Using the PowerDial The BTZS
PowerDial is a handy calculator that produces exposure and development recommendations that work well with either spot or incident metering methods. Camera settings (f-numbers and shutter speeds) can be read directly from the dial, and development recommendations, in the form of Average Gradient (G-bar) numbers, are provided on a separate scale on the back of the slide. Exposure compensation for reciprocity effects is included in the dial calibration, and development compensation for reciprocity effects can be calculated easily from chart data supplied on the back of the sleeve. The recommended exposure and development data also include average compensation for camera and lens flare The PowerDial was originally designed for use in large-format photography, and will be of greatest value for sheet film users who can vary film development for best effect for each subject; but rollfilm users who are willing to commit an entire roll of film to some specific subject condition can work similarly. Photographers who prefer to standardize on “normal” development will not be able to exploit all of the PowerDial’s capabilities but can still take advantage of the reciprocity and flare compensation features that it provides. EV Numbers The PowerDial must be used with EV numbers. Most modern meters will provide EV numbers, either directly or on an auxiliary scale, but the numbers, as given, may or may not be usable. This is because, in some meter designs, changing the film speed setting changes the EV numbers; in other designs it does not. To avoid this confusion, and possible serious error, it’s always best to set your meter on ISO 100 and set the rated film speed into the PowerDial. For example, if you’re using Delta 400 film (ISO 400) set your meter on ISO 100, but set the PowerDial for ISO 400. This procedure is the recommended one because it’s easy to remember and it will work with any meter.
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The PowerDial Slide; Incident or Zone The PowerDial slide has three faces. If you remove it from the sleeve and unfold it you’ll see that there are patterns on both sides. The Incident System fan with its associated Subject Brightness Range (SBR) and Average Gradient (G-bar) scales are on one side; the Zone System fan, with a duplicate SBR scale and G-Bar chart set on the other. You can select either metering method by folding the slide so that your chosen pattern is on the outside. Insert the folded slide into the sleeve with the SBR and G-bar scales in back so that your chosen fan pattern appears in front, under the circular dial. Paper Grade, ISO Range, and ES Before the PowerDial can provide you with useful development information, you’ll need to tell it what sort of negative you want to produce, and you do that by specifying the contrast grade of the paper you plan to print on. On the back of the slide, along the top border of the G-bar chart, you’ll see “ES” (Exposure Scale) values labeling the vertical chart lines. Above them are the more familiar paper grade numbers, 3 through 0. The ES numbers indicate paper contrast much more specifically than the traditional grade numbers do. As you can see, for example, the grade 1 category includes papers whose ES values extend from 1.15 to 1.39.Similarly, grade 3 includes ES values from 0.8 to 0.94; grade 2 includes values from 0.95 to 1.14, and grade 0 begins at ES 1.4 and extends to 1.69, beyond the limit of the fan chart. Of course you won’t find numbers such as 1.16, 0.95, 1.15, and 1.69 on any paper package or instruction sheet but you may encounter "ISO Range" numbers which are really paper ES values, multiplied by 100 and rounded to the nearest multiple of 10. For example, an ES of 1.23 becomes ISO Range 120 and ES 0.97 translates to ISO Range 100. If you have discovered the ES value of your paper by testing it yourself or having it tested for you, you can estimate its position between the marked ES lines to improve the accuracy of your average gradient choices. If you don't know your paper's ES, but do know the manufacturer's ISO Range rating for the paper, convert it to an ES value by dividing it by 100.
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If you don't know either the ES value, or the ISO Range rating, use the average ES value in the appropriate grade number range. For example, if you have a "grade 2" paper, it can presumably have an ES value of anything from 0.95 to 1.14. Split the difference and use 1.05; it probably won't be exactly right, but it's better than a simple guess. Setting Up Again, set your meter at ISO 100. Set the PowerDial to the nominal ISO rating of the film in use unless you’re using either Ilford FP4-Plus or Kodak Tri-X Professional. I suggest derating both of those films: try 250 for Tri-X and 100 for FP4 Plus. But don’t use your “personal film speeds” yet because the PowerDial includes generic compensation for film speed correction. If you discover by experience that some other film speed adjustment is necessary, then make that adjustment, but give the PowerDial a chance first. When you’ve set the film speed, fasten the two dials together with a bit of tape to keep the film speed setting from shifting as you rotate the dials. Zone System With Sheet Film Take your usual spotmeter readings of the subject, selecting one low luminance value and one high one. Slip the slide in and out of the sleeve and turn the dial until both luminance readings are aligned with your selected zone lines. You can work with fractional zones if you want to; for example, if you want to use zone II-1/2, center your luminance reading between the lines representing zones II and III. When you’re satisfied with both zone assignments you’ll find the appropriate camera settings displayed along the lower margin of the dial. Note that these settings may not agree with the readings you’ll get by setting up these zone placements on your spotmeter dial. The main reason for this is that the PowerDial calibration includes a small compensation for camera and lens flare as well as some generic compensation for the film speed adjustment that’s appropriate for development time changes. Second, because zones and stops are equal only when the subject range is normal, the PowerDial’s recommendations will include some error as the subject range departs from
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normal, and/or as the chosen low zone is increased above the minimum value of zone I-1/2. This error is not usually great enough to be serious; it’s negligible for zone II placement, and for zone III placement it varies from about 1/2stop underexposure for a 10-stop subject range to about 1/2-stop overexposure for a 5-stop range. Notice, however, that there is no error at all with any zone choice when the subject range is the normal 7 stops; and there is no error for any subject range if the low luminance reading is “placed” in zone I-1/2 as represented by the right hand boundary line of the zone fan pattern. Although it’s theoretically possible to base exposure and development on any pair of zones, it’s best when using the PowerDial to use the lowest zone possible—ideally III or lower. For Example: Finding Exposure For the purpose of illustration, let’s assume you’re using a 100-speed film (ISO 100 set on both the meter and the PowerDial) and planning to print on a paper whose ISO Range value is 100 (ES 1.0). A spot reading of your chosen low value is 7 EV and you place it in zone III. You visualize your high reading of 11 EV as falling in zone VII-1/2. Slip the slide in or out and turn the dial as necessary to align those EV values with your chosen zones, then find f/11 @ 1/4 second, approximately, as one of the recommended camera settings displayed along the lower border of the dial. Finding Development: G-bar Next, without moving the slide, turn the PowerDial over and you should see that the SBR is about 6 1/2 stops. Then find the appropriate G-bar value by locating the vertical line representing the ES value of the paper you plan to print on, and following it down to the edge of the sleeve. In this example, if you’re using an ES of 1.0 the appropriate G-bar value is about 0.53, indicated by the fact that the vertical 1.0 line falls between the 0.5 and 0.55 diagonal lines, right at the sleeve edge. You can also estimate the values that are appropriate for other papers: for ES 0.8 the G-bar value is about 0.43; for 0.9 the G-bar is about 0.48; for 1.1 use G-bar 0.58; and for 1.2 use G-bar 0.65. I recommend that you mark the appropriate G-bar value on
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the side of the holder that contains the exposed film. Then, even if you don’t keep any other records, you’ll have the specific development information for each sheet of film when you get back to the darkroom. Incident System With Sheet Film Refold the slide to expose the Incident pattern. You’ll notice that there are only two lines on this face. Be sure your meter is set on ISO 100, and set the PowerDial film speed to the rated ISO value of the film you’re using. Like the Zone System, the Incident System requires two meter readings, one low to establish the exposure and one high to determine the subject range. Normally you should take the low incident reading in a shaded area of the subject, pointing the meter cell directly at the camera. Take the high reading in full light, again pointing the meter at the camera. Adjust the PowerDial slide and dial so that these light readings are aligned with the curved fan lines, and find the recommended camera settings along the lower margin of the dial. Read the exposure and set the camera. Then, without moving the slide, turn the PowerDial over and find the G-bar value for film development. For Example Let’s assume that you’re using an ISO 400 film, and your readings are 9 and 14. Align these values with the curved lines on the slide. A useful exposure setting is now seen to be about 1/2 second at about f/32. Turn the PowerDial over and you’ll see that the SBR is 10 stops, and the appropriate G-bar value for an ES value of 0.9 is about 0.32. Incident Metering Technique Successful use of the Zone System requires that you carry an image of the various zone grays in your mind and assign them to selected areas of the subject. “Placing” the low meter reading in the chosen low zone determines film exposure; the point where the high reading “falls” on the zone scale determines the “N-number,” which establishes development. The Incident System also requires visualization, but of a dif-
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ferent sort. Instead of expecting standard tones of gray to be assigned to selected subject areas, the Incident System expects you to imagine how the near-black and near-white extremes of subject tone will be rendered in different light conditions. In general, the low incident meter reading will provide sufficient exposure to record a black object at the meter position as borderline-black in the print, and the high reading will provide sufficient exposure to record a white object, in that light condition, as borderline-white in the print. Because the incident meter measures only illumination, and ignores the subject entirely, it doesn’t matter whether these black and white “reference” objects actually exist at the metering points or not; the meter simply guarantees adequate exposure to record them in their proper values in case they are there. Although this may sound haphazard it’s actually quite sensible because satisfactory gradation in the middle print values depends on establishment of appropriate extremes of tone. If you can imagine a black object accompanying the meter in the subject’s shadow area (you can use the meter’s own black case as a visual reference) you can see that as you move the meter deeper into the shadow area the subject tones in the shadows appear to get lighter—relative to the reference black. On the other hand, moving the meter out into the light makes the shadow details appear darker when compared with your imagined black reference, or the actual meter case. Measurement of the high illuminance is similar; imagine a white object (or use a sheet of white paper or some white cloth) at the meter position and compare the brightness of this reference object with the highlight areas of the subject. Usually this single reading in full light is satisfactory but if you should want to emphasize the subject’s highlights, you can move the meter into partial shade for this reading, thereby “overdeveloping” the subject enough to increase highlight brightness. Similarly, you can effectively subdue highlight brilliance by using an EV value somewhat higher than the actual high reading. In other words, if the high reading is, say, 12, you
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can gray the subjects highlights a little by using 12-1/2, or reduce highlight brilliance significantly by using EV 13 (or more) for the high reading. For a more complete explanation of this Incident metering method, see “Beyond The Zone System,” by Phil Davis, published by Focal Press. Reciprocity Compensation When the film illumination is dim enough to require an exposure of about 1 second or more, the film’s sensitivity decreases and some additional exposure must be supplied to form a satisfactory image. At the same time, almost all films will appear to gain in contrast to some extent, so it’s almost always desirable to reduce development somewhat. The PowerDial includes the necessary exposure compensation directly by simply replacing the usual exposure times of 1 second or more with corrected values; so, for example, you’ll see 1.1 in place of 1 second; 3 in place of 2; 8 replacing 4; etc. This makes it possible to use the exposures just as you read them without the need to apply additional correction. While it’s true that different films are affected somewhat differently by reciprocity failure, these generalized values are derived from the averages of many tests and will provide useful exposures for most films, in almost all cases. Development compensation is a bit more complicated but the principle is simple enough. Because the PowerDial provides development recommendations in the form of G-bar numbers, corrections can be made by adjusting the normal G-bar number by some percentage. Again, different films require different development compensation so the PowerDial provides average correction for groups of films whose characteristics are somewhat similar. Use the development chart on the back of the sleeve to find the correction factor for the film you’re using, then multiply the indicated G-bar number by that factor to find the corrected value. For Example If you’re using TMX film, your paper’s ES value is 100, the indicated G-bar value is 0.35 (for an SBR of 10 stops), and the corrected exposure time is 20 seconds, read up to the
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TMX line, and across to the left to find the factor 0.97 (approximately). Multiply 0.35 by 0.97 and find the corrected G-bar value of 0.34. Notice that there are specific development compensation recommendations for TMX, TXT, TMY, Delta 100, and FP4 Plus; all other films get the generic “Everything else” treatment. These reciprocity recommendations are based on average or “typical” behavior for the materials mentioned but they should be good enough to keep you out of serious trouble. As you use the PowerDial you’ll undoubtedly learn how to adjust the data to accommodate the characteristics of various unusual materials combinations. Using the PowerDial With Rollfilm Ideally, film development should be related to subject luminance range. But rollfilm exposures are developed together, so unless you purposely shoot the whole roll of the same, or very similar subjects, whatever development the roll receives will almost certainly be inappropriate for at least some of the negatives. Happily, in most cases, we tend to shoot subjects of more or less “normal” range so variations in negative contrast can usually be taken care of satisfactorily by using papers of various contrast grades or by using VC papers with appropriate filtration. The most important consideration in rollfilm use is providing adequate exposure. Spotmetering With Rollfilm If you want to use your spotmeter with rollfilm and the PowerDial, fold the slide to display the Zone fan and set the slide at the 7-stop position (as indicated on the SBR scale on the back of the slide). Because development won’t be varied you don’t need to know the subject range, so it isn’t necessary to take a high luminance reading at all. Set your spotmeter ISO scale on 100 and take a luminance reading of the darkest area of the subject in which you’d like to retain a trace of tonality—in other words, a value just above accent black. Turn the dial to place that EV reading over the right hand boundary line of the Zone fan, corresponding to Zone I-1/2.
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Choose your camera settings from the pairs displayed along the lower margin of the dial. and make the exposure. Then, leaving the slide at this 7-stop position, turn the PowerDial over to find the G-bar value for the paper of your choice. For Example Assume you’re using Fuji Neopan 400 film, and plan to print on a paper whose ISO Range is 110 (ES 1.1). Set the slide to its 7-stop position; then, if the dark luminance reading is EV 6, place EV 6 on the right hand boundary line of the fan. Then you’ll see that one of the useful camera settings is about 1/15 @ f/11 or 1/8 @ about f/14. Turn the PowerDial over to find the “normal” (7-stop) Average Gradient of 0.55 for the ES value of 1.1. Metering borderline blacks is probably the safest way to insure good shadow detail, but you can meter subject tones other than black and use a limited version of the Zone System if you want to. For example, you can “place” a very dark gray area in zone III—that is, align its luminance value with the zone III line on the fan. Actually, as long as the slide is set at the 7-stop SBR position, zones are equal to stops, so if the subject range really is 7 stops, you can meter any subject value and align it with an appropriate zone line. Remember, though, you should not use any low zone higher than zone III if the SBR is not normal, or if the slide is not set at the 7-stop SBR position. Incident Metering With Rollfilm Before you begin, be sure that the meter is set for ISO 100, and be sure the PowerDial ISO value is set for the nominal speed of the film you’re using. Fold the slide to reveal the Incident fan and set the slide at the 7-stop SBR position. Because you will probably develop rollfilm “normally” there’s no need to determine the subject range, so a single incident reading taken in open shade, while pointing the meter cell at the lens, is all you need. Simply align the indicated EV value with the right hand fan line and select any aperture/shutter speed pair from those displayed at the lower margin of the dial. This simple procedure will work well most of the time but, as
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was pointed out above, deep shadow detail is determined directly by meter placement so, in some cases you’ll have to decide whether to place the meter farther into the shaded area (to capture details in deep shadow) or move the meter out farther into the light (to deliberately lose deep shadow detail). Under flat light conditions when there’s very little illuminance difference between shaded and unshaded areas, placing the meter in the deepest shade is likely to provide more exposure than is necessary to preserve detail in the shadows (unless there are actually black objects present there). In fact, you may find that the “shadow” reading can be taken in full light. You’ll probably have to experiment a little to discover how to position the meter to provide the results you want but it will be worth the effort. Incident metering, based on shadow readings, is a relatively quick and reliable way to determine exposure for any negative film, and the PowerDial’s unique design makes it easy by avoiding a special speed adjustment that’s required when using the meter alone. Why G-bar Numbers? As mentioned previously, the PowerDial provides development recommendations in the form of Average Gradient (Gbar) numbers. These numbers are more reliable than the more usual tables of development times because they simply define the slope of the film’s characteristic curve, so they’re independent of printing conditions and developer type (development time charts assume the use of “normal” paper contrast, “average” printing methods, and specified temperatures). That means that if you need a G-bar value of, say, 0.55, you can use any film and any developer (at any temperature) that will provide that curve slope. In other words, the instruction “Develop for 8 minutes” begs a lot of questions; but “Develop to a G-bar of 0.55” defines the necessary development extent precisely. “Normal” G-bar For Rollfilm Use Remember, if you’re using rollfilm in the usual way—that is, mixing subject types on a single roll, and planning to use “normal” development— be sure to set the slide at the 7-stop SBR position before finding the G-bar value.
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