A simplified Zone system for making good exposures
by Norman Koren

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Table of
contents
Slides, negatives, and digital
Introduction to the Zone system
Zones
Good exposure
Determining exposure
Colors   |  Links   |  Imatest
What exposure range can film capture?
Equations for zones
This page presents a simplified version of Ansel Adams' Zone system, suitable for 35mm and medium format photography, color or black & white. The bulk of the page was written when film was dominant. It was revised in October, 2005 to include material on digital photography. Digital is different. See Tonal quality and dynamic range in digital cameras for additional advice on exposure for digital cameras.

Polish language version
  Correct monitor calibration
is vital for viewing and making fine prints. Two key parameters, Black level (often labelled Brightness) and Contrast, are set on the monitor. A third, Gamma, is set in the video card lookup table (LUT). Contrast is normally set to maximum. Color temperature should be set to 6500K or lower 6500K looks best on most CRTs. It can be set on the monitor (preferably) or with the video card/color management software, but not both. Color quality should be set to 24 or 32 bits. The room should be dimly lit; no direct light should shine on the screen. Gray images should look subjectively gray to your eye. For flat screen (LCD) monitors, Screen resolution (right-click on the wallpaper, Properties, Settings) should be set to the monitor's native resolution.

Gamma defines the curve that relates the pixel levels in your computer to the luminance (brightness) of your monitor and prints, using the equation,

Luminance = (pixel level/255)gamma + black level
You can estimate gamma from the pattern on the left side of the chart on the right by viewing it from a distance and observing where the average luminance across the pattern is constant. Gamma should be set to 1.8 for older Macintosh systems (2.2 seems to be the current standard) or 2.2 for Windows systems and the Internet sRGB color space using techniques in Monitor calibration: Setting gamma. Gamma is extremely sensitive to viewing angle in most Laptop LCD screens.This chart is only for monitors; it doesn't work on printed media.

You can set your monitor's Black level using the mostly black pattern on the right of the chart. This pattern contains two dark gray vertical bars which increase in luminance with increasing gamma. (If you can't see them, your black level is way low.) The left bar should be just above the threshold of visibility opposite your chosen gamma— it should be invisible where gamma is lower by about 0.3. The right bar should be distinctly visible, but still very dark. Black level interacts with gamma; you may have to go back and forth two or three times.

The pattern provides a good indicator of display quality. Cheap LCDs can't achieve a constant neutral gray appearance. But good flat screen displays can be excellent.

For more detail and a larger chart, see Monitor calibration: Gamma and black level.

I encourage you to load this chart on your computer and check it occasionally.I'll be happy to grant permission to reproduce it on your website if you e-mail me, give me credit and a link to this page.

If you are new to photography and still unclear on the basic concepts of exposure, Accurate Exposure with Your Meter from Eastman Kodak is a good introduction.

Slides, negatives, and digital

Back in the days when film ruled, most professional professionals worked with color slides instead of negatives because clients demanded them. Why? WYSIWYG! What you saw was what you got; it was easy to judge the quality of slides, and when they were good, they were really beautiful. Projected slides can reproduce a brightness range of 1000:1 (a logarithmic density range of 3); the best you can do with a print is about 100:1. It's difficult to judge anything from a negative; prints from camera shops or drug stores seldom 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 are as sharp as slide films of comparable speed (possibly sharper), though not as fine-grained. With affordable scanners, image editing programs and color printers, you can produce inexpensive prints of impressive quality— often better than custom darkroom prints, especially when difficult manipulations (dodging, burning, selective color shifts, etc.) are required. 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 an edge. And difficult lighting can sometimes produce the finest, most dramatic images.

Since this article was first published, digital has overtaken 35mm and medium format film, though it still can't equal large format for detail. Digital SLRs (which have pixel sizes of at least 5 microns; larger than compact digital cameras) have impressive dynamic ranges: 10 or more f-stops, comparable to negatives and far better than slides. But as we shall see, digital images need to be exposed more like slides than negatives to avoid losing highlight detail. "Expose for the shadows" applies only to negative film (color or B&W); not to digital.

Introduction to the Zone system

The 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.
 
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.

Ansel Adams developed the Zone system to cope with this situation (see history, at the bottom of the page). 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,

"Expose for the shadows; develop for the highlights."
(Warning: This rule applies only to negative film. It does NOT apply to slides or digital images; more later.)
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Adams used a view camera and developed each sheet film negative individually, based on the scene contrast and the previsualized print tonalities. The greater the contrast in relation to the print, the shorter the development time. His system is very elegant— it produces prints of exceptional tonality, and most images print on a single grade of paper (Adams preferred grade 2), but it requires a tremendous amount of testing, calibration, record keeping, and time. It's not practical unless you have access to a darkroom and plenty of time.

We present a simplified Zone system that will enable you to make excellent exposures. The technique is identical for color and black and white negatives, but different for slides and digital, where it can be used to find a starting point for bracketing (for critical work). Once you get the hang of it, you'll find it's not all that complex.

 
The difference between the "simplified" and full Zone system
is that in the simplified Zone system you don't have control over development; you control only the exposure. You give up the idea of making most of your prints on a single grade of paper. This is no problem if you plan to scan your film and print digitally because you have total control over print contrast. The goal of the simplified Zone system is to expose film for optimum image quality— to make sure all the critical information is present.

Zones

Tones in scenes and prints are divided into nine zones, numbered 1 through 9 from dark to light. Zone 5 is, by definition, subjective middle gray.

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 8 steps between the nine zones represent a luminance range of 256 (28), typical for landscapes on sunny days and somewhat less than negative film can capture. The actual tonal range of scenes can, of course, be very different.

In a print, zone 1 is pure black, zone 9 is pure white, and zone 5 is subjective middle gray, corresponding to a reflectance of about 18%. On good photographic paper, pure white is a little over 90% reflectance and pure black is about 1 – 2% reflectance. The maximum luminance range is around 50 to 100, equivalent to about 6 zones; a good deal less than the 8 hypothetical zones in the original scene. Since the difference between subjective middle gray (18% reflectance) and white (a little over 90%) is a factor of 5, equivalent to only 2.3 f-stops, highlight tones in a print tend to be compressed with respect to tones in a scene. A zone chart printed on paper therefore involves a degree of compromise involving the application of an "S" curve.

The charts below are derived from an equation intended to make zone 5 on a properly calibrated monitor appear as subjective middle gray— about 21% of the maximum screen brightness level, similar to middle gray on a print (18% reflectance referenced to 90% white). The equation is in a box near the end of the page. It may interest math geeks, but it is unnecessary for understanding the Zone system. The charts for gamma = 2.2 and 1.8 came out closer than I expected. The major differences are in the lower zones. The zones below closely resemble those in my 1959 edition of Ansel Adams' "The Negative." [A piece of irresistable trivia: Adams received exactly ten cents royalty for each copy of the book.] On a properly calibrated monitor the zones here are more accurate than those in the book.

1 2 3 4 5 6 7 8 9
               
 

 

Zone system chart for gamma = 2.2 (PC's, sRGB color space)>
               
 

 

Zone system chart for gamma = 1.8 (Macintosh)
Note 1. To display these tables correctly in Netscape, the Always use my colors, overriding document box must be unchecked. Click Edit, Preferences, Appearance, Colors) In Firefox, click Tools, Options, General, Fonts & Colors. To print in Internet Explorer 5, Click on Tools, Internet Options..., Advanced. Scroll down and check the box, "Print background colors and images." You might want to uncheck it afterwards.
Note 2. The best way to print these charts, which are HTML tables, not image files, is the following.  (1) Adjust the width of the window for proportions you like. (2) Copy the window into the clipboard by pressing Ctrl-PrintScreen on your keyboard. (3) Paste the image into your image editor. (4) Crop it and otherwise adjust it in the editor. (5) Print it from the editor.

You will need to have a good feeling for zone 5— middle gray, so if you don't entirely trust your monitor— if zone 5 doesn't correspond to your idea of middle gray—  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 usually isn't practical for landscapes— the light at the photographer's position must be the same as the light on the subject. Incident metering produces excellent results when it can be used. Meters built into cameras measure reflected light.

Ansel Adams' description of zones (geared towards black and white printing)
Low values
Zone 0
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. Omitting zone 0 makes little practical difference.
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.
Middle values
Zone 4
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.
High values
Zone 7
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.)

Good exposure

Negatives— You 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— detail is absent; 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. This 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 around f/5.6 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 often degraded in overexposed negatives.

Slides— 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.

Digital— As with slides, overexposure resulting in blocked highlights is the most common problem. Digital sensors are linear, and like all linear devices, they have an abrupt cutoff. According to Kodak Image Sensors – ISO Measurement, when a digital sensor is exposed for an 18% reflectance gray card, 106% reflectance will saturate (reach pixel level 255 out of 255). This can result in blocked highlights in contrasty scenes. Many digital cameras have tonal response "S" curves that reduce the severity of the blocking. These curves are applied when the RAW files are converted. Even so, digital photographers must be ever vigilant; they must pay attention to highlights when setting exposure.

Capture RAW images whenever the lighting is challenging and you want the best image quality. Converting RAW files to standard formats on a computer (off the camera), gives you tremendous control over the results. You can correct color, adjust contrast, and apply tonal response curves. You can also convert to 48-bit color, which allows you to do extensive manipulation (dodging, burning, etc.) without degrading the image or causing "banding." For more details, see Tonal quality and dynamic range in digital cameras.

  A beginner's glossary of confusing photographic terms
Shutter speed 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.

The heart of the matter: determining exposure

The 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 (middle gray).

For example, suppose 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.
Lauterbrunnen, Switzerland, illustrating zones 3, 5, and 7
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 and digital, 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. For slides, you may want to take several zone-adjusted readings and use them as the basis of bracketing. For digital, you can make a test exposure, then look at the histogram to be sure highlights are not blocking. 

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. (He now has a Canon digital SLR; no more problems.)

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. (Digital is ever so much easier; you can examine the histogram in the camera, immediately after the exposure.) 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.

 
In Ansel Adams' full Zone system, you would expose to maintain shadow detail, then develop to place highlights in zones of your choosing. For example, in the scene above you would meter the shaded mountain mountain on the left and expose for zone 3— two f-stops below the meter's reading. You would then meter the highlight area— the sunlit snowy mountain. Assume you chose to place it at zone 7, 4 zones above the shadow area. If it metered 4 zones higher, you would give it a "normal" (N) development. If the scene were contrasty and metered 6 zones higher, you would give it "normal minus 2" (N-2; shorter) development. Similarly, if it were a flat scene and metered only 2 zones higher you would give it (N+2; longer) development. By using this technique, Ansel Adams could make most of his prints on a single grade of paper with beautiful tonalities. The process of determining development times is beyond the scope of this article (it takes a lot of work); I've included this brief description to clarify the difference between the simplified and full Zone systems.

I'll offer one tip for those of you who do black and white darkroom work. Adams targeted his development times to print on grade 2 paper. When I was doing 35mm and medium format darkroom work, I found that targeting development times so an average scene would print on grade 3 (higher contrast) paper resulted in better image quality. The dense areas of negatives optimized to print on grade 2 paper can get noticeably grainy in small film formats. This is not an issue with 4x5 and larger formats.

 
Spot meters are reflected light meters with narrow sensitivity angles, from 10o to 1o for classic models. They are the most accurate way to implement the Zne system. Without one you have to compromise, but with practice (know your meter's sensitivity pattern well!) you should be able to make good exposures most of the time, and know when to bracket (contrasty situations).

If you don't want to use an external spot meter with your 35mm camera (I don't), the next best thing is a manual exposure camera with match-needle spot metering. The Canon FTb and F-1 (both old and "new" models) meter a rectangular area covering 12% of the frame— close to ideal for the Zone system. The "New" F-1 is my favorite; its modern meter responds quickly, even in dim light. The FTb and older F-1 have slower CdS meters that work well in most situations. They can be purchased used at reasonable prices. An important tip: the PX625/PX13 mercury battery is no longer available (turns into incredibly toxic waste when thrown out). The MR-9 battery adaptor from CRIS Camera Services in Phoenix (800-216-7579) allows you replace it with the widely available Silver 76 battery. 

Click here to link to LightZone from LightCrafts
If you enjoy working with the Zone system, you should check out LightZone, a new image editor based on Zone system concepts. LightZone has a built-in raw convertor and a highly intuitive interface, You may never need to open Photoshop again.

  Simplified Zone system summary
  1. Know your camera's metering pattern.
  2. Select a portion of the scene to meter. Shadow areas are generally preferred for negatives and middle to highlight areas for slides and digital, but the most important part of the scene is often the best choice. You'll learn from experience.
  3. Decide the zone in the final image to place this portion of the scene. (For example, you may want to place dark foliage at zone 4 or snowy mountains at zone 7.)
  4. Meter the selected portion.
  5. Determine the exposure. Adjust the meter's exposure by the difference between your chosen placement and zone 5. For example, to place a region on zone 7, increase the exposure by 2 f-stops over the meter reading. For slides or digital cameras, highlight areas where detail is to be maintained should be placed no higher than zone 7.
  6. Practice. Examine the exposure of your slides or negatives (but not the prints). Learn to interpret the histogram on your digital camera. Keep refining your technique and calibrating your equipment.
TIP: You can expand the effective dynamic range of film for photographs of stationary objects taken with a tripod. Make two exposures: one for the shadows, one for the highlights. Techniques for combining them are presented by Jonathan Sachs for Picture Window Pro and Michael Reichmann for Photoshop.

Zones and colors

Bright colors can confuse your estimates of zone values. It's easy to pick out a middle gray and say, "that's zone 5." But what arout red (seems to be in the middle)? Yellow (seems brighter)? Blue (seems darker)? To clarify the effect of colors on zone estimates, I printed out the following chart on Epson Matte Heavyweight paper and measured each square's reflectivity on an overcast day with my Canon "new" F-1.
 
 Z 1  Z 3  Z 5  Z 5  Z 7  Z 9
           
 R  Y  G  C  B  M
           

The top row contains zones 1, 3, 5, 7 and 9. Tonal values are compressed in the monitor display and print. (The tonal range is 5, rather than 8, zones.). The second row contains dark colors, the third contains pure primaries (R,Y,G,C,B,M), and the fourth contains pastels. HTML color names are shown in brackets [...]. In some cases they differ from conventional names.

This chart is fairly simple to use. If you are metering off a surface that subjectively resembles one of the colors, make the exposure adjustment (relative zone 5) shown on the right. For example, if you are metering off a pure yellow surface, increase the exposure by 1.5 f-stops above the meter reading (zone 5), i.e., place it at zone 6.5. The values on the chart are accurate to about +/-0.25.

 
Zone 1
-2.5
Zone 3
-1.5
   Zone 5
   0 
Zone 7
+1.5
Zone 9
+2.5
[Maroon]
-1
[Olive]
0
Dark green
[Green]
-0.5
[Teal]
0
[Navy]
-1.5
[Purple]
-1
[Red]
+0
[Yellow]
+1.5
Green
[Lime]
+0.5
Cyan
[Aqua]
+1.5
[Blue]
-0.5
Magenta
[Fuscia]
+0.5
+1 +2 +1.5 +2 +0.5 +1

To download the above chart for printing, shift-click here.

Use the chart for a rough estimate of how to meter off bright colors in the field. Note that the subtractive primaries (Y, C, M) are lighter than the additive primaries (R = Y + M; G = Y + C; B = C + M). Green as we perceive it in nature (typical leaves and grass) is generally darker than spectrally pure green (early spring foliage). Blue as we perceive it in nature (except for rare deep blue skies in mountains above 3,000 meters) is generally lighter (and closer to cyan) than spectrally pure blue— much lighter for typical "blue" skies.

Imatest

The Imatest program, written by this author (it's kept him busy since 2003), provides a staightforward means of calculating digital camera response curves (log10(pixel level) vs. log10(exposure)), which are comparable to the film and paper response curves shown below. Imatest can also calculate a camera's dynamic range, either from a single image of a transmission step chart or multiple images of reflective step charts, which typically  have lower density ranges than transmission charts.  For more detail, refer to the following links:  Using Stepchart (especially the Dynamic range section), and Dynamic Range (a postprocessor for Stepchart). Here is an example of Dynamic Range results for the Canon EOS-20D. The DR numbers are based on the signal-to-noise ratio. Imatest Studio, which measures a wide variety of image quality factors, is available to individual photographers at a very attractive price. 


Imatest Dynamic Range
results

Links

Beyond the Digital Zone System— by Russell Cottrell  An exploration of the dynamic range of his digital camera.
A Simplified Zone system... (this article) translated into Polish by Radek Przybyl.  (I'm "Normana Korena" in Polish!)
Accurate Exposure with Your Meter  from Eastman Kodak. A good introduction to the basics of exposure.
Zone2Tone (Tech notes)  Les Meehan's images and introduction to the Zone system. Excellent for exploring the subject in greater depth.
The 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 correction  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.
Curvemeister, written by Mike Russell, is an interesting curves adjustment plugin for Photoshop— something like the Picture Window Pro curves adjustment with zones added. I haven't tried it.
 
  What exposure range can film capture?
This is the characteristic curve for Ektachrome 100VS slide film— Kodak's answer to Fuji's highly saturated Velvia. E100VS responds to Log Exposures between -2.0 and -0.3: a range of about 1.7 on the log scale or 101.7 = 50 = 5.6 f-stops (or EV or zones).   This is the characteristic curve for Kodak Supra 100 negative film (now discontinued, but typical). Supra 100 responds to a Log Exposure range of about 3, equivalent to a (linear) range of 103 = 1000 = 10 f-stops, far more than slide film.
 

Fujichrome Astia 100 (left) has a significantly larger exposure range than Velvia 50, Provia 100F, or Ektrachrome 100VS. This should make it superior in many instances for scanning, even though the reduced contrast makes the slides themselves less "snappy." It gets a lot less press than Provia or Velvia (which has an even shorter range than Provia). I haven't tried it.

The exposure range film can respond to, particularly negative film, is further limited by flare light— light that bounces off the interiors of lenses and between the elements. Flare light fogs shadow regions. It tends to be worse for zoom lenses than for primes (single focal length lenses) because zooms have more elements. The primary purpose of lens coatings is to reduce flare. Flare light originates from bright areas— often light sources— inside or outside the frame. It it's inside the frame, you just have to hope the lens is well enough designed to keep it under control. It it's outside, a lens shade is helpful. You can use your hand or a hat to shield the lens from glaring light sources like the sun. When flare is taken into consideration, the actual exposure range of negatives is reduced to between 7 and 9 f-stops. Because of flare light, it is generally unwise to meter from regions darker than zone 3.

Of course Ansel Adams could capture longer ranges 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 to control flare. He was shown next to one mounted on his view camera in a 1970's TV commercial. Nobody knows how many Toyotas (Datsuns?) he sold, but camera shops around the country sold out their tiny stocks of bellows lens shades.

Canon EOS-10D: converted with Capture One "Film curve"Characteristic curves of digital cameras can now be measured, thanks to the Imatest program. The total dynamic range, shown here for the Canon EOS-10D at ISO 400, is almost 9 f-stops: nearly as good as negative film. (It may be better; the measurement was limited by the 10 f-stop range of the Kodak Step Tablet.)  The practical dynamic range— specified by the maximum allowable noise for a specified level of image quality— depends on the ISO speed. See the Imatest Stepchart tour for more details.

The characteristic curves provide valuable information on contrast or gamma, which is proportional to the average slope— the change in Density for a corresponding change in Log Exposure— over the film's sensitivity region. The green (G) curve is most appropriate because the eye is most sensitive to green. For EV100VS slide film, gamma = (0.3-3.3)/(-0.3+2) = -3/1.7 = -1.76. For Supra 100 negative film, gamma = (2.5-0.8)/(1.0+2.0) = 1.7/3 = 0.57. 

EV100S is three times contrastier than Supra 100, making it much contrastier than the original scene. Very punchy. Film gamma is closely related gamma in digital displays, where brightness = (pixel level)gamma, and hence log(brightness) = gamma * log(pixel level).

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, giving slides an advantage for softly lit, low contrast subjects. Since I now use a digital SLR, which combines the best features of slides (low noise) and negatives (large exposure range), I've put this particular debate behind me.

 Equations for zones
The equation is expressed in the following Matlab statements.
zone = 1:9;  gamma1 = 2.2; gamma2 = 1.8;  f1 = 5.25;
y1 = (exp(f1*sin(pi*(zone-1)/16))-1)/(exp(f1)-1);  % Normalized screen levels.
z1 = y1.^(1/gamma1);  z2 = y1.^(1/gamma2);  % Normalized pixel levels.
px1 = round(255*z1);  px2 = round(255*z2);  % Pixel levels for gamma = 2.2, 1.8
y1 is the normalized screen luminance. There's nothing sacred or unique about this equation— it's simple and does the job. It's designed so y1 = 0 for zone 1, y1 = 1 for zone 9, and y1 = 0.21 (21%) for zone 5 (what I perceive to be middle gray). The sin function is responsible for the compression required at high pixel levels. f1 is set to 5.25 to make y1 = 0.21 (21%) for zone 5. z1 and z2 are the normalized pixel levels (0-1) for gamma = 2.2 and 1.8, respectively. px1 and px2 are the corresponding actual pixel levels (0-255).

The results are screen levels y1 = { 0.0000  0.0094  0.0341  0.0922  0.2107  0.4097  0.6688  0.9035  1.0000 }. Pixel levels for gamma = 2.2 are { 0  31  55  86 126 170 212 244 255 Decimal} = { 0  1F  37  56  7E  AA  D4  F4  FF Hexidecimal}.  Pixel levels for gamma = 1.8 are { 0  19  39  68 107 155 204 241 255 Decimal} = { 0  13  27  44  6B  9B  CC  F1  FF Hexidecimal}.

The history of the Zone system, or was Ansel Adams the sole inventor? Here's what Adams says in his Autobiography. The setting is the Art Center School, Los Angeles, 1940. "With the cooperation of Fred Archer, instructor in photographic portraiture, I set out to plan a way by which the students would learn the 'scales and chords' to achieve technical command of the medium. It took several weeks in refinement before I could teach it to the students. I called my codification of practical sensitometry the Zone System."

I found no mention of Archer in my 1964 edition of Adams' classic text, "The Negative," but Christian Boesgaard wrote, " In my 1981 edition, Adams elaborates on the origins of the zone system in the Introduction. He states that it was done with the "cooperation of an important instructor, Fred Archer," and, "We based our first plan on articles by John L. Davenport that appeared in U.S. Camera in the Autumn and Winter edition of 1940" (citations from page xi)."

Images and text copyright 2000-2013 by Norman Koren. Norman Koren lives in Boulder, Colorado, where he worked in developing magnetic recording technology for high capacity data storage systems until 2001. He has been working on Imatest since 2003. He has been involved with photography since 1964.