Memory Lane: Reciprocity Failure

Reciprocity failure. Copyright: Jake Levin, USA, 2007

As a result of Charlie’s article about his black & white photography course, I found myself thinking about all those weekends I spent hidden away from daylight, processing film and prints. I realized there are all sorts of fancy terms and technical issues which those of you who came to photography in the digital age will know nothing about.

The first one which sprang to mind, was “reciprocity failure”. Reciprocity is the relationship between shutter speed and aperture and the resulting exposure value. When we double the amount of light passing through the lens by opening the aperture one stop, we maintain a constant exposure value by halving the shutter speed.

The difference between film and digital is that reciprocity remains true and constant in digital cameras at all exposure times, whereas film requires adjusting to compensate as exposure time increases. Thus, reciprocity fails beyond “normal” exposure times (I’m not going attempt to define “normal”!).

Possibly not the most useful piece of information in this digitally dominated world, but a nice little stroll down memory lane for me. Now, can I interest anyone in “reticulation”, I wonder?

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