There are 4 factors that go into achieve accurate developing of film and paper. Whether or not your are developing your film at home in a tank or you are using/operating a C-41 film developing machine such as those from Fuji or Noritsu, the concepts are the same.
- time
- temperature
- agitation
- concentration (dilution or strength of the chemicals)
As in all processes (C-41, B&W, RA-4 and E-6) the developer itself is the most critical in terms of applying these four principles.
All of the processes include a developer step. B&W has a stop to arrest the development, then a fixer to remove the remaining silver and clear the film. Color negative (C-41) and color paper (RA-4) have either a separate bleach and fix or a combined Blix step. These two steps convert the silver metal reduced in the developer back to a silver solution that is soluble and removable, leaving only a dye image behind and no silver.
E-6 has a separate color developer where the dyes are formed, a reversal to make the negative image positive, and then a bleach/fix step.
For color negative processing, the developer time is specifically 3:15 or 3:30 depending upon the temperature. In a tightly controlled process, that temperature is 100 degrees F. A variance of +/- .25F can affect the color balance of the film. You can use the developer at 102 and 104 degrees F and adjust the time accordingly. I use a kitchen stove and a pot of water to create a warm bath. Once the chemicals are in the 100 degree range, they are ready to go. Film processing machines use heaters, similar to aquarium heaters, submerged in the tanks of chemicals. These heaters maintain the perfect temperature throughout the process.
The agitation factor comes in the form of the developing tank being inverted during the developing cycle. This allows fresh developer to come into contact with the emulsion of the film to act on the silver halide to reduce it to silver metal. If there were no agitation and the film were to sit in the developer, the developer itself would exhaust at the location point on the emulsion where it does its magic. Typically, in a developing tank scenario, you would invert the tank about 3 times every 30 seconds or 10 times every 60 seconds.
In the case of automated machine processing, the agitation comes in two forms.
1: the movement of the film through the chemicals. The film is typically taped to a plastic card. The center of the card is outfitted with gear holes, similar to sprocket holes. The card is inserted into the machine and contained in each tank of chemicals is rack containing gears and rollers. These gears grab the card and pull the film through the tanks and through each chemical step. The movement of the film through the tanks allows fresh chemicals to come into contact with the film. This movement action coincides with the time factor. The tanks are a certain depth and the racks are a certain depth inside the tank. As the drive motor moves the gears which in turn pull the card with the film attached, the film enters and exists the tank for exactly 3:15. The time in which the film is developed is not adjustable. The motor only operates at one speed.
2: Circulation. In order to keep the chemicals from becoming exhausted, a pump is used to move the chemicals from the top of the tank, through a filter and down to the bottom of the tank and back up again.
Finally, concentration is the last factor in quality control. The dilution of the developer along with each chemical in the processing steps. Most home developing kits for color are prepackaged as powders. Some are in liquid form. These are then diluted in water to make a final "working" solution ready to use for developing. In the case of C-41 processing, once the chemicals are mixed, there is no further diluting as is an option in B&W developing. C-41 is a fixed process. Most photo labs purchase the chemicals for film developing in pre-mixed containers. Some have on-board mixing, where you add the developers directly to a tank on the machine and then the machine adds water. This eliminates user error in mixing.
As you can see, if any of the above 4 factors are altered, you can get inconsistent results. Too much or too little agitation, time, too low or too high temperatures and finally, too strong or too weak chemicals will either over or underdevelop the film.
So, how do you keep track of these factors and maintain QC? For the photographer that processes film at home, the simplest way is to keep very good notes. If you mix the chemicals properly, you can eliminate concentration as variable. The time you develop the film is critical. Keep your timing accurate between steps. A good thermometer will provide you with the proper temperature. Finally, following a consistent agitation scheme to keep fresh chemicals in contact with the film. Take notes on each roll of film you develop that include data for time, temperatures and total rolls developed. Your agitation and concentration* should not alter. For the home darkroom user, the time and temperatures will be the biggest factor in getting good results.
In the photo lab industry, there is a much more technical way to keep up with QC. A small strip of film called a control strip is used. The control strip is pre-exposed by Kodak or Fuji and contains certain patches of exposure. The control strip is developed and then the developed film containing the patches is read by a machine called a densitometer. This passes light through the film and onto a light sensitive optical eye. The densitometer then calculates the amount or Red, Green and Blue light that is recorded as the light passes through those individual and different density patches. These RGB values are converted to a number. This number is then plotted on a graph. The graph contains upper and lower limits. After the values are plotted, the operator can look at the lines and determine if the control strip has been developed properly within the designed limits. The RGB lines in the graph can converge, split, raise or lower depending upon one or more of the above 4 processing factors. An experienced QC technician can determine which one or more of the 4 factors are contributing to an incorrect processing of the film. Each one of the patches on the control strip has an identification such as LD, HD, etc that is used to form a corresponding RGB plot line on the graph as seen below.
The home darkroom user would likely not go to the expense and training necessary to purchase control strips from Kodak of Fuji (and they aren't cheap!) along with a densitometer (also, not cheap!) to maintain tight QC. Simply keeping good notes will help you maintain good developing habits at home.
* now for the caveat. When developing film at home the developer solution will with time and usage begin to exhaust. Developers are active solutions that are sensitive to light and oxygen, both will kill a developer over time. In addition, the more a developer is used the more silver it reduces to metal during development. Eventually, after X number of rolls processed, it can no longer reduce silver effectively. As this happens, the quality of the development decreases. In the home darkroom, you will need to alter one of the 4 factors to continue getting acceptable results until the results are no longer desirable. Usually, you will simply add more time to the development, which is what I do when processing color film at home. Once I add more than 1 minute additional time, or total about 30 rolls developed per 2-Liter mix, I discard the chemicals. The photo lab does not have the same issue as the machines automatically add fresh chemicals to the working tanks at certain volume intervals, keeping concentration consistent.
There are several more incidental factors that occur downstream to achieving good QC. These include the concentration of the bleach and fixer, aeration of the bleach along with the time steps for each of these chemicals and so forth that are too detailed for explanation on this blog and really only apply to photo labs and not the home darkroom user. But because the developer itself is so critical, I concentrated on this step for basic educational purposes.
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