Your Personal Reference Speed Point
by Phil Davis

The Plotter now has a new option named “Personal Speed Point” that’s enabled by default and assigned the value 2.4. You can either use this default value, or change it to some other number, or disable the feature entirely by unchecking it.

To understand what this number stands for and why it’s important, consider how the plotter displays film curves. First, the horizontal location of the film curve family in the graph space is determined by two factors: the speed of the film and the amount of exposure your test provided. If your test is overexposed the curve family will be displaced to the left in the graph space and some or all of its toe contour may be lost. If underexposed, the family is displaced to the right and the useful curve length may be shortened excessively.

The same displacement will occur if films of different speeds are tested using the same exposure. In that case the faster film’s family will move to the left and the slower film’s family will appear farther to the right in the graph space. In other words, if you want the family to be ideally positioned in the graph space, you must modify your test exposure to compensate for the film’s speed.

My test setup provides an exposure that is appropriate for testing ISO 100 films. Assuming that all films have been assigned honest ISO speeds by their manufacturers, I should be able to reduce the exposure by two stops to test a 400-speed film, or increase it by one stop to test a 50-speed film, and have their families appear superimposed over the 100-speed family at the “ideal” location in the graph space. In fact, though, that rarely happens because effective film speeds (EFS) for any given film/developer combinations can be quite different from the manufacturer’s claimed ISO values. This means that these films’ families will be displaced laterally from the “ideal” graph location, and we can measure this displacement to calculate their EFS values quite precisely.

To do this, remember that the graph’s x-axis is calibrated in one-third-stop intervals that (conveniently!) correspond to units in the common film speed sequence. For example, if we’re testing an ISO 400-speed film and its family appears 2/3 stop to the right of the “ideal” family location, we know that the real speed is 250, not the 400 claimed.

Of course, to make this work we have to know where that “ideal” family location is, and that requires careful calibration of the film test exposure procedure.

To set up your test conditions to establish this “ideal” curve location, set your enlarger lens aperture at f/11 and adjust the enlarger height until you can read EV 4.0 on a white card laid on the printing frame. If this setup produces an illumination circle on the baseboard of less than about 15” in diameter, open the aperture to f/8 and readjust the height. It’s important that the light circle be large, and that the printing frame be placed in the center of it where the illumination is most uniform. When you’ve completed these adjustments and your spotmeter reads EV 4.0 on the white card, set your enlarger timer to provide an exposure of 1/2 second. You’re now ready to begin calibration for films of ISO 100 speed.

I recommend that you begin with Ilford Delta 100, developed in either D-76, 1+1, or Xtol, 1+1, at 70°F., because these combinations have been shown to produce an honest 100 speed. Expose three sheets of film identically under a 21-step step tablet and process the films together in the BTZS tubes for individual times of 4, 8, and 16 minutes. When the films are dry, read their densities into the Plotter and select the Analysis tab. The Plotter’s gray “ISO triangle” will identify the family’s “ISO” speed point and it will appear as a small gray circle on the x-axis of the graph.

If the ISO mark falls on 2.4 on the x-axis your test is complete and your test setup is calibrated to provide useful film speed information. If the mark does not fall on 2.4, I recommend that you adjust the enlarger height or the aperture, or both, to shift it into place. Since each mark on the x-axis represents a 1/3-stop increment of exposure, and since exposure increases from left to right, it’s easy to estimate the necessary exposure correction. For example, if the “ISO” mark falls on 2.2, the test was underexposed by 2/3 stop. Readjust the test illumination until you read EV 4.67 (a 2/3-stop increase) and run another 3-sheet film test. This should place the “ISO” mark quite precisely on 2.4.

When that’s been accomplished, your test setup is calibrated. To test other than 100-speed films you simply have to adjust the test exposure to compensate for their different ISO ratings. For example, if EV 4.0 is appropriate for 100-speed films, then use EV 2.0 for 400-speed films, EV 2.3 for TXT (ISO 320), EV 3.67 for FP4 Plus (ISO 125), EV 3,0 for Bergger 200, etc. If you make these exposure adjustments, and if the films are honestly rated by their manufacturers, all of your film test families will be ideally located with their ISO speed points on, or very close to 2.4.

But of course this won’t happen very often, because many film/developer combinations don’t provide the speed that their ISO ratings claim for them. And here’s where a real problem can arise; the Plotter program has no way to know whether a film is accurately rated or not, so it has to assume that the manufacturer’s ISO value (which you enter in the Curve Family window in the Data tab) is correct. All the program can do on its own is identify, in the curve family, a curve that satisfies the ISO specifications — approximated by a curve gradient of 0.62. Having found that curve mathematically (the curve may not actually exist), the program marks its speed point in the x-axis and labels it with the rated ISO speed. It then calibrates the EFS charts using this ISO speed point as a reference.

More likely than not, that reference speed point value is wrong; and if it is, the EFS chart data are wrong, too. This is why it’s important to calibrate your test setup. You need to provide the Plotter with an accurate point of reference to use in calculating the real speed of the tested materials.

If you have calibrated your test setup as I’ve recommended above, your Personal Reference Speed Point is 2.4. This is not a magic number; if you prefer to use 2.3 or 2.5 — or some other number — the Plotter will be able to work with it. But don’t stray too far from 2.4 because higher numbers may lop off some of the curve toe contours and lower numbers will limit the curve length the Plotter needs to complete its analyses. I recommend 2.4 as a reasonable standard, and that’s the value you should enable in the Options window if you plan to use any of the test files that are supplied with the Plotter.

As an illustration of how this personal reference speed point works, compare figures 1 and 2. In figure 1 the reference speed point has been disabled and the Plotter (in the absence of a reference) has assumed that this 100 speed film (Agfa APX 100 in D-76 1+1) is accurately rated. Notice that the EFS values range from 40- to 160-. In figure 2 the reference point has been enabled and its location is marked on the x-axis with a dark circle/triangle combination very close to 2.4. In this case the Plotter has recognized the ISO reference and has derated the film accordingly by about one stop. Now you can see that the family’s EFS values range from 16+ to about 64+ — a significant difference, and one that could cause you to underexpose if you were working with the unadjusted Plotter data.

© 2002 Phil Davis