RECIPROCITY RULE FAILURE
As mentioned earlier, the reciprocity rule allows fortremendous flexibility in exposure settings due to theinterconnectedness of shutter speeds and apertures. A change in onerequires an equal change in the other in the opposite direction inorder to compensate
FIGURE 16-1
Sample Reciprocity Rule Variations
Aperture I----I----I----I----I----I----I----I----I----I----I
1.0 1.4 2.0 2.8 4.0 5.6 8.0 11. 16. 22. 32
Shutter I----I----I----I----I----I----I----I----I----I----I
1 2 4 8 15 30 60 125 250 500 m
MORE LIGHT <----------------------> LESS LIGHT
Suppose, for example that a meter indicates a correctexposure at 1/30 and f-5.6. The reciprocity rule states thatthe film will also be correctly exposed at all the othershutter speed and aperture combinations listed in FIGURE 16-1, such as 1/250 and f-16 or 1/8 and f-2.8. The reciprocityrule has definite limitations however, and breaks down atextreme exposure times
Most films are designed to be exposed at around 1/30 ofa second. There are notable exceptions to this, but exposingmost films at shutter speeds far removed from 1/30 leads tounderexposure and/or a change in contrast. Visualizing what happenson the film may help clarify this condition. When very long shutterspeeds are used, for example in night photography, there arenormally extremely low light levels. Under these conditions the filmis less likely to reach its exposure threshold especially in theshadow areas of the scene, even with exceptionally long shutterspeeds. Underexposure is the result; the shadow areas are morelikely to be more underexposed and the contrast on the filmincreases. Generally speaking, exposures of 1/2 of a second orlonger require compensation in order to avoid underexposure
Shutter speeds in this range are the frequent companion of theavailable light photographer and the close- up shutterbug. Workingwith fine-grain, slow-speed films will also necessitate frequentreciprocity rule failure compensation. At the other extreme, withexceptionally short exposure times, for example, 1/2,000 of asecond, the light although intense, is so brief that the film cannotadaquately respond. Again, underexposusre or a change in contrastcan be the result. Shutter speeds at this end of the continuum,usually anything shorter than 1/500 of a second, are encounteredunder very bright conditions and whenever using electronic flash
Although the shutter speed knob is set at longer than 1/1000 duringa flash exposure, the film is only exposed for the duration of theflash, usually 1/000 to 1/50,000 of a second
Whenever using very long or very short shutter speeds,compensation is needed to achieve correct exposure, but how much tocompensate depends on the type of film and how far off the shutterspeed is from the optimum setting for that type of film. Films areconstantly being introduced and improved by the manufacturers, andtherefore, rather than outdate this book by listing all the specificcompensations needed by the dozens of films that are currentlyavailable, a ballpark guide compiled from the reciprocity failuredata from several major manufacturers is listed below. If youregularly encounter long or short shutter speeds, it is prudent towrite to the manufacturer for more accurate, up- to-date informationbecause for any individual film the numbers here may be quiteinaccurate
It is important that the necessary exposurecompensations for longer exposures be made by changes in theshutter speed rather than by an assumed equal change in theaperture. That is, if the meter indicates a correct exposure at 4seconds and f-2.8 compensate by adjusting the shutter speed to 15seconds rather than by attempting to open-up the lens an equivalentamount, in this case, two f-stops. First, in such low lightconditions, the lens is probably already at or near the maximumf-stop and cannot be opened much without loss of quality anddepth-of-field. More importantly, in instances such as this wherethe reciprocity rule breaks down, a change in the aperture is notequivalent to a change in the shutter speed
FIGURE 16-3
Sample Reciprocity Rule Calculations
Hypothetical I------I------I------I------I------I
f-stop 1.4 2.0 2.8 4.0 5.6 8.0
HypotheticalMetered speed
1s 2s 4s 8s 15s 30s I------I------I------I------I------I
Compensated speed 2s 5s 15s 35s 80s 180s Figure 16-3 displays a hypothetical situation where ameter indicates that a correct exposure of a scene is f-2.8and 4 seconds. The meter indicates that other combinationsof settings are also useable, for example f-1.4 and 1 secondor f-8.0 and three seconds. If the photographer selects f-2.8 the meter recommended shutter speed of four seconds mustbe increased to 15 seconds in order to compensate for thereciprocity rule failure, rather than opening up the lens by twof-stops in an attempt to compensate. Opening up the lens to f-1.4to compensate and exposing the film at the original metered shutterspeed of four seconds will overexpose the film, as seen in Figure16-3 for the correct shutter speed for an exposure at f-1.4 is twoseconds, not four seconds
With very short shutter speeds it is necessary tocompensate for reciprocity rule failure by adjusting theaperture instead of the shutter speed because fineadjustments are not available on most shutter speed knobs
Similarly, when using an electronic flash, compensation forreciprocity rule failure must be accomplished using a change in theaperture because the electronic flash burst is too short to beregulated by the shutter speeds. With flash exposures, in order tocompensate for reciprocity rule failure it is necessary to know theduration of the flash burst, which is the actual exposure time ofthe film but is rarely known. In the automatic flash mode, there isno easy way to compute the duration of the burst because it changesaccording to the needs of the scene. In the manual mode, however,the flash duration is constant and may be listed in the owner'smanual of the flash. Depending on the flash you can expect flashduration to be around 1/500 to 1/2000 of a second in the manualmode. This will be decreased if a variable power unit is employed;each time the power output is halved, the flash duration isshortened by 50%
Color films present an additional problem forphotographers attempting to compensate for reciprocity rulefailure. Unlike B&W films which have one emulsion, colorfilms have three emulsions, one for each color. Notsurprisingly, each emulsion in the color film responsesdifferently to the effects of reciprocity rule failure. Someare more or less underexposed relative to the others duringlong or short exposures, and consequently color shifts canoccur especially in the shadow areas. Therefore with colorfilms, it is not only necessary to compensate for reciprocityrule failure by giving additional exposure, it is alsoimperative to add filtration to correct for any color shift
Film manufacturers usually specify a CC filter(s) to use ateach rated shutter speed, but with some extra long exposuresthe color shift may be too severe to be corrected and theexposure is not recommended. When using color films, writeto the manufacturers for the specific changes that are neededor perform your own tests
Reciprocity rule failure can also lead to contrastchanges in the film as shadow areas are more greatlyunderexposed relative to the highlight areas. This requiresa change in development in order to compensate, and again itis best to obtain the latest information from themanufacturer of the film being used. Remember that whenusing roll film that individual frames may experiencedifferent exposure times and need different development, butthat the entire roll is developed for the same time. As aresult it is best to avoid very long or very short exposuresor try to get these types of exposures on the same roll of film
FIGURE 16-2
Ballpark Exposure Compensation for Reciprocity Rule Failure
Metered or calculated Compensated
Shutter Speed Shutter Speed (or duration of flash) 1/125,000 1/125,000 + 1 f 1/60,000 1/60,000 + 3/4 f 1/30,000 1/30,000 + 2/3 f 1/15,000 1/15,000 + 1/3 f 1/8,000 1/8,000 + 1/4 f 1/4,000 1/4,000 1/2,000 1/2,000 1/1,000 1/1,000--------------------------------------------------- ---------- 1/500 (no compensation 1/500 1/250 is needed in 1/250 1/125 development in 1/125 1/60 this range with 1/60 1/30 most films. Out- 1/30 1/15 side this range, 1/15 1/8 it is more likely.) 1/8------------------------------------------------------- ------ 1/4 1/4 + 1/2 f 1/2 1s 1s 2s 2s 5s 4s 15s 8s 35s 15s 80s 30s 3m 1m 9m 2m 30m 4m 1.5h 8m 5.5h 15m 19h
To return to the main index for the Sub Club click here.
COPYRIGHT @ 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005 by Joe McGloin. All Rights Reserved.