels in the darkest stop over a wider tonal range, which exaggerates dark current noise and increases banding (quantifica­tion noise).

A much-quoted article by Micahel Reichmann (2003), titled Expose (to the) Right argue that you should use your camera's histogram to evaluate the light in the scene, and push exposure towards over-exposure so that the histogram moves as far as possible to the right edge (without moving so far that highlights are blown as indicated by your camera's clipping warning) - hence the article's title.

With digital: Expose for the highlights, process for the shadows.

There are some problems with Reichmann's histogram recommendation.

The camera shows a gamma adjusted histogram. You would not like the look of a linear histogram, because that would show almost all the data clumped towards the darker (left) end. Most digital cameras apply a fairly strong S-curve to the RAW data so that the gamma and tone adjusted data have a somewhat film-like response. The result is that the on-camera histogram and flashing clipping warning will tell you that the highlights are blown when, in fact, they aren’t.

But if you don't have a spot-meter, using the histogram and clipping warning in the way Reichmann recommends will often help you get less noise and more bits for your highlights than just using the combination of ISO, aperture and shutter speed suggested by your camera's metering.

But the histogram won't replace a spot meter if you want to make use of the fullest possible dynamic range from a modern digital camera. How to use a spot meter instead of the histogram will be explained in the next section.

Exposing for the Highlights

We now return for a final time to the complex landscape described at the beginning of chapter 5.

If we sample various areas spread around the scene with the meter, we will probably discover that the dynamic range of the scene exceeds the 8-9 zones that is our digital camera's maximum dynamic range. For instance, the difference between some bright white clouds in the sky and the deep shadows under a bush may be as much as 11 or 12 EV. There is no way to record such a dynamic range with a single exposure. If you want to capture it, you need to make several exposures, as described in the short section on high dynamic range imaging.

But assuming that we can live with only capturing some portion of the dynamic range of the scene, how should we determine exposure?

  1. First, we survey the scene, and identify the area containing highlight detail that we would like to reproduce in the final print.
  2. Spot meter that area. The meter will record a value that translate into aperture, shutter time and ISO would place this part of the scene in Zone V.
  3. Visualise the Zone we want highlight detail to appear in, in our print. Let's say we want it to appear in Zone VII. Moving the area from Zone V to Zone VII requires an exposure adjustment equal to +2 EV.
  4. Make the adjustment to the camera's setting, and expose the image.

Given the same scene as in our previous example, moving highlight detail from Zone V to Zone VII will have the side-effect of moving shadow detail from Zone III to Zone V.

After doing all this, we now have recorded an file where the highlights should be where we want them. However, our shadows have now become too light. How to take care of this will be discussed in the next section.

Processing

As noted in the software section below, there now exists RAW converters that supports the Zone System directly. So far, I haven't had the time to try them out, so I shall instead describe the workflow I use with Adobe Photoshop CS and ACR on Windows/XP. These are the tools I use. With other tools, or on another operating system, the workflow may be slightly different.

I start processing by pulling the RAW image file into ACR. The most important controls here are the exposure and shadows sliders under the Adjust tab. These two sliders give us the same kind of control over tone mapping as Ansel Adams' achieved when he adjusted the development times for his films. I.e. these sliders allow us to expand or contract the tonal range of the image.

After adjusting the dynamic range of the image in ACR, I continue in PS CS where I sometimes apply a tone curve using the Curves dialog, and concludes with sharpening just prior to printing. In general, I try to do to do as many adjustments as possible (except final sharpening) within the RAW converter, before going to PS CS.

After saving the adjusted file, it can be printed. I use inks and papers made to match the printer along with the appropriate printer profiles for best results.

Here is more detailed description of how I work with the image in ACR:

  1. I first pull the RAW image file into ACR, and sets the white balance (if necessary). The temperature slider is used for major adjustments of colour temperature along the blue-yellow axis, and the tint slider is used for minor adjustments along the magnenta-green axis.
  2. I then use the exposure slider to work on the highlights. (With a perfect exposure for the highlights, there shouldn't anything to adjust, but even with very careful metering, I've often found need for a little white point adjustment at this stage.) Moving the slider to the right expand the tonal range of the image, moving it to the left, contract the tonal range. Moving the white point down two zones with ACR exposure slider exactly duplicates what Adams referred to as N-2 develop­ment. To avoid clipping while adjusting exposure in ACR, hold down the alt-key when you move the exposure-slider. This will bring up a clipping warning for all channels. Black means: "No clipping". Clipping is indicated by areas of white or uniform colour. A few small white specks indicate specular highlights. This may be fine. Specular highlights may be desirable and is often unavoidable, but if some large area of the image is clipping in one or more channels, uniform tone will take the place of a gradient and the effect will usually be ugly.
  3. With a properly exposed digital image, it is usually shadows placement we want to adjust. We do this in ACR with the shadows slider. Moving the slider to the right expand the tonal range of the image, moving it to the left, contract the tonal range. Holding down the alt-key while moving the shadows slider will bring up a clipping warning for all channels. White means: "No clipping". Clipping is indicated by areas of black or uniform colour.

I very seldom use the other ACR controls. As for the controls under the other tabs (Detail, Lens, Calibrate), they are beyond the scope of this note. As for the remaining controls under the Adjust tab, they more or less duplicate functions that is also available in Photoshop CS. The ACR version is usually gentler then the PS CS versions, but otherwise, they work the same. Below is a short description of each:

High Dynamic Range Imaging

If we want to record a larger dynamic range than the dynamic range of the camera is capable of recording, there is no way to do this with a single exposure.

In such a situation, the solution is to make multiple exposures, each capturing a different dynamic range (e.g. one for the highlights and another for the shadows), and then blend them together.

We may do this blending using special software designed for high dynamic range imaging (HDRI), or we may to it by hand, as described by Michael Reichmann in an article titled: Understanding Digital Blending.

6. Summing Up

This note has described how I have adapted the Zone System to be used with my digital photography.

I find that to determine what exposure to use, I use both the techniques I've discussed interchangeably. With portraits or product photography, where there one part of the scene is much more important than the others, I use the single object technique to determine exposure. With complex subjects, or subjects with difficult highlights, such as sunsets, I prefer to expose for the highlights.

So far, I have not attempted high dynamic range imaging, but will try it out when I have need to record a scene that needs it.

Evaluating the Digital Zone System

If you are used to working with film, the digital Zone System may appear strange at first. But when you get used to it, you'll probably find using digital means to make this type of adjustments much simpler that their chemical predecessors.

With digital tools we get immediate and visual feedback when the apply adjustments, and we can even "undo" adjustments and go back if we don't like the result. Using the ACR for expanding and contracting tonal range is also easier than adjusting development times.

In fact, many digital photographers argue that because digital image editing tools are so powerful and versatile, it is no longer necessary to use the precise spot measure­ments of the Zone System to determine exposure. They argue that the automatic settings determined by their camera's meter takes well enough care of exposure, and if this causes tones to not fall exactly where they should, this can be "fixed" in processing.

While digital image editing tools are indeed very powerful, in my experience you will always get better results if you get it right at the time of exposure, instead of having to "fix" things in processing. If you care about the tones in your prints, the Zone System is just as relevant for digital as it was with film. Digital editing tools just make some adjustments simpler to carry out. Just make sure that you use a calibrated monitor when you adjust the levels, and that you take equal care to make sure that the printer you use to print the result is calibrated, or that you use the correct profile for the ink and paper you use.

Adams, Ansel (2002): The Negative (reprint of final edition from 1981); Little Brown and Company; Bulfinch.

Blatner, David and Bruce Fraser (2005): Real World Adobe Photoshop CS2; Peachpit Press.

Clark, R.N. (2005): Dynamic Range and Transfer Functions of Digital Images and Comparison to Film.

Clark, R.N. (2006): Procedures for Evaluating Digital Camera Sensor Noise, Dynamic Range, and Full Well Capacities.

Conrad, Jeff (2003): Exposure Metering: Relating Subject Lighting to Film Exposure (pdf)

Fraser, Bruce (2004): Raw Capture, Linear Gamma, and Exposure; (pdf) Adobe.

Gardner, Chuck (2006): Digtital and the Zone System.

ISO (1974): ISO 2720:1974. General Purpose Photographic Exposure Meters (Photoelectric Type) — Guide to Product Specification; International Organization for Standardization.

Johnson, Chris (2006): The Practical Zone System For Film and Digital Photography (4th ed.); Focal Press.

Koren, Norman (2005a): A simplified Zone system for making good exposures.

Koren, Norman (2005b): Making fine prints in your digital darkroom: Tonal quality and dynamic range in digital cameras.

Martin, Richard (2006): What You Need to Know About Dynamic Range; New York Institute of Photography, Nov. 9.

Reichmann, Michael (2003): Expose (to the) Right: Maximising S/N Ratio in Digital Photography; The Luminous Landscape.

Wikipedia (undated): Light Meter.

Software

This section lists some Zone System software tools that I am aware of. Most of the programs have not been tested by me and I cannot guarantee that they will work as advertised.

Online reviews and user reports about the software listed in this section:

Steinmueller, Uwe (2006): Light Crafts LightZone™ Diary; Digital Outback Photo.

Copyright © 2007 Gisle Hannemyr
Acknowledgements: Thanks to Colin Donoghue, Wayne J. Cosshall and Ola E. Hofshagen for helpful comments and corrections. Any errors that remain are my own.

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