Sorry i was being lazy, i should have said reciprocity 'failure' but forgot to type the failure bit.
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However, during very long exposures, sensors can respond much more slowly than usual, resulting in underexposure. It is said then, that reciprocity failure has occurred, i.e. the conventional relationship between aperture and shutter speed has broken down. Most film manufacturers publish data on the latitude of their films (how much they can be pushed or pulled beyond their normal exposure range), and also reciprocity corrections. For example, if a light meter indicates a required EV of 5 and the photographer sets the aperture to f/11, then ordinarily a 4 second exposure would be required; a reciprocity correction factor of 1.5 means that the actual exposure must be extended to 6 seconds.
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This was exactly what i was referring to. And as usual explained perfectly!
As a rule, do you reckon you get about 30 seconds of open shutter before reciprocity failure starts to effect the exposure?....shooting RAW!