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
Selective Latent Image Manipulation Techniques (hereafter, SLIMTs) 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 daytoday use.
Although highly versatile, my other two new contraction techniques, the "Monobath Method" and the "Highlight Method," do not lend themselves well to the daytoday 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 twobath 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 Vestals, 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 SLIMTs 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. Heres 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 emulsions 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 emulsions 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.
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 SLIMTs 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:
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 filmdeveloper 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:
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 filmbleach developer combination and compensate when necessary. Its 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 dont recommend doing so to start out, but it is an option to consider when youre more experienced with SLIMTs.
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 (34 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 baths 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.
Contrastwise bleaching can be used to get a continuous tone, normal density range negative from any high-contrast negative material including Kodaks 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:
In "Zone System Expansion Film," I talked about using Kodaks 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 films 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.
SLIMTs 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.
SLIMTs do not yet work with color transparency films. But, one day..........
Potassium ferricyanide, without the need for potassium bromide, is extremely effective with both CP-2 (now almost completely obsolete) and RA4 color printing papers. DO NOT add potassium bromide to your bleach for RA4 paper or you will destroy your RA4 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.
SLIMTs do not yet work with color reversal papers. But, one day..........
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 bleachfilmdeveloper or bleach paperdeveloper combinations. If a bleach doesnt 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.
1. When I originally began my research, I was investigating the use of what is known as the Herschel Effect as a contrast reduction tool for contraction. This involves exposing film to infrared radiation prior to development to reduce contrast. I was finally able to make the Herschel Effect work marginally well. However, it required use of a desensitizer that was not widely available, and therefore not practical for use as a contraction technique. My research into desensitizers led me finally to SLIMTs.
2. Modern emulsions produce surface latent image almost exclusively.
3. Subsequent work since these findings were first published suggest that bleaches are more stable in this regard than initially thought.
4. More recent evidence suggests that shorter times may not produce uneven development with negative materials.
5. When I originally announced SLIMTs in 1990, I believed that thorough rinsing was necessary before film or paper could be transferred to developer. Subsequent testing and reports from early SLIMT enthusiasts proved that rinses are completely unnecessary in all cases except where acid is added to the bleach, such as for pull-processing / speedwise bleaching with potassium dichromate and acid. It is necessary to rinse the acid from the emulsion before immersing the emulsion in an alkaline developer.
6. Its only fair to point out that calibrating for expansions greater than about N+3, at the most, provides only a general indication of the degree of expansion that will actually be achieved when making real photographs. Traditional Zone System theory begins to get a little weak beyond about N+2, and calibration becomes less than reliable along with it. With expansions of N+3 or greater, it is very wise to shoot several identical negatives and process them one at a time, so that alterations to the processing scheme can be made as needed.
7. My thanks to Robert Anderson and Bertram W. Miller for their kind assistance in testing SLIMTs with color materials. Their generous cooperation while I was developing SLIMTs was most invaluable.