Zone System Expansion Film
Achieving N+4 and Beyond

©Copyright 1989 thru 2008 David Kachel

Article First Appeared in Darkroom & Creative Camera Techniques in Sept/Oct 1988

Author's Note Dec. 2004: The film described in this article is no longer manufactured, making the technique obsolete. However, I am a firm believer in the idea that old information germinates new ideas. Many of the ideas for my techniques were sparked by VERY old information from as far back as the early 1900's. So, though you can no longer use this technique, perhaps its description will give you the spark for a new idea.

Aesthetic considerations aside, fine B&W photography boils down to one thing: control. Without contrast and tone control, excellent results become a matter of luck. Control depends not only on how well you understand your medium and materials, but also on how flexible those materials can be. If your film can’t do what you need, you either have to pass up some photographs or look for other options.

Most Zone System photographers are aware that current medium-contrast films are highly limited when it comes to contrast expansion. A few modern films will surrender an N+2 expansion, but the majority will barely squeak up to N+1 or 12. Even N+2 is nothing to celebrate, since greater expansion is often needed, especially outdoors.

Traditional Approaches

Many photographers have had to resort to other methods for getting contrast increases, but these techniques are limited and have some serious drawbacks.

Typically, a photographer will fall back on a higher paper grade when negative development is insufficient. According to White, Zakia, and Lorenz in The New Zone System Manual, graded paper offers roughly the equivalent of N+1 to N+2 when changing from grade 2 to grade 4. However, this varies greatly among paper manufacturers.

Unfortunately, higher contrast papers are high-contrast throughout their entire tonal range, including the dark tones. Most negatives already have adequate separation in the shadows and don’t require additional contrast there. This is especially true for negatives that received prolonged development. As a result, a high contrast paper may only serve to make shadows too harsh or too dark. Finally, the extreme exposure sensitivity of a high-contrast paper makes it very difficult to control subtle shadow and highlight detail while printing. A slight exposure change can result in a big density change. Though higher paper grades can often be used as an effective expansion substitute, the degree of expansion they offer is limited and some negatives will simply not respond well to the increased shadow contrast. For me, a high-contrast paper is a good expansion tool to hold in reserve, but not a viable first line approach to expansion.

Negative intensification generally provides a boost to about N+1. However, the negative must have nearly normal starting contrast for the procedure to work. Since most intensification methods are dangerous to the negative, photographer, or both, intensification should be reserved for emergencies.

The exception to this rule is of course Kodak Rapid Selenium Toner. Selenium is the only relatively safe negative intensifier of which I am aware. Mixed 1:2 or 1:3 with water, Selenium toner is a fast and very effective method of achieving about an N+1 expansion. The negative must of course have a nearly normal density range to begin with, as Selenium does not add much density to areas below about Zone VI. Like employing higher paper grades, this tool is effective and useful, but hardly a primary expansion tool.

Using Traditional High Contrast Films for Expansion

Ansel Adams and others have suggested using high or moderately high contrast films when more than N+2 expansion is required. This would certainly seem to be a sensible approach. Need more contrast? — Use a higher contrast film!

Unfortunately, high and moderately high contrast films have no significant toe, and therefore can’t render shadows as we are accustomed to seeing them.^{footnote 1} The visual effect on the print is one of overly dark and contrasty shadows, an exaggerated version of what is sometimes seen when high contrast papers are used for expansion purposes. (See above.) For these specialty films to be useful for expansion, they would need medium-contrast in the toe of their characteristic curves to prevent overly harsh shadow tones.

Nonetheless, there are those who employ high-contrast films for expansion, and even for normal contrast work.

The general approach taken with all of these high-contrast films is to treat them as negatives requiring contraction, which of course, they are. In other words, by using a high-contrast film, we are turning an expansion subject into a contraction subject! An excellent example of one of these films is Kodak’s Technical Pan.

Technical Pan is by nature a high-contrast film. It is currently in popular usage as a normal film, medium contrast being achieved by employing specialized low contrast developers provided by Kodak and other companies. What these developers are really doing is applying an extreme contraction to Technical Pan, thereby achieving a relatively normal density range from a high-contrast film. (By nature, Technical Pan is a very high contrast film.)

To use Technical Pan as an expansion film, all that is required is to give it less contraction than the above mentioned specialized low-contrast developers ordinarily provide. This can be done simply by augmenting the development time in whatever developer is customarily used to get a normal density range from Technical Pan. You can also accomplish this by using a lesser dilution. The problems inherent with this approach to expansion are the same problems (magnified by the high contrast nature of the film) always encountered with traditional contraction techniques — uneven development, mottling, etc.

Another, perhaps more effective approach, is to employ a Selective Latent Image Manipulation Technique (SLIMT). This is a group of contrast control techniques of my own invention. (See "Selective Latent Image Manipulation Techniques" — (SLIMT’s). With a SLIMT bleach it is unnecessary to use specialized developers to control the contrast of high contrast films. You can apply normal development time, temperature, dilution and agitation in your normal film developer, and still achieve the desired reduced contrast results.

SLIMT’s work by destroying latent image prior to development. The latent image is reduced to a normal (potential) density range before development occurs. With SLIMT’s the risk of uneven development and mottling are significantly reduced. The ability to employ your normal developer also saves the expense of the more exotic developers associated with Technical Pan and other high-contrast films.

Kodak Professional Copy Film — Type 4125

My search for a solution to the limited expansion capabilities of modern B&W films and the difficulties in dealing with conventional high-contrast films, led to the surprising discovery that a film I consider to be ideal for expansion purposes is readily available.

Someone at Kodak had a truly inspired moment some years ago, and designed a film tailor-made for expansion, although the original intent was to solve a completely different problem:

If you make a copy negative of a B&W photograph using a conventional film, the dark tones reproduce quite well, but the midtones, and especially the highlights, will be a poorly separated muddy gray. To remedy this problem, Kodak invented Professional Copy Film Type 4125 (PCF). But, as effective as it may be as a copy film, PCF is nothing less than exceptional as an expansion film for Zone System photography.

Kodak solved the copy negative problem by coating PCF with two emulsions that are radically different. The first is a slow, medium-contrast emulsion that records shadow detail, like any general-purpose film. The second emulsion records the midtones and highlights, and has moderately high contrast.

With what could be considered approximately normal development for ordinary films, PCF automatically produces the equivalent of about an N+2 to N+3 expansion. This is considerably less contrast than that produced by the same development of Technical Pan and other higher contrast films. Development times shorter than normal produce N and N+1 negatives with PCF, while longer than normal development times produce N+3 or N+4 negatives. In other words, expansion is PCF‘s natural state.

PCF is a fine-grain, orthochromatic film on Estar thick base, and is currently available only in sheets and 70mm long rolls.

It would be nice if PCF were panchromatic, but I don’t consider its orthochromatic character to be a major drawback. Frankly, I feel that outdoors an orthochromatic film produces better foliage rendering and shadow detail than a panchromatic film. Since yellow and green filters work just fine, producing dark skies is no problem with PCF. Of course, orange and red filters are out of the question, and filter factors are different. The following filter factors have been effective for me:

I base my film speeds solely on a Zone I density of 0.10 greater than film-base-plus-fog.^{footnote 2} Using this criterion and a 10-second exposure, PCF produced EI 12 in daylight and EI 6 in tungsten light.^{footnote 3} For practical purposes, these speeds apply to exposures in the range of 3 to 30 seconds. For exposures of 2 seconds or less, PCF gains one-half stop, for a daylight EI of 18. For more detailed reciprocity data see Kodak Publication F-5, Professional Black and While Films.

Working With PCF

According to traditional Zone System practice and Ansel Adams, an N+1 expansion means prolonging negative development such that a given subject tone placed on exposure Zone VII acquires a Zone VIII density in the negative. N+2 means that a tone exposed on Zone VI also acquires that same Zone VIII density, etc. Unfortunately, the characteristics of ordinary B&W camera films limit expansion much beyond this point.

PCF is not a traditional film. You can increase PCF‘s contrast in two ways: First by increasing development, and second by increasing exposure.

Additional exposure with PCF moves more of the subject tones onto the higher contrast portion of PCF‘s characteristic curve, significantly increasing contrast. The higher contrast part of PCF‘s curve is not a straight line, but rather a progressively sharper upsweep, much like the curves of long-toed medium-contrast films, but much steeper. (See Figure 1.) As a result, more exposure does not just produce greater density, but also a sharp increase in overall density range and contrast. While increased exposure will give you a moderate increase in shadow separation, it will also produce a marked increase in midtone and highlight separation. In other words, more exposure equals higher contrast. This characteristic is what makes it possible to go beyond N+4 with PCF. (Negatives will of course require a moderate increase in print exposure time.)

To achieve an N+5 expansion with PCF, simply increase exposure by one stop (i.e., place lightest tone on Zone V) and process for N+4. For an N+6 expansion, increase normal exposure by two stops (i.e., place lightest tone on Zone VI) and process for N+4.

Further expansion would seem to be possible through additional exposure increases and/or development manipulation. Frankly, I find it difficult to imagine a situation (outside of the graphic arts) where a photographer would have need of expansion greater than N+6, and have not tested PCF beyond that point.^{footnote 4} It is likely that expansions greater than N+6 would produce harsh shadow tones, substantial grain, and diminished resolution.

I discovered that you can manipulate PCF‘s twin contrast controls (exposure and development) to produce different tonal interpretations of the same subject. You can achieve N+4 using an N exposure and N+4 development, or through E+1^{footnote 5} exposure and N+3 development. These two negatives will have the same density range but very different tonal distribution. I’ll take you through an example:

If you exposed sky on Exposure Zone III and clouds on Exposure Zone IV, and give N+4 development, the sky produces Print Zone III and the clouds Print Zone VIII. Alternately, with one stop more exposure, the sky falls on Exposure Zone IV and clouds on Exposure Zone V. With N+3 processing the sky ends up on print Zone VI and the clouds on Print Zone VIII. These two N+4 treatments produce radically different interpretations of the same subject.

Having now used PCF for several years, I could write an entire article about just such variations but exposure/development controls seem tame compared to what you can do with filtration.

Filter Tricks With PCF

Objects of identical or similar reflectance (i.e., the same meter reading), but having different colors, can be separated tonally by applying a filter that darkens or lightens one of the colors. Using the same sky/cloud example, it’s common to find a one-half stop or less difference between them on a hazy, cloudy day. Under such conditions, the sky typically has very low color saturation and a filter would have only a very slight effect with a panchromatic film. However, with PCF you could place the sky and cloud on Exposure Zone IV and add a yellow or green filter. With the filter, enough blue light is absorbed to lower the sky about one-half to one exposure Zone. In effect, you’ll be splitting the sky and clouds between PCF‘s two emulsions. The sky, now on Zone III, will remain there while the clouds can be developed to Zone VIII, separating two similar tones by five Zones. This extreme example shows just how remarkable PCF can be. With ordinary films, the most you could hope for would be a difference of 1 to 12 Zones.

Exposure & PCF

You must exercise special caution when determining exposure with PCF. Normally, photographers pay particular attention to the lowest Zones when determining exposure. Such care is required to avoid losing shadow detail through underexposure. Since PCF is two different films in one, it helps to think of it as having two toes. The second toe (found between Zones III and IV) is the beginning of the moderately high contrast portion of PCF‘s characteristic curve. You must pay equal attention to both toes. A simple example will underscore the need for caution.

With an N+4 expansion, it’s likely that you’ve placed the lightest subject tones on exposure Zone IV for expansion to Zone VIII. The middle of Zone IV is only one-half stop into the moderately high contrast part of PCF‘s curve. A one-half stop error on the underexposure side will place that tone on the medium-contrast portion of the curve. The result will be no significant expansion.

This is only a critical problem with N+4 expansions, and avoiding a disappointing negative is quite simple. If you have any doubts about the accuracy of your Zone IV exposure calculation, give one-half stop of additional exposure. If the additional exposure turns out not to have been necessary, you’ll get an N+42 instead of an N+4 negative, which is easily corrected when printing.

When you calibrate with PCF, don’t use a textured target. The texture can easily lead you into this N+4 dilemma.

Also, if your PCF negatives consistently demonstrate substantially lower or higher than expected contrast, adjust your effective EI up or down accordingly, instead of altering development. In this case, you’re probably missing PCF‘s second toe by a small but significant degree.

Processing PCF

PCF requires no exotic or expensive developer formulas. Kodak recommends HC-110 Dilution E when using PCF for making copy negatives. For me, HC-110’s short processing times build up the highlights too quickly,^{footnote 6} leaving the shadows with insufficient time to catch up.

I initially used two developers for PCF: one low-contrast (Kodak D-23) and the other normal contrast (Ansco-47 formula)^{footnote 7}. I did not choose these developers specifically for PCF. I used them for all of my film processing while living in South America (I had to mix my own developers from whatever chemicals were commercially available, and these were the best options under the circumstances). Whatever developer you’re currently using will probably work just as well. D-23 gave me longer developing times when processing to N or N+1, and also offers slightly finer grain.

Why process PCF to N or N+1 when you can get the same density range from your regular film? Because PCF will give you different tonal relationships, first because it is an orthochromatic film, second because it favors highlight contrast so heavily, and third, because it will split shadow and lower midtones so well. Try some comparison shots to see the subtle, yet valuable, differences.

If you want to use another developer, nearly any should work well. Start your tests by processing PCF using your standard N development time, and expect to get approximately an N+2 to N+3 expansion. For a one Zone change in density (contrast), modify your development time by about 20 percent with normal contrast developers, and by about 25 to 35 percent with low contrast or highly dilute developers.

Some developers offer more or less maximum contrast than others. If the developer you choose will not produce an N+4 expansion with PCF, try a different developer, or simply take advantage of the contrast boost provided by increasing exposure. One-half stop additional exposure should be sufficient.

Conclusions

Don’t fail to run you own film tests of PCF, for speed and contrast. It is improbable that you’ll get results identical to mine.

I also highly recommend shooting multiple negatives and bracketing exposures by a half-stop when using PCF. It is a somewhat tricky film because of its high-contrast nature and its two toes, and requires a cautious hand. If you’ve got extra negatives to process, dealing with suprises is no problem. If you’re a “bag ‘em with one shot” kind of photographer, you may have trouble with this film!

The developing times and exposure compensations in Table I are based on tray-processing PCF sheetfilm at 20°C. I cycle one sheet through the stack every 30 seconds. My standard Zone VIII transmission density is 1.30 above film-base-plus-fog. Also, I print with a cold (grid) light enlarger light source.^{footnote 8}

You’ll be pleased to know that the clearing time for PCF is about 30 seconds with an ordinary sodium thiosulfate fixer. Rapid fixer may produce pinholes. Otherwise, processing is conventional.

My use of Kodak Rapid Selenium Toner as an intensifier for PCF has shown it to be substantially more effective than with conventional films, broadening considerably the already phenomenal contrast control options available with PCF. Use selenium exactly as you would with your regular film.

I have found PCF to be so versatile that I now load at least one-third of my film holders with it whenever I go into the field. Subjects I formerly passed up as impossible, or with which I would have to do battle in the darkroom, I now handle with confidence and ease. PCF has opened up a whole new realm of subject matter for me. I think you’ll quickly find it just as indispensable a tool in your work.

David Kachel

1. Compared to the slope of the rest of the characteristic curve, the toe section of medium contrast films is comparatively flat. However, it must be remembered that shadow tones print on the higher contrast region of the paper’s characteristic curve. This compensates the lower contrast shadow region of the negative and produces more or less normal contrast for that area of the print. Subsequent use of a high-contrast paper can sometimes raise the shadow contrast above a visually acceptable level.

2. During the 1980’s I went along with the majority of Zone System photographers and used .10 above B+F as my film speed determination. I have since realized the fallacy in this approach and currently use a method based on Zone III shadow detail.


3. This can and does vary from batch to batch. I have had boxes of PCF with a daylight speed as low as 6. Be careful with new batches of film and bracket your first few shots, or test it in the darkroom first.


4. As with contraction, Zone System theory and terminology get more than a little weak beyond about an N+2 expansion.Though from a practical standpoint, expansions of N+4 and greater are certainly available, they are hard to substantiate scientifically and completely impossible to calibrate for. In other words — You can do it. You just can’t prepare for it in advance or prove you did it! Fortunately, as photographers we’re only interested in producing photographs, not in proving theories.


5. I now refer to this as “E+1” exposure rather than “N+1” which of course is too easily confused with N+1 film processing. It means, your base exposure — “E”, plus the number of additional stops of exposure you’ve added — “+X .”

6. More recent experience since this article was first written suggests that HC-110 may not be too harsh with PCF after all, if you are careful with dilution.


7. I am no longer using these developers since returning from South America in 1989.


8. As of this writing, I am using a variable contrast head from Beseler Corp..