A Simplified Zone System
By: Norman Koren
This tutorial on the Zone System, written by Norman Koren, will be of primarily interest to newcomers to photography who use negative film — either colour or B&W. Even experienced transparency users will find it of interest as a review of an often misunderstood topic. — Michael Reichmann
Why Negatives?Most professionals work with color slides instead of negatives because clients demand them. Why? WYSIWYG! What you see is what you get; it's easy to judge the quality of slides, and when they're good, they're really beautiful. It's difficult to judge anything from a negative, and prints from camera or drug stores rarely do justice to the information on negatives.
Why then would a serious photographer choose to work with negatives? Because
negatives can capture a much larger exposure range than slides. (See What
exposure range can film capture?, below.) You can make excellent images in
difficult lighting that would defeat slide film. Modern negative films, such as
Kodak's new Supra
series, are as sharp and fine grained as slide films of comparable speed. And we
now have affordable scanners, Image editing programs and color printers that
enable you to produce inexpensive prints of impressive quality — as good as
custom darkroom prints most of the time, and better when difficult manipulations
(dodging, burning, selective color shifts, etc.) are required. These scanners
work well with properly exposed slides of low to moderate contrast, but if you
work in the field with difficult or contrasty lighting, negatives have the edge.
And difficult lighting can sometimes give you the finest, most dramatic images.
Introduction to The Zone SystemThe first thing you need to know is that exposure meters are dumb, really dumb. They are clueless about what they're photographing. They assume that all scenes have the same average tonal value — middle gray — roughly Zone 5 in the chart below. If a scene is different, for example a snow scene, it will be exposed incorrectly. It will come out middle gray — underexposed. If you follow the meter's reading, every scene will have the same average middle gray density. You'll get lots of bad exposures, especially if you photograph in difficult light.
Ansel Adams developed the zone system to cope with this situation. His technique was to carefully study a scene, visualize the final print, then determine the correspondence between portions of the scene and tones in the print. He would then meter, expose and develop the negative accordingly. His basic rule was,
Ansel Adams used a view camera and developed each sheet film negative individually. His system is very elegant — it produces prints with exceptional tonality — but it requires a tremendous amount of testing, calibration, record keeping, and time. It's not practical unless you have a darkroom. With my day job I don't have that kind of time. Luckily there are shortcuts to making good exposures.
"Expose for the shadows; develop for the highlights."
(Warning: This rule applies to negatives, not slides; more later.)
We present a simplified zone system that will enable you to expose your image
correctly most of the time. It works equally well with color and black and white
negatives; exposure technique is identical. Once you get the hang of it, you'll
find it's not all that complex.
|An exception to the dumb exposure meter rule can be found in modern single lens reflex cameras with matrix metering, which tend to be expensive high-end models. A good example is the Nikon F100, which has a 10 segment meter. It meters each segment individually, then uses a computer program to determine the optimum exposure. The program employs artificial intelligence or fuzzy logic-- it's been taught to respond correctly to a wide variety of scenes. Of course there will always be exceptions-- a reader's comment in the photo.net review of the F100 indicates that it can be fooled by backlighting, but it can be trusted most of the time.|
The zone system begins with a zone chart. Zone 1 is pure black; zone 9 is
pure white; zone 5 can be described subjectively as middle gray. On paper, zone
5 corresponds to a reflectance of 18%. Since computer monitors and printers vary
greatly — Macintoshes have different brightness curves (gammas)
from PC's — what you see here may not be an accurate representation of the
zone chart. You will need to have a good feeling for Zone 5-- middle gray, so if
what you see doesn't seem right, you should go to a camera store and buy a Kodak
18% reflectance gray card. These cards are used by professionals for exposure
metering in the studio: They place the card next to the subject and meter from
it. This is equivalent to incident light metering: measuring the light
that reaches the subject. With incident metering, the exposure is independent of
the subject's reflectance: dark subjects come out dark and light subjects come
out light. Incident metering works very well in studio environments and for
close-ups, but it isn't practical for landscapes. Meters built into cameras
measure reflected light.
|Complete lack of density in the negative image, other than film base density plus fog. Total black in the print. We will omit zone 0 from the remainder of this tutorial; zone 1 will be considered pure black.|
|Zone 1||Effective threshold. First step above complete black in the print. Slight tonality, but no texture.|
|Zone 2||First suggestion of texture. Deep tonalities, representing the darkest part of the image in which some detail is required.|
|Zone 3||Average dark materials. Low values showing adequate texture.|
|Average dark foliage. Dark stone. Landscape shadow. Recommended shadow value for portraits in sunlight.|
|Zone 5||Clear north sky (panchromatic rendering). Dark skin. Gray stone. Average weathered wood. Middle gray (18% reflectance).|
|Zone 6||Average Caucasian skin value. Light stone. Shadows in snow in sunlit snowscapes.|
|Very light skin. Light gray objects. Average snow with acute side lighting.|
|Zone 8||Whites with textures and delicate values (not blank whites). Snow in full shade. Highlights on Caucasian skin.|
|Zone 9||Glaring white surfaces. Snow in flat sunlight. White without texture. (The only subjects higher than Zone 9 would be light sources; they would be rendered as the maximum white value of the paper surface.|
In a scene — in the field — each zone represents a doubling or halving of the luminance — the light reflected from the subject — or equivalently, a difference of one f-stop. The eight steps between the nine zones in the chart (1-9) represent a luminance range of 256 (28). On paper surfaces, this difference is considerably compressed. On good photographic paper, pure white is about 90% reflectance and pure black is about 2% reflectance. The maximum tonal range is around 45, equivalent to about 5.5 zones (log245). Reflectance differences between zones are less than a factor of two. The difference between zones at the ends of the scale (1 and 2 or 8 and 9) is much less than between zones in the middle (4, 5, and 6).
|A beginner's glossary of confusing photographic terms|
is the amount of time the shutter is open during exposure. It is the
inverse of the number on the dial. The standard sequence is 1, 2, 4, 8,
15, 30, 60, 125, 250, 1000, ..., where 125 represents 1/125 second, etc.
Aperture is the variable opening in the lens that admits light.
F-stop is a measure of the aperture opening, defined as a lens's focal length divided by the aperture diameter. F-stops are sequenced in multiples of the square root of two: 1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, etc. Increasing the f-stop by one step halves the light reaching the film. F-stop also refers to a change in the aperture by one step, which doubles or halves the light reaching the film. "Opening up" or "increasing" the aperture means admitting more light; reducing the f-stop. Similarly, "stopping down" means admitting less light; increasing the f-stop. The speed of a lens is its maximum aperture, i.e., smallest f-stop.
Film speed is a measure of a film's sensitivity to light. Faster films tend to have more grain and less resolving power.
Exposure is the total light reaching the film. It is a function of the luminance of the subject (the light it emits), the aperture setting, the shutter speed and the film speed. Photographers often say, "increase the exposure by one f-stop," or "stop down by two f-stops." They don't literally mean change the aperture. They mean adjust either the shutter speed or the aperture. We will use this terminology below.
Proper ExposureYou probably know what a well exposed slide looks like — it's pretty obvious. Colors are rich and saturated where you expect them to be, and no important areas are washed out or too dark. But it's less obvious what makes a well exposed negative. First, there should be detail in all important shadow areas. In underexposed negatives, shadow areas look clear — they lack detail; there is nothing to print. Information is lost. Shadow detail is extremely important in tonally rich, satisfying fine prints. But shadow areas shouldn't be too dense. If no areas of the negative approach the film base density; if the entire negative is thick, it's probably overexposed. Now overexposure isn't necessarily disastrous. Negative film can capture a huge tonal range, and the detail you need to print is probably still present. But if you overexposed, you either used a longer shutter speed than necessary (more chance of camera shake or blur) or a wider aperture (less depth of field and possibly less lens sharpness; in 35mm cameras, lenses are sharpest between two stops down from maximum aperture and f/11). Portions of the negative may be denser than the Dmax of the scanner (the highest density it can respond to, typically 3 to 4 on a logarithmic scale). In black and white film, dense areas of negatives become grainy and lose sharpness; color negative film isn't quite as bad. Nevertheless, image quality is generally degraded in overexposed negatives.
The situation is reversed is slides, where overexposure is the
cardinal sin. Overexposed areas are washed out and lacking in detail. Slides
capture a much smaller brightness range than negatives, hence they require very
careful exposure. Some detail may be lost in contrasty scenes, even in
well-exposed slides. Most professionals bracket their slide exposures:
expose at nominal, ± 1/2 f-stop, ± 1 f-stop, and sometimes more. They wouldn't
think of risking a $1000 job to save $20 of film. In landscapes, where the sky
and scenery at the top of the frame is often much brighter than at the bottom, a
graduated neutral density filter (dark on top; clear on bottom) can be
invaluable for reducing the brightness range. These filters come in several
gradients and maximum densities. It takes practice to use them effectively.
The Heart of The Matter: Determining ExposureThe key to the zone system is visualizing how you want areas of the scene to appear in the final image, or in the language of the zone system, where you want to "place" them. The zone system can then be expressed in a single statement: Select an area of the scene, meter it, then adjust the exposure by the difference between the zone you want in the final image and zone 5. For example, say you want to place the snowy mountains in scene below (Lauterbrunnen, Switzerland, with the Jungfrau range in the background) at zone 7. Meter them, then increase the exposure by two f-stops, i.e., if the meter says 500 at f/16, expose at 250 at f/11. Or if you choose to place the shaded mountain on the left at zone 3, meter it, then decrease the exposure by two f-stops. I probably pointed the camera, a Canon FTb with a narrow angle meter, down slightly towards the Zone 5 region, then exposed as indicated. It's that simple. Of course the devil is in the details. And there are several.
|The first devilish detail is
that you must be aware of your meter's sensitivity pattern. The zone
system was designed for narrow angle meters, such as 1o spot
meters (see sidebar). Typical SLR's have center-weighted meters, whose
sensitivity is greatest near, or slightly below, the frame center (below
to respond less to bright skies in horizontally oriented landscapes),
falling off gradually towards the edge. These meters are not designed
for the zone system and must be used with care. You have to average over
the region of sensitivity. If you're shopping for a new camera, look for
one with a narrow angle metering mode.
The second devilish detail is how to determine the region to meter. For negatives you should bias your choice towards shadow regions — light shadows, not deep ones. The Zone 3 mountain on the left is a good example. But it's OK to take shortcuts. If a scene is not too contrasty, it's safe to meter from middle tones; shadow detail will be maintained. If it's very contrasty, try to meter off a shadow area. If your meter's sensitivity pattern doesn't allow you to isolate shadows, consider opening up one or two f-stops or bracketing, i.e., making several exposures: nominal, one stop over, etc. You'll learn from experience.
For slides, where overexposure must be avoided, you should bias your choice towards middle to lighter regions, with emphasis on the most important part of the image. You may want to take several zone-adjusted readings and use them as the basis of bracketing.
The third devilish detail is how to actually set the exposure. If you have an old-fashioned camera with manual f-stops and shutter speeds, or an automatic camera with full manual override, you have no problem. If you have a automatic camera that allows exposure compensation, go ahead and use it, but be forewarned — be mindful! It's easy to forget that you've compensating, and your camera won't remind you. My neighbor lost most of the images from a trip to the Canadian Rockies because he forgot to turn off an exposure compensation mode in his ultra-sophisticated Canon EOS 3.
The final detail, not so devilish, is that you must carefully examine each negative or slide when you get it back from the processor. A lightbox and loupe can help with the evaluation. Remember that films, shutters, apertures, and meters all vary, and they can change with time. The film speed on the box isn't absolute; it's only a suggestion. If your negatives or slides are consistently underexposed, decrease your camera's film speed setting. Conversely, if they are overexposed, increase the setting. Don't bother trying to figure out exposure from prints. They're entirely misleading because most photo labs use programs to expose them, and the density of the print is unrelated to the negative. Underexposed negatives result in washed out gray shadow areas that beginners often misinterpret as overexposure. As we engineers say, you must "close the loop," that is, keep adjusting your technique based on recent results. If you do, you'll be rewarded with beautifully exposed negatives and slides nearly every time.
|Simplified zone system summary|
|What exposure range can film capture?|
Of course Ansel Adams may have been able to capture longer ranges, particularly by using short development times (normal minus; N-). He also used small aperture view camera lenses with few elements, and he often used an adjustable bellows lens shade, which is a precise means of controlling flare. He was shown next to one mounted on his view camera in a 1970's TV commercial. Nobody knows how many Toyotas he sold, but camera shops around the country reported selling out their limited stocks of bellows lens shades.
Ultimately the number of f-stops the films responds to is not very critical. The main purpose of this tutorial is to help you determine the correct exposure so you can make full toned prints. For negatives that means shadow areas that are neither too thin nor too thick.
The characteristic curves provide valuable information on film contrast, which is proportional to the average slope of the film's sensitivity region, (y2-y2)/(x2-x1). The green (G) curve is most appropriate because that's where the eye is most sensitive. For EV100VS slide film, contrast = (0.3-3.3)/(-0.3+2) = -3/1.7 = -1.76. For Supra 100 negative film, contrast = (2.5-0.8)/(1.0+2.0) = 1.7/3 = 0.57. EV100S is three times contrastier than Supra 100, and is in fact contrastier than the original scene! Very punchy.
For negatives, scanner contrast is set much higher than for slides. (Papers for printing negatives also have much higher contrast.) That makes the scanner more sensitive to dust, scratches, and grain. This can give slides an advantage, particularly for softly lit, low contrast subjects. I say this with some trepidation because I continue to use negative film for most of my work. Contrast can be pretty extreme in those Rocky Mountain canyons. I'm considering carrying a second camera body with slide film for low contrast images like close-ups of flowers in shade, but, alas, that means more weight and more fumbling with gear.
LinksThe Zone System by Steve Roberts of SR Photography. A nice introduction.
The Zone System by Lars Kjellberg. Fine work from creator of the website with the best lens reviews.
Exposure compensation by Klauss Schroiff. A simple explanation of exposure without zones.
The zone system by Cicada Photography Resource. Very detailed. Probably more than you want to know.
Books on the zone system.
Copyright © 2001 by Norman Koren.
Norman Koren lives in Colorado, where he develops
magnetic recording technology for high capacity data storage systems. He has
been involved with photography since 1964.