Introduction
One of the most used methods for black and white printing in the darkroom is based on the so-called test strips. By applying this method, a gradation of the paper and an aperture of the diaphragm are chosen, after which the paper is exposed with increasing times following a sequence at regular increments, or at increments of submultiples of f/stop. Examine the strips and find the most satisfying one for the highlights. If the shadows are also rendered satisfactorily, the test is over. On the other hand, if the shadows are rendered satisfactorily for longer times, the gradation of the paper must be increased, vice versa the gradation of the paper must be reduced. If you change gradation, you have to start over with a new test strip. After the right gradation and the best time have been found, it may be appropriate to refine the latter by repeating the strip with smaller increments of time, in order to achieve the desired result. This procedure involves a waste of time and paper
consumption, although the operator's experience can lead to a significant reduction in both. In the past, many electronic tools have been produced to automate the choice of printing parameters. Even more recently, devices based on functionality similar to those of the discontinued items have been placed on the market. These products have a significant cost and are related to the assistance and technical support of manufacturers. They usually provide the possibility for the user to introduce their own paper profiling parameters. There are no calculation tools, like this, that allow using standard darkroom light meters to obtain the same results without purchasing dedicated electronic tools. In the procedure proposed in this document, we explore the possibility of selecting the gradation-time pair without going through the test strips. The procedure is based on the brightness levels of the various parts of the negative projected by the enlarger. A calculation tool developed for this
purpose is responsible for optimizing the choice of printing parameters.
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The measuring tool
In order to measure the various areas of a negative in a repeatable manner, it is necessary to use a darkroom light meter. In the case treated here, a Gossen Luna-Pro F was used with the accessory called LAB attachment.
The light meter measures the light level (in lux). The graph below shows the relationship between lux and EV100 of the instrument (points) together with the analytical expression.
The EV scale is linear, while that in lux is logarithmic. It is important to take into account the fact that light meters measure an intensity of illumination per unit of surface. Therefore the measured EV value should not vary from instrument to instrument, as can be seen from the graph, which also shows the lux-EV relationship of the Minolta Auto Meter III F, which can be superimposed on that of the Gossen. Ultimately, even if different exposure meters have different sensor sizes, for the simple fact that the measurement in lux (from which the EV is derived) is expressed per unit area, the result should not change by changing the reading instrument.The major limit of the instrument is related to its sensitivity, which usually corresponds to EV200 = 0. This means that, if the lens diaphragm is closed and a reading of a dark area of the negative is taken, negative EV200 values can be found, and therefore not entirely reliable because they are below the sensitivity of the
instrument. This problem can be solved in two ways. The more rigorous approach involves taking the exposure meter readings of the negative at full aperture, and then recalculating the print exposure based on the diaphragms normally used. An approximate approach is based on some simple experiments, with which a correction coefficient is calculated to take into account the measurement error below the threshold of the instrument. Let us see how to proceed to calculate the correction coefficient. We know that closing the lens diaphragm by one stop reduces the EV200 value by one unit. This is easily verified by positioning the light meter in the center of the projected image, even without a negative, by moving the enlarger head until reading, for example EV200 = 3 at full aperture, for example at f/4. If we close at f/5.6 we will read EV200 = 2, at f/8 we will read EV200 = 1, at f/11 EV200 = 0. So we verified that the lens diaphragm correctly selects the variation of light. Now we can
raise the head of the enlarger until, at f/4, we read EV200 = 0 with the light meter. By repeating the previous steps, we can note the values of EV200 measured by the instrument, which generally do not follow the sequence that we expect: EV200 = 0, -1, -2, -3. Plotting the measured and theoretical values in a graph, we see that the points align themselves approximately on a straight line, for which it is possible to calculate the angular coefficient, which represents the desired corrective coefficient to be used in the measurements.
Another aspect to be taken into account in the light measurements on the printing surface is that of the physical dimensions of the instrument. This means that the light level is measured at a certain distance from the paper and therefore, especially if the head is low enough, an error is made, which can be taken into account knowing that the light level decreases with the square of the distance. Finally, we can take into account the so-called cosine-correction term, representative of the fact that when the angle formed by the direction of the luminous flux that hits the instrument cell with the normal to the plane of the cell is different from 0, the quantity of measured light decreases with the cosine of the angle. Then a corrective factor can be applied to derive the light intensity that would have been measured under ideal conditions.
The abore reported calibrations have been applied to the lightmeters selectable with the corresponding listbox of the app.
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Paper profiling
The purpose of profiling a printing paper is to find a functional relationship between the amount of light to which the paper is exposed and the grey level. The amount of light depends on many factors: the negative, the height of the enlarger head, the lens diaphragm and the printing time. A variable that further increases the factors to be considered is also given by the gradation of the paper or filter. The chemicals used also affect the final result, but if you follow the manufacturer's standard guidelines their influence is less pronounced than the other factors listed above. A sample strip with 11 gray levels (1 = white to 11 = black) was used in the procedure described here. It is immediate to find the correspondence with the Zone System which, as is known, goes from Zone 0 = black to Zone X = white. The procedure requires 6 sheets of the paper to be profiled. Each of the 6 sheets is printed with the different grades of the filter (from 0 to 5). The enlarger head must be
positioned in such a way as to project a light beam that covers the printing area with a certain abundance. It is advisable to use a negative in which the entire frame has a uniform level of gray. In the profiling, the negative and the height of the head have been selected in order to measure exactly EV200 = 0. For each gradation of the filter and for each printing time, a strip is obtained to be compared with the sample gray levels. Each paper and each gradation express specific characteristics, which can be appropriately selected by the operator during the printing phase. The usefulness of this representation of the relationship between light intensity and gray level lies in the fact that it is possible to measure a limited area of a negative and obtain the EV200 exposure value. If a precise gray level is wanted to be associated with this area, it is possible to enter into the profiling curve with the gray level and obtain aperture, printing time and gradation of the paper.
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Description of the calculation tool
We have previously discussed how you can derive the printing time, the aperture and the gradation of the paper, once the brightness level of the negative is known, depending on the desired gray level of the final printing. This operation, however, normally does not refer to a single area of the negative, but to several areas, for each of which a specific level of grey is desired, depending on the expressive result to be obtained. In mathematical terms, the problem can be defined as an optimization problem, as we seek the gradation-time combination that best satisfies the desired result. To speed up the procedure it is advisable to use this calculation tool. For several areas of the negative in which you intend to impose different grey levels, perform a reading with the light meter at ISO 200, eventually noting an eccentricity, i.e. the distance between the axis of the lens and the point where the measurement is made. Knowing the aperture of the diaphragm at which the light reading
is made, the code finds the best time-gradation pair that respects the desired levels (expressed with the Zone System). The obtainable result is shown next to each cell, while the overall square error provides a global indication. When a Zone value is not requested in correspondence with a value of EV200, the code simply calculates the grey level corresponding to the optimization performed, which instead is guided by the cells for which a Zone value is provided. If the printing opening calculated by the code is not convenient, it can be forced, resulting in a recalculation of the printing time. It is possible to deactivate some gradations of the filters, to force the optimization on the desired ones.
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Application example
To illustrate the usefulness of the code with a practical case, we start from the negative shown on the following.
Exposure readings were taken with a Durst Neonon 105 mm 1: 5.6 lens at full aperture. Highlights reading provide EV200 = 2.67, while deep shadows provide EV200 = 4. By imposing a Zone VII for the former and a Zone I for the latter, the code calculates a grade 3, with a time of 21 sec at f/16 aperture. Ilford MGFB paper was used. Compared to the required values, the obtainable Zones are respectively 7 and 0.9, therefore very close to those in input. The result is shown below.
By associating Zone VIII and Zone 0 to the highlights and to the deep shadows, respectively, the code calculates a paper grade 5, with a time of 12 sec at f/16 aperture. Ilford MGFB paper was also used in this case. Compared to the required values, the obtainable Zones are respectively 7.8 and 0.7. The result is shown below.
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Conclusions
The printing procedure most commonly used in the darkroom for black and white is based on test strips, with which, by successive approximations, the correct gradation-time coupling is found to achieve the desired expressive result. Each paper is characterized by a profile, defined as the relationship between the intensity of the light flux and the grey level. Knowing this profile, and effectively measuring the light projected by the negative on the paper, it is theoretically possible to predict the final result. The procedure proposed here is based on the use of a fairly common instrument, namely a light meter equipped with an accessory to measure the light level of different areas of a negative. Some aspects related to the accuracy of the measurement were investigated, as it was observed from the various experiments that this accuracy plays a very important role in the possibility of actually obtaining the results expected from the calculation. Subsequently, the profiling of the
paper was discussed, carrying out test strips and associating the corresponding grey levels to the various times, and to the various paper grades. Profiling is incorporated into a calculation tool, with which we can optimize the two parameters of interest (grade and time) to obtain the smallest difference between desired and achievable grey levels. Some experiments, including an example discussed in detail in the text, have shown an acceptable accuracy of the instrument. Obviously, the achievement of the optimal result could also require subsequent steps, such as dodging and burning, as well as further refinements of the printing time, starting however from a solution not too far from the final one. Thus, a significant saving of time and material is possible. One way to derive the burning or dodging time is described below. For simplicity, let's imagine we want to calculate the burning time. Let's suppose we have imposed some printing parameters on the highlights, but having renounced
a darkening of the highest lights. From reading with the light meter on these areas, freeing all the others, imposing a VII-IX gray level and forcing the calculation to the previously determined grade and aperture, a longer time is obtained. The difference between the new time and the calculated one gives the necessary burning time. The same can be said for dodging. In this case we will get a shorter time. The difference between these two times will tell us how much we have to dodge to lighten the areas of interest.
Contacts: support@photo-paper-profiler.com
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Release notes
Rel. 8 june 2022: Added Bergger Prestige Variable CB Style photo paper (developed using Ilford Multigrade 1+9, 20 Celsius degrees, 120 sec, stopped for 60 sec and fixed using Ars-Imago Fix 1+7, 180 sec).
Rel. 30 may 2022: Added a lightmeter based on Arduino project, including digital light sensor VEML7700.
Rel. 30 apr 2022:
Supported lightmeters: GOSSEN LunaPro F with enlarging accessory; GOSSEN VARIOSIX F lighmeter (reflected light reading, without diffuser); for this product a darkroom accesory is not available.
Rel. 14 apr 2022: Ilford MGFB developed using Ilford Multigrade 1+9, 20 Celsius degrees, 120 sec, stopped for 60 sec and fixed using Ilford Rapid Fixer 1+4, 180 sec. Ilford MGRC developed using Ilford Multigrade 1+9, 20 Celsius degrees, 60 sec, stopped for 60 sec and fixed using Ilford Rapid Fixer 1+4, 60 sec.
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