Calibrating Dichroic Color Heads for Variable Contrast Black and White Printing

by Paul Butzi

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Introduction

It's common knowledge that you can use a dichroic enlarger head to adjust the contrast of variable contrast printing papers. Lots of darkroom workers know to dial in more magenta to get higher contrast, and to dial in yellow for less contrast. However, few people use a systematic way to go about it.

Paper manufacturers often are little help; hampered by the differences between brands of enlargers and between individual enlargers, they only offer vague guidelines. Even when paper manufacturers offer tables of filtration settings, they miss one essential feature: preserving exposure consistency when adjusting contrast. Since these adjustments are made easily, this should be part of the convenience of using a dichroic head.

The value of adjusting print contrast without having to adjust print exposure must be experienced to be believed. Minor changes in print contrast are made as easily and quickly as minor changes in exposure. Because the paper appears to have a constant speed, different parts of the print can be exposed with different contrast settings easily. Likewise, the contrast of burns can be adjusted, and the burn exposure will still relate to the overall print exposure in a sensible way. In addition, the ability to adjust exposure and contrast rapidly leads to understanding the effects of altering the overall print contrast.

What the photographers need is a system for calibrating a dichroic head so that it can be used as a Variable Contrast, Constant Exposure light source.

How the system works

Many darkroom workers follow the practice of setting the highlights of the print by adjusting the exposure, and altering the print contrast to adjust the shadows. Like the adage "Expose for the shadows, develop for the highlights" regarding negatives, this practice is the result of working with the natural properties of the materials.

Unfortunately, with variable contrast papers, two effects make this difficult. First, as you adjust the filtration at the light source, you change the intensity of the light. Second, as you adjust the filtration (and thereby the color of the light) the apparent speed of the paper changes. These two effects combine to produce a shift in the print exposure; this change can force you to determine the print exposure anew each time you alter the contrast.

However, two essential observations allow us to solve this problem. First, introducing neutral density (ND - that is, equal amounts of magenta and yellow) will change the print exposure without altering the contrast. Second, the change in exposure as we change the contrast is easy to measure, and follows a simple pattern.

This system eliminates exposure changes by measuring the change in speed between different contrast settings, and then introducing ND to offset any changes, so that when you change contrast, the print exposure remains unchanged. In other words, we find the contrast setting where the paper appears to be the slowest, and then use contrast-neutral filtration changes to 'slow down' all the other contrast settings to match that slowest one.

Printing using the system

To illustrate how the system works in practice, let me describe my basic printing procedure.

I start by using test strips to choose an exposure that gives me the highlight density I want. Once I have the highlight exposure that I want, I either make a full print or a largish test patch to evaluate the overall contrast. In some cases, I adjust the contrast to get the shadow density I want; in others, I adjust the contrast to get the right ‘feel’ to the print. In either case, all the contrast adjustments I make will produce exactly the same highlight density. By simply adjusting the filtration according to my handy graph, I can get the contrast I want, and I never have to adjust my exposure.

Setting The System Up

There are several things you’ll need if you want to follow my method and calibrate your dichroic enlarger head for use as a VCCE light source.

First, you’ll need an enlarger with a dichroic filtration head.

Next, you’ll need a step wedge. I use a 31-step wedge, with steps of 0.1 density (part number T3110, made by Stouffer Graphic Arts Equipment, 1801 Commerce Avenue, South Bend, IN 46628, 219-234-5023). This wedge is ¾" by 8", and I contact print it. Strictly speaking, I should probably use a step wedge sized to fit my negative carrier, and projection print it. However, since my dichroic enlarger is a diffusion light source, it makes very little difference in practical terms. The instructions that follow can be performed with a wedge with steps other than .1 density, but some changes will have to be made.

Next, you’ll need some sort of baseboard enlarging meter or comparator. My comparator is the Ilford EM10, but there are many similar inexpensive units on the market, such as the Jobo Comparator 2. Other more sophisticated devices for measuring light intensity at the baseboard, such as color analyzers, will work as well, as long as you can detect when the measured light intensity matches a previously measured value.

Finally, you’ll need a supply of the VC paper you intend to use, cut into strips suitable for contact printing the step wedge, and the usual print processing setup. You’ll want to use the print processing that you use for prints, since changes in development time or print developer can change the paper speed and contrast.

The procedure for calibration is fairly straightforward:

1. Make step wedge prints at various yellow and magenta filtration settings.
2. Using these prints, we determine the paper speed and exposure scale (overall contrast) that the paper shows at that filtration setting.
3. Determine how many 'cc' of ND it takes to produce a given change in exposure.
4. For each filtration setting, calculate how much neutral density we're going to add to 'slow' the paper down to match the slowest filtration (typically maximum yellow or maximum magenta).
5. Produce a graph, with exposure scale along the independent (horizontal) axis, and 'cc' along the dependent (vertical) axis. On this graph, we'll chart how much magenta and yellow to use to produce a given exposure scale. By always using the combinations from this graph, we can change the contrast of the print at will, and the highlights of the print will remain unchanged. Figure 2 shows the graph that I have for Kodak PolyMax Fine Art paper and my enlarger.

Generating Set Of Contact Prints

Getting Exposure Right

Set the dichroic head to maximum yellow, zero magenta, and zero cyan. Now, set the enlarger head to a comfortable height, and work out an aperture and exposure time that gets the gray scale centered on the image of the step wedge. It helps to know that a density change of .1 corresponds to one third of a stop; thus, a one stop change in exposure will move the gray scale three steps on a step wedge which has .1 density steps. In addition, it’s a good idea to have the lens at least three stops from fully stopped down; this gives you more maneuvering room for the rest of the contact prints.

Ideally, you’ll end up with an exposure the produces at least three or four paper-base white steps at one end of the strip, and at least three or four maximum black steps at the other end.

Making Entire Set Of Prints

Now, go ahead and make a set of contact prints, adjusting the filtration by about 35-70cc each time, running from maximum yellow to no filtration, and then from no filtration to maximum magenta. Getting the steps exactly the same is not necessary; we’re going to interpolate between the points, so you just need them close enough together to get a good interpolation. It’s important, however, to have one strip with no filtration, because that point can't really be interpolated from the other data points.

On the back of each contact strip, record the paper type, the filtration, the exposure time, the enlarging lens aperture, and any exposure adjustments (which are discussed below).

Making And Recording Adjustments To Exposure

As you make the contact prints, you’ll discover that as you reduce the amount of yellow filtration, the gray scale will no longer be centered on the strip; if it gets too far off center, you can adjust by stopping the enlarging lens down by one stop. With a.1 density step wedge, the gray scale will shift three steps for each stop of change. You may also need to adjust the aperture when going from no filtration to maximum magenta; in this case, you will probably need to open up by a stop to keep the gray scale centered. As you record the exposure adjustment, record it in units that match the steps of the step wedge you’re using. For a wedge that has .1 density steps, for instance, you would record a one stop decrease in exposure as 3 steps.

You don’t need to be a fanatic about keeping the gray scale centered; just make sure that you have a few paper-base white steps at the white end, and a few maximum black steps at the black end. For each contact print, note the adjustments made; this information is essential to sorting out the data later.

When you record any exposure adjustments, convert the data to match the steps of the step wedge you're using. Note that the first contact print you made will have an exposure adjustment of zero. All changes in exposure are all recorded relative to that step. For example, in Figure 1, the contact prints corresponding to the second and last rows both received one stop less exposure than the strip for the first row. One stop corresponds to 3 steps on the step wedge, so the exposure adjustment is recorded as 3 steps for both of those entries.

Figure 1 - Data and Calculated Values

Extracting Data From Contact Prints

Once the contact strips have been washed and dried, you can go ahead and extract the needed data from them. The chart I use for recording my data is shown in Figure 1, filled in with my data for PolyMax Fine Art and my Saunders enlarger.

The first five columns (labeled Magenta (cc), Yellow (cc), First Non-Max Black Step Number, Last Non-White Step Number, and Exposure Adjustment (in steps)) contain data extracted from the contact prints. The remaining columns contain values calculated in later steps.

Sort the contact strips into order, from maximum yellow filtration, through no filtration, to maximum magenta filtration. Examine the contact prints in light appropriate for viewing prints. Each print shows steps that are the maximum paper black, followed by a number of steps that run through the shades of gray, and finally several paper-base white steps.

On different strips, the last step that is not paper-base white falls on different steps; this is because the apparent paper speed changed because of the changes in filtration and the exposure changes made to keep the gray scale centered on the print. After we compensate for the changes you've made in exposure, we'll know exactly how much we need to adjust the exposure to keep this lightest non-white tone constant as we vary filtration. This is half the goal of this whole process!

You'll see that the gray scale covers different numbers of steps on different contact prints. This is visual evidence that changing the filtration changes the paper’s exposure scale. The prints made with yellow filtration will have the gray scale cover more steps than those made with magenta filtration. Since paper grades vary from manufacturer to manufacturer and paper to paper, I use the measured exposure scale to express how hard or soft paper is.

Make up a blank chart like Figure 1, with one blank row for each contact print. For each print, record in the chart the filtration used for that strip (in the appropriate "Magenta" or "Yellow" column), the number of the first step that is not maximum black (in the "First Non-Max Black" column), the number of the last step that is not paper-base white (in the "Last Non-White" column), and any exposure adjustment used for that strip (in the "Exposure Adjustment" column). If you note that the first non-black step or last non-white step is lighter or darker than usual, feel free to interpolate values - I find that I get good results guessing at when to call something a half or one quarter step.

Computations

Now, you’ll need to perform a series of computations for each row of data. I’ve set up an Excel worksheet that performs these calculations for me; this simplifies the entire process. However, it isn’t so complicated that it can’t be done by hand. I’ll describe the required calculations in order.

Adjusted First Non-Max Black Step - This column contains the numbers from the "First Non-Max Black" column, adjusted to compensate for any exposure changes you made to keep the gray scale centered. To do this, you add the entries for that row's "First Non-Max Black" column and "Exposure Adjustment" column to produce the entry in the "Adjusted First Non-Max Black Step" column. For example, in the second row of Figure 1, I calculated the "Adjusted First Non-Max Black Step" by adding 6.50 (the last non-white step number for that row) and 3.00 (the exposure adjustment in steps for that row) to get 9.50. I recorded 9.50 in the " Adjusted First Non-Max Step" column for the second row.

Adjusted Last Non-White Step - This column is calculated similarly. Again, the goal is to produce a column that contains the data from the "Last Non-White" column, adjusted to compensate for any exposure changes. This is done by adding the "Last Non-White Step" value for the row to the "Exposure Adjustment" value, and entering the result in the "Adjusted Last Non-White Step" column.

Exposure Scale - Exposure scale is the measure of the overall paper contrast. In my system, the exposure scale is the difference in exposure needed to change from the last non-white tone to the first non-max black, expressed as the number of steps on the step wedge. To calculate the exposure scale, subtract the Adjusted First Non-Max Black step number from the Adjusted Last Non-White step number. For example, in the last row of Figure 1, I subtracted 20.50 from 29.00 to get 8.50.

Needed Exposure Adjustment - Find the row that has the lowest step number for the Adjusted Last Non-White column - this is the row that represents the slowest point for the paper. We’re going to even out the exposure across all the different exposure scales by slowing all the faster entries down (by introducing neutral density) to match this one. To do this, subtract the Adjusted Last Non-White entry from the slowest row from the Adjusted Last Non-White entry for each row, and enter it on the chart. In Figure 1, the lowest step number in the Adjusted Last Non-White Step column occurs in the first row, where the value for that column is 27.00. So, to get the value for the second row, I subtracted 27 from second row's value (which is 28.00) to get 1.00, which I entered in the "Needed Exposure Adjustment" column in the second row.

Neutral Density Calibration

Before we can compute the final filtration values, we need some way to relate neutral density introduced by dialing in equal values of Cyan, Magenta, and Yellow to the neutral density steps of our step wedge.

The way to do this is to measure how much neutral density we need to introduce to exactly match the change in light intensity we get when we adjust the enlarging lens aperture by three stops. Then, since we know how many steps on the step wedge are equivalent to the three stop change (nine), we can just divide and we have the amount of neutral density we need to introduce to get the same effect as one step on our step wedge.

With your dichroic head set to no filtration, your enlarging lens aperture set to fully stopped down, and no negative in the carrier, put the enlarging comparator on the baseboard and adjust it until it reads nulled. Now, open the aperture three stops, and introduce neutral density by dialing in equal amounts of the three colors until the comparator again reads nulled.

Recall that one stop is equal to .3 density units; three stops is equal to .9 density units. By dividing the filtration we needed to dial in to match the three stop change by 9, we can deduce how much filtration it takes to match .1 density units (which happens to match one step on the wedge). In my case, I needed to dial in 78 cc of neutral density to match a three stop change, so for my enlarger I need 78/9 or 8.61 cc of neutral density to match one step on my step wedge.

Finally, recall that black and white printing papers are insensitive to red light. Adjusting cyan filtration adjusts how much red light strikes the paper (because transmission filters subtract their complementary color). Because of this, for black and white printing purposes, cyan filtration makes no difference at all, and we can ignore the cyan contribution to neutral density. As a result, cyan filtration is ignored in the final calculations.

It’s important, however, to remember that your enlarging comparator very probably is sensitive to red light, so it’s important to use cyan filtration when determining how much ND you need to match one step on the step wedge.

Final Calculations

Now that we have a relationship between the step wedge density units and the amount of ND filtration dialed in on our dichroic head, we can calculate the aggregate filtration values, which include both the yellow or magenta filtration needed to alter the paper’s exposure scale, and the neutral density needed to even out the changes in paper speed. We do this by filling in the last four columns in the chart in figure 1.

Neutral Density Needed for Exposure Adjustment - To calculate the entry in this column, you multiply the "Needed Exposure Adjustment in Steps" by the amount of neutral density in CC you need to dial in on your enlarger to match one step on your step wedge. For example, recalling that for my enlarger, 8.61 CC of ND matches on step on my step wedge, I calculated the value for the fifth row by multiplying 6.00 by 8.61, giving me 51.66, which I then round to the nearest unit to get 52.

Final Magenta (cc) - The entry in this column is the sum of the original "Magenta (cc)" column and the neutral density column. For instance, in the last row of figure 1, I added 170 and 17 to get 187.

Final Yellow (cc) - The entry in this column is the sum of the original "Yellow (cc)" column and the neutral density column. For instance, in the second row of figure 1, I added 135 and 9 to get 144.

How is it used?

To use the system in practice, suppose you want to make a print with the 'normal' exposure scale; the one that matches the majority of my negatives. In my case, my film gives me negatives that usually print nicely when I use an exposure scale of around 12. This corresponds to the exposure scale I'd get printing on Polymax Fine Art with no filtration at all.

First, set the enlarger’s dichroic head on my enlarger to match the chart lines corresponding to 12 on the chart. On Figure 2, this would be about 52 for both magenta, and yellow. You'll see that at this exposure scale, which corresponds to no contrast filtration at all, we're just dialing in ND to keep the exposure constant.

After getting the highlight exposure right, take the print and think about the dark areas. If the darker parts of the print are too dark, I would reduce the contrast, that is, lengthen the exposure scale of the paper. To do this, try changing the exposure scale to, perhaps, 15. The chart tells us that this requires a filtration of about 16 magenta, and 92 yellow. So, I reset the dichroic filtration to those values, and make another print, keeping the print exposure exactly the same. The resulting print will have the highlight density unchanged.

Similarly, if I felt the shadows weren't dark enough, I'd reduce the exposure scale, perhaps to 10, by changing the filtration to 77 magenta, 35 yellow.

No matter what exposure scale you choose, your highlight exposure will remain the same. You can make a series of prints with incremental changes in contrast without adjusting.

In some instances, I choose the exposure scale that gives the print the right textural feel. In this case, I'm not helped so much by the constant highlight exposure, since the texture is probably set by the midtone contrast and density. However, the system does provide a predictable, repeatable way of producing known changes in contrast, since it's calibrated using a measured exposure scale rather than nebulous paper grades.

Suppose I liked the way the print looked, but felt that one particular area could benefit from increased contrast. In that case, I might dodge that area back while making the base exposure at normal exposure scale, then dial in the filtration for the higher contrast area, and make another exposure (exactly the same as the first one) while exposing only the area that was dodged back before. Voila! The highlight densities match, but part of the print has been done with higher contrast than the rest, all without hit and miss trials to get the dodging and burning times right.

Variations

There are some variations on this basic system that might make interesting avenues to explore.

Extending the Filtration Range

To reach the extremes of the range of possible exposure scales with modern VC papers, it's sometimes necessary to use more filtration than your dichroic head can provide. In that case, you can extend the system by supplementing the dichroic head's filtration with magenta or yellow gelatin filters. One of each filter should be sufficient.

It's simple to produce another chart for each additional filter. Run a set of step wedge contact prints running from no dichroic filtration and the magenta filter, through maximum magenta and the magenta gel. The resulting chart will extend your ability to reach the shortest exposure scale (maximum contrast) that paper can provide. Running a set for allows you to achieve the longest exposure scale of which the paper is capable. Note that many papers will fail to reach a maximum black under very heavy filtration; you may want to note where that occurs on your chart so that you can avoid that region when printing if you want full blacks.

Different Print Value Held Constant

There’s nothing magic about picking the lightest non-white tone on the gray scale as the print value to hold constant; it’s just that that’s the way most printers like to work.

You could, however, calibrate the system using the darkest tone, or some selected mid-tone. This will produce a different chart, which will hold exposure constant for the selected print value.

Just add a column to the chart for recording the step number for the print value you want to hold constant. As you extract the data from the contact prints, identify the step number that matches that print value, and record it. Then, add another column, this time for the adjusted value; you adjust for exposure compensation just like you did for the "First Non-Max Black" and "Last Non-White" columns. I'd call this new "adjusted CE step". Now, instead of using the "Adjusted Last Non-White" column when computing the entries for the "Needed Exposure Adjustment" column, use the new "Adjusted CE step". All the remaining calculations are unchanged.

Almost Constant Exposure

Some manufacturer’s variable contrast filters provide constant exposure through most of the contrast range, but for some filters (typically, the filters providing the most contrast), you must increase exposure by one stop. There’s a reason for this - the difference between the unfiltered speed of VC papers, and the speed with maximum yellow or magenta filtration can be more than two stops. This system produces its constant exposure characteristics by slowing everything down to the match the slowest point. My enlarger, a Saunders/LPL 4550xl, is exceedingly bright, so I don’t find that this is a problem for me. Other people may find this to be a real annoyance, particularly if they rarely use the extremes of the available range of exposure scales. In that case, it would be a simple matter to take the approach used by the paper manufacturers, and produce constant exposure over the most used portions of the range, and increase exposure by a stop or two for the less used and slower portions.

All that's needed is to decide what region of the chart you'd like to speed up, and subtract neutral density from those values to equal a one stop change. Recall that when we calculated how much neutral density we had to dial in to match one step of the step wedge, we first measured the amount needed to match a three stop change. One third of that three stop change would be one stop. If you subtract that much ND from all the columns of the chart that include at least that much ND, you'll speed up those columns (and all the values we interpolated between them when we drew the graph) by exactly one stop.

For example, it takes 26cc of neutral density on my enlarger to equal a one stop change. Referring to Figure 1, we can see that the fourth through the seventh rows include at least that much ND. By subtracting 26cc of each of the three colors from the "final' columns, I produce a chart that ranges from an exposure scale of 13 to 9.75 (the rest of the chart would contain impossible negative filtration) and is exactly one stop faster than the original. Alternatively, I can just read values off the original graph, and subtract 26cc from each value on the fly.

Acknowledgements

As with all things, credit for this system should be shared with several people.

Howard Bond's article Finding Paper Contrast and Exposure Changes, Darkroom and Creative Camera Techniques, Nov./Dec. 1992, described how to use a step wedge to measure changes in paper speed and exposure scale. That article provided me with the basic tools to measure paper speed and exposure scale needed to put my system together.

Barry Sherman provided another key in one of his excellent notes posted on the Internet newsgroup rec.photo. In the post, Barry described how he produced a set of filtration settings for his dichroic head by using his color analyzer to generate settings that matched his set of PolyMax filters. Barry's post made me realize both that VCCE printing with a dichroic head was possible, and egged me into working out how to do it.

Several of the members of the rec.photo community provided the invaluable service of reading drafts of this article and providing excellent feedback and suggestions - Barry Sherman, Danny Altschuler, and Myron Gochnauer all labored the reading of early versions and gave me both a better article and deeper insight into printing. Thanks to all of you.

Naturally, these people all share credit, but not blame. Any errors are mine alone.