Reciprocity Failure

Reciprocity refers to the interrelationship between shutter speed (time) and aperture (amount of light). As you know, for example, you obtain the same exposure with each of the following combinations: 1/125 at /716 . . . 1/250 at //ll . . . 1/500 at f/8 . . . 1/1000 at ft5.6.

This interrelationship breaks down, unfortunately, when the shutter speed is either extremely fast or extremely slow. Typically, with black-and-white films the breakdown — called reciprocity failure — occurs at exposure speeds faster than 1/1000,or slower than 1/2 second. In either case, the resultant negative is underexposed by a normal exposure meter reading.

For example, if a proper exposure is 1/1000 at fj5.6 and you decide you want to shoot at f/4 to get more selective focus, if you shoot 1/2000 at f/4 you will get underexposure.

At the other extreme, if a proper exposure is 1/2 second at fill, and you decide you want to shoot at //16 to get greater depth of field, if you shoot 1 second at //16 you will again get underexposure.

Another effect of reciprocity failure is a change in contrast. During very long exposures, you will get higher-than-usual contrast.

Finally, reciprocity failure becomes increasingly significant the further you deviate from "normal" exposure times. In other words, the underexposure and lowered contrast is barely noticeable at 1/2000 of a second. But, if you are using a strobe producing an exposure of 1/10,000-second, the effect becomes pronounced; at 1/50,000 it is extreme. Conversely, at 1 second, reciprocity failure is minimal. At 10 seconds, it's pronounced; at 100 seconds, it's extreme.

How do you handle reciprocity failure?

The easiest way is to change your exposure time, where possible, to shoot within the "normal" range of shutter speeds. For example, if you are shooting 1 second at f/4, can you shoot instead 1/2 second at /72.8? Or if you're shooting 1/2000 at f/8, can you shoot 1/1000 at fj 11; or better still, 1/500 at fl 16? Where possible this is the best way.

Where it's not possible, you should compensate for reciprocity failure by adjusting the exposure time (to compensate for underexposure) and the development time (to compensate for the change in contrast). How much should you adjust? This varies from film to film. Each film has its own reciprocity characteristics. Since these are usually not set forth in the instruction sheets that accompany the film, you can write the manufacturer if you need the information, or consult technical manuals such as the Photo Lab Index published by Morgan and Morgan.

Here is a general chart that is approximately correct for many B&W films.

Compensating for Reciprocity Exposure

Indicated

Additional

Altered

Exposure

Exposure

Development

1/10,000-second

Vt stop more

15% more

1/2,000-second

'A stop more

10% more

1/1,000-second

none

none

1 second

1 stop (or 2 seconds)

10% less

10 seconds

2 stops (or 40 seconds)

20% less

100 seconds

3 stops (or 800 seconds)

30% less

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