Zone System Contraction Part III
Selective Latent Image Manipulation Techniques (SLIMTs)

©Copyright 1990 thru 2008 David Kachel

Article First Appeared in Darkroom & Creative Camera Techniques in Sep/Oct 1990

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Selective Latent Image Manipulation Techniques (hereafter, SLIMT’s) are a whole group of contrast control techniques for both B&W and color photography that were the surprising result of my search for a simple Zone System contraction technique for day–to–day use.

Although highly versatile, my other two new contraction techniques, the "Monobath Method" and the "Highlight Method," do not lend themselves well to the day–to–day small contraction needs of most photographers. They are also not applicable to color photography. People who seldom need contraction, and even then require only N-1 or N-2, might find the Monobath and Highlight Methods overly cumbersome for all but a few negatives.

When I started researching new contraction methods, I realized I had to devise not one, but a variety of significantly different techniques for contraction that could appeal to a broad diversity of temperaments and skill levels. I have long recognized the need for a fast and simple method, easily applied even by beginning workers, that would solve some of the problems inherent in older contraction methods. I felt such a method should also appeal to the Zone System photographer not inclined to complicated tinkering.

The four basic contraction techniques used by Zone System photographers for nearly 50 years — water bath development, curtailed development time, high dilution compensating development, and two–bath development — are, in my opinion, either obsolete, unwieldy, or both. They need to be replaced. Films have changed drastically over the years and they simply do not respond to these techniques the way they once did.

Modern films are designed to be developed normally and any attempt to significantly curtail development from normal can only lead to uneven development and speed loss.

Knowing that normal development is the only development to which film is designed to respond well (expanded or push development notwithstanding), I searched for a way to develop a negative in a normal contrast developer, at normal dilution, agitation and temperature, for the normal time, and yet achieve a contracted negative! I wanted a technique that allows contraction negatives to be developed without special developers or dilutions, without producing an artificial shoulder (see "Zone System Contraction — A New Theoretical Approach") without the risk of uneven development, and with a minimum of speed loss.footnote 1 Finally, I wanted to be able to process contraction and normal negatives together in groups, thus saving the photographer considerable time and aggravation.

As far fetched as such a technique may sound, the seeds for it have been right under our noses, albeit in a different form, since 1904. Unfortunately, the concept was incorrectly applied and not fully understood back in 1904, and has therefore been resting unnoticed among what might be called, “photo curiosities” ever since. The secret lies in manipulating the latent image prior to development, an idea never before explored by Zone System photographers.

The amazing 1904 technique to which I refer was called the Sterry Method and is still in use by a few photographers today. It is a technique for reducing print contrast so that otherwise unprintable, long density range negatives can be accommodated. With the Sterry Method — after exposure and before development, a print is treated in a solution of potassium dichromate, rinsed, and then developed. The resulting print has much reduced contrast. Unfortunately, it also has substantially reduced speed and lengthened developing time. (For an excellent treatise on the Sterry Method, see David Vestal’s, The Art of B&W Enlarging.) Due to the advent of graded papers and its serious shortcomings, the Sterry Method never gained much popularity.

Lying dormant in this ill-favored, and outdated method is the secret not only to a new technique for contraction, but to a wide variety of tone control techniques for both B&W and color photographers. I refer to these techniques collectively as Selective Latent Image Manipulation Techniques (SLIMTs).

The key to these techniques is in understanding how the Sterry Method and all the SLIMT’s work. Sensitometrists have understood these effects for decades, but apparently never saw the usefulness of this information to working photographers. Even among sensitometrists, this information would appear to be quite obscure. I have found it discussed only in one text among all that I have searched.

On page 123 of the 1966 (only) edition of Theory of the Photographic Process, authors Mees and James discuss the topics of “contrastwise bleaching” and “speedwise bleaching.” Here’s what they are all about:

Latent image is formed both on the surface of silver halide crystals and in their interior. In order to study the proportional distribution of latent image between the surface and interior of an emulsion’s grains, sensitometrists have used bleaches to destroy the surface latent image prior to development. This leaves only the remaining internal latent image to be developed.footnote 2 Scientists can then develop one emulsion with the surface latent image intact and another with the surface latent image bleached out. In this way they can determine the ratios of surface to internal latent image.

In devising an “ideal” bleach for this purpose, sensitometrists noticed that latent image silver reacts quite differently to being bleached than does an already developed silver image, and in two distinct ways.

The first type of response to latent image bleaching is called, contrastwise bleaching, and is illustrated in Figure 1. Here, image contrast is substantially reduced, while speed is only moderately affected. Since contrastwise bleaching prior to development selectively destroys latent image in areas of greater exposure and is increasingly more active as exposure increases, an overscaled negative exposure range simply no longer exists at the time of development. Such a negative can be developed normally.

In speedwise bleaching, the potential CI of an exposed negative is left unchanged, while its speed is reduced. See Figure 2. In this case an overexposed negative of a normal tonal range subject can be developed normally after bleaching, and the resulting negative will not be too dense. Hence, the first technique in history for truly pulling film speed.

I should explain in more detail how contrastwise and speedwise bleaching work.

As a silver halide grain is exposed, the first few atoms of reduced silver tend to be amorphous (without form), which for reasons unknown, makes this latent image silver largely immune to the action of some bleaches. As additional silver is formed by continued exposure, it begins to take on a crystalline shape and this crystalline silver structure is much more susceptible to bleaching. Heavy latent image silver (highlights) is crystalline and can be bleached. Small amounts of latent image silver (shadows) are amorphous and not easily bleached. This is what produces contrastwise bleaching.

In the case of speedwise bleaching, all levels of exposure are equally susceptible to bleaching.

In short, a contrastwise bleach can be used to reduce an emulsion’s contrast and is therefore an effective contraction technique without the inherent flaws of traditional contraction techniques.

A speedwise bleach can be used to reduce the actual exposure of an overexposed emulsion, without altering its contrast.

Basic Information

Before presenting all the current SLIMT techniques, some helpful information is in order:

If you do not have chemicals and a scale at your disposal and just want to try SLIMT’s out, purchase a Kodak Sepia Toner Kit. Dispose of Part “B”, then dissolve Part “A” (consisting of potassium ferricyanide + potassium bromide) in 1 liter of water to make a main stock solution. (This will be a lower concentration than the stock solutions recommended above, but will still serve quite well.) You can use this with any B&W or color material except RA-4 papers. The potassium bromide content will destroy your RA-4 chemistry.


Well, now that you have the basic concept, here are the SLIMT variations that exist so far:

B&W Film Contraction

Potassium ferricyanide works as a contrastwise bleach for the films I have tested, and will very likely work with any B&W film, All B&W films require the presence of potassium bromide to prevent fogging. Follow the previously mentioned method to determine bleach concentration with your film—developer combination. You will very likely find substantial differences in required bleach concentrations when you change developers or films.

Use dilution changes for rough contrast manipulation and time changes (not less than 4, or more than 10 minutes) for fine tuning. I recommend always using a first trial bleach time of 5 minutes. Zeroing in on a final dilution and bleach time is extremely easy, even if your first test is very far off, as it is likely to be. Proper steps for processing contraction negatives are as follows:

  1. If you use a pre-soak, apply it as usual. However, you may use the SLIMT bleach in lieu of a pre-soak.
  2. Place film in bleach which is at the same temperature as your developer and agitate normally, exactly as you would in developer, for either roll or sheetfilm.
  3. When bleach time is up, transfer film directly to the developer, without a rinse.
  4. Develop normally in your standard film developer at the same concentration, temperature, agitation and for the same time as you process normal non-contraction negatives! That’s right, process normally. In fact, after bleaching, you can mix your contrastwise bleached negatives with your normal, unbleached negatives and develop them all together.
  5. Run your tests on film or test strips exposed specifically for this purpose. Make note of bleach concentrations and times for N-1, N-2, etc., for future use. If you change developers or film, you will have to repeat these tests.

Some minor loss of film speed may occur with this technique, especially at greater contractions. You will have to test for this loss with your film–bleach– developer combination and compensate when necessary. It’s been my experience that speed loss is considerably less than that which occurs with older contraction techniques, but may require some exposure compensation.

You may find you can actually regain some lost speed by increasing bleach time and at the same time, slightly increasing development time. In fact, the above described approach to contrastwise bleaching of B&W film does not take into consideration the possible variable effects that could be gained by using a SLIMT contrastwise bleach and altering development. I don’t recommend doing so to start out, but it is an option to consider when you’re more experienced with SLIMT’s.

Pull Processing

Until now, pulling overexposed film has always been a myth. Film could not be pulled, but was given diminished development in order to reduce contrast. It thereby appeared to have had its speed reduced.

The first genuine “pull processing” ever is done with speedwise bleaching in a potassium dichromate and acid bath.

Start with a solution of 0.01-percent (.1 grams per liter) potassium dichromate and 0.01-percent hydrochloric acid and bleach 5 minutes prior to normal development. Other acids, such as glacial acetic, may also be used, but concentrations may vary.

The bleach, development and testing process is exactly the same as for B&W film contraction as described above, with one very important exception: Since the bleach contains acid, you must thoroughly rinse the film (3–4 changes of water) before transferring it to the developer.

Potassium dichromate at about a 10-percent (100 grams per liter) solution will work without the presence of hydrochloric acid. The acid serves only to speed bleach action. This allows smaller bleach concentrations. Therefore, increases and decreases in concentration (of both acid and bleach) will produce increases and decreases in the bath’s effectiveness. You will likely find that radically different bleach formulations (10-percent potassium dichromate by itself, or 0.01 percent dichromate with 0.01-percent acid, or 0.001-percent dichromate with 0.1-percent acid, etc.) all produce acceptable results. Aim for a 4-minute or longer bleach time with normal agitation.

Change concentrations or times until you find the right one to pull film speed by the amount you desire. Obviously you will have to test this on unimportant film or test strips before using it with irreplaceable negatives.

Technical Pan Processing

Contrastwise bleaching can be used to get a continuous tone, normal density range negative from any high-contrast negative material including Kodak’s Technical Pan.

You can now process Technical Pan (and other high-contrast films) to continuous tone in your ordinary film developer, at the same time, temperature and dilution you employ for your regular B&W films, without resorting to expensive specialized developers. The technique is similar to that for contraction with medium-contrast films with some exceptions:

  1. A pre-soak before the bleach bath is a necessity with Tech Pan and other high-contrast films. Presoaking several minutes in Photo-Flo instead of plain water may provide even more uniform development.
  2. Use a potassium ferricyanide and potassium bromide bleach in the same 1:1 ratio of main stock solutions already discussed under Basic Information. Experiment in the same manner as previously described to find a bleach concentration and time that gives you the density range you want. With Tech Pan and other high-contrast films however, you’re looking for a bleach that will give you normal continuous tone, rather than a contraction, as in the case of SLIMT contraction of ordinary B&W films. Try bleach times from 4 to 5 minutes to start.
  3. After bleaching, use a development time you would normally employ to get maximum contrast from the high-contrast film in question. For my tests with Tech Pan I used 12 minutes in HC110, dilution B (not a typo) after the bleach, which produced normal, continuous tone contrast with Tech Pan. This approach produces maximum possible speed, so you may find you have to increase your EI slightly.
  4. Use an agitation method you have previously found effective with Tech Pan. Use it in the bleach and developer, or results will be uneven. If you are not experienced with this film, try a slight variation on Phil Davis’ agitation technique described in his November/ December 1989 article. Use a two-reel tank and keep the top reel empty. Fill the tank only half way with developer. Invert the tank for 5 seconds, then right it for 10 seconds (four agitations per minute).

High-Contrast FIlms as Expansion Films

In "Zone System Expansion Film," I talked about using Kodak’s Professional Copy Film — Type 4125 as a Zone System expansion film, because I feel it is ideally suited to this purpose. I also mentioned that more conventional high-contrast films can be used as expansion films if a SLIMT bleach is employed to reduce their contrast to usable levels. The procedure is simple.

To use any high-contrast film as an expansion film, simply follow the same procedures as those above for Tech Pan, but use a weaker bleach. Calibrate the film for N+1, N+2, N+4, 5, or even 6.footnote 6 Since the natural state of high-contrast films is “N+ more than we can ever use”, by using a SLIMT bleach we are simply reducing the film’s contrast to “N+ whatever contrast level we need.”

In my opinion, these films lend themselves much better to being used in this way (as Zone System expansion films), than for example, Tech Pan lends itself to use as a normal contrast film.

The nature of these films is high-contrast, so using them as expansion films makes more sense, and doing it with SLIMT bleaches makes it easy.

SLIMT’s and Color Negative Filmfootnote 7

SLIMT’s work with all C-41 color negative films. Use them exactly as you would with B&W film, except of course that the bleach must be at the same

temperature as your color developer and will therefore probably require a higher dilution bleach than for B&W film.

You must use potassium bromide along with the potassium ferricyanide, exactly as with B&W films, to prevent fog.

Start with a 3 or 4 minute trial bleach time.

SLIMT’s and Color Transparency FIlms

SLIMT’s do not yet work with color transparency films. But, one day..........

SLIMT’s and Color Negative / Positive Papers

Potassium ferricyanide, without the need for potassium bromide, is extremely effective with both CP-2 (now almost completely obsolete) and RA–4 color printing papers. DO NOT add potassium bromide to your bleach for RA–4 paper or you will destroy your RA–4 chemistry.

Use the potassium ferricyanide bleach for an initial three minute trial time. Use the same temperature and agitation for the bleach bath as you employ for the color paper developer.

No rinse is required between bleach and developer.

Experiment with different bleach concentrations and times exactly as described previously for other materials.

SLIMT’s and Color Reversal Papers

SLIMT’s do not yet work with color reversal papers. But, one day..........

The “New Sterry Method”

Mr. Sterry had the right idea back in 1904, but the wrong bleach. The proper bleach is not potassium dichromate, which acts on B&W paper as something midway between a contrastwise and speedwise bleach. This is what produces the speed losses and increased development times I mentioned earlier. However, potassium ferricyanide (potassium bromide is unnecessary with all B&W papers), and 3 to 6 minutes bleach time, makes an excellent contrastwise bleach for paper. I always try a bleach time of 3 minutes with B&W papers to start testing. Experiment with different bleach concentrations, as previously described.

With B&W papers, the New Sterry Method can produce anything from slight to extreme reductions in shadow densities and little loss of highlight detail (speed) even with drastic contrast reductions. No increase in development beyond your normal time is necessary with the New Sterry Method.

This new Sterry Method is much more effective than the old, and provides some interesting possibilities for print tone manipulation, such as printing a normal negative on a SLIMT bleached, grade 5 paper to alter its tonal relationships. It can also be used to turn ordinary graded papers into variable contrast papers. See "Variable Contrast from Graded Paper," for more details.


With all of the methods in this article you are largely on your own with regard to bleach concentration and time. It would be impossible for me to test or predict the results for all the different bleach–film–developer or bleach– paper–developer combinations. If a bleach doesn’t work for you, try a drastically different concentration or even a different bleach or developer. SLIMTs are very simple and reliable, and should be workable with almost any material.

Manipulation of latent image prior to development (SLIMTs) is a whole new realm of contrast/tone control. There are a great many possible uses for SLIMTs I have yet to touch upon. I believe they have enormous potential. I know many readers will come up with new applications that have not occurred to me, since I have barely scratched the surface. I encourage you to pursue and write to me about those ideas.

David Kachel