Bruce Barnbaum - The Art of Photography - An Approach to Personal Expression

Another problem with long exposures is the fact that the gray meter does not calculate the film’s reduced response to low light levels. Within the negative’s normal response range you can determine your exposures as explained in this chapter and the previous one; but for long exposures (beyond one second) you must factor in additional time to compensate for reciprocity failure. Then you must alter contrast in negative development to compensate for the contrast increase during exposure.

Table 9-1. Exposure Table for Reciprocity Failure of Tri-X Pan Film (ASA 320)

Metered Exposure

Required Exposure

Contrast Increase

2 sec.

3 sec.

5 sec.

8 sec.

N + ½ *

10 sec.

18 sec.

15 sec.

30 sec.

20 sec.

45 sec.

N + 1 *

30 sec.

75 sec.

1 min.

3 min.

2 min.

7½ min.

N + 1½ *

4 min.

16 min.

10 min.

50 min.

N + 2 *

20 min.

2 hrs. 20 min.

N + 2½ *

30 min.

4 hrs.

N + 3 *

Table 9-2. Exposure Table for Reciprocity Failure of T-Max 100 Pan Film

Metered Exposure

Required Exposure

Contrast Increase

2 sec.

2½ sec.

5 sec.

7 sec.

10 sec.

15 sec.

N + ½ *

15 sec.

24 sec

20 sec.

35 sec.

30 sec.

50 sec.

1 min.

2 min.

N + 1 *

2 min.

4½ min.

4 min.

10 min.

10 min.

28 min.

20 min.

65 min.

N + 1½ *

30 min.

1 hr. 50 min.

In both tables (Table 9-1 and Table 9-2), the asterisk (*) indicates the approximate amount of contrast increase due to reciprocity failure and extended exposure. Thus, if you use Tri-X film and have a metered reading of 30 seconds, the table indicates that you must expose the negative for 75 seconds; but normal development of that negative will increase the contrast as if you had N+1 development (Figure 9-6–Table 9-4), so you must give the negative an N–1 to compensate for the contrast increase during the long exposure .

The reciprocity failure tables show the approximate exposure increase needed for extended exposures for both Tri-X and T-Max 100 films. The Tri-X table is my own personal table derived from years of practical experience (identical to Minor White’s own determination 40 years earlier). Kodak’s table for Tri-X is wildly incorrect. The T-Max table is based upon Kodak’s numbers, which are correct but incomplete.

It’s easy to interpolate between the stated times. The following example for Tri-X illustrates the proper method for using the table with the zone system. Suppose you want to place an object in Zone 7, and your gray meter gives you a five-second exposure reading. Of course, the reading places the object in Zone 5 (as always!), so first you double the exposure to 10 seconds in order to place it in Zone 6. Then you double the shutter speed to 20 seconds to place it in Zone 7. By consulting the reciprocity failure table, you determine that 20-second metering requires a 45-second exposure for Tri-X film. With T-Max 100 film, the metered 20-second reading requires a 33-second exposure.

If you’re using color film, reciprocity failure can be fascinating. Color film is made up of three separate emulsions, one for each primary color. Each has its own rate of reciprocity failure, so the color balance of the film tends to shift as exposures progressively lengthen. Colors that do not exist in reality may show up during long exposures, and some colors may be greatly enhanced, while others may be lost. Anything can happen. The effect can be wonderful or awful, but it’s usually surprising. It also tends to vary with each emulsion batch, so it’s never consistent from roll to roll or sheet to sheet. Reciprocity failure could open up areas of wonderful creativity for those interested in pursuing its eccentricities.

Examples of Decreasing and Increasing Contrast

As a first example, let’s return to the sunlit snowfield of Chapter 8. As before, you see the modulations of the snow, its hills and valleys. You want to photograph it so that those delicate tones are visible in the print. If nothing but snow is in the scene, it’s easy. All you have to do is decide where to place the snow on the scale and shoot. My choice would be an average placement of Zone 7½ or 8, which would yield a range of light tones from pure white to light gray and would show the modulations.

But suppose a large, dark boulder sits in the center of the field, oriented in such a manner that it’s shaded. Let’s agree that the rock should be about a Zone 3 tonality, and that the snow should still be placed at about Zone 7½ or 8 on the average. Suppose the gray meter gives a reading on the snow of f/16 @ second, while the reading on the rock is f/5.6 @ second. How many zones separate the snow and the rock? The separation is determined by adding the differences between the two readings in both aperture and shutter speed. F/16 represents 3 stops less exposure than f/5.6 as follows: f/5.6––f/8––f/11––f/16. At the same time, second represents 2 stops less exposure than second as follows:

The difference is 5 stops, or 5 zones. No matter how you expose the negative, there will be a 5-zone spread between the snow and rock, which cannot be changed in the exposure . It may not be wise to expose at either the metered reading on the snow or the rock; if you choose the former, the snow would come out at Zone 5 and the rock at Zone 0, while if you choose the latter, the rock would be exposed at Zone 5 and the snow at Zone 10. But you know how to place the rock at Zone 3: close down stops from the gray meter’s reading on the rock. With the rock in Zone 3, the snow is automatically placed at Zone 8 on the exposure scale, and normal development brings the density of each to the desired level. (Further on I explain why I actually prefer the shadow placement to be in Zone 4 rather than Zone 3.)

Suppose, however, that the meter reading on the rock is f/4 @ second rather than f/5.6 @ second. Now the spread between the rock and snow is 6 zones, so that exposing the rock in Zone 3 automatically places the snow in Zone 9—too high for any texture. But since cutting the development time lowers contrast, if you place the rock in Zone 3 (which automatically places the snow in Zone 9) and develop less than normally, the higher zone drops significantly more than the lower zone. By cutting the time so that the Zone 9 exposure drops to Zone 8 (perhaps about six minutes, based on Figure 9-3), the Zone 3 exposure hardly drops at all. At most, it will drop to something above Zone 2¾. By exposing just a little higher on the scale, say Zone 3½ on the rock and 9½ on the snow, and cutting the development time a bit more, the snow can be developed to a density of Zone 8 (a drop of 1½ zones from the exposure) while the rock will drop less than ½ a zone to a point just above Zone 3. Now the spread of zones between the two objects is just what you wanted. You have achieved the precise contrast and tonalities you sought by thoughtful use of the zone system coupled with your knowledge of negative development.