Gordon that is kind of true until you get into very long exposures (over 20 seconds) and then you need to take in reciprocity law failure when you have to actually extend your exposure time. I know this is true with film and it's properties not sure about digital.
http://silvergrain.org/Photo-Tech/reciprocity.html Film has this because of the chemical properties in the film not sure what the properties of digital chips and if they are consistant across all types of chips are.
Went on a photo shoot with a group in Upper Antelope Canon in Page Az and the photographer doing the workshop spent a good 20 minutes before the days shoot going over the rules and how we had to add additional time to our exposures to compensate. We were all film back then (this was close to 20 years ago).
http://www.fatali.com/index-frame.php
Look at his stone cathedrals portfolio. Yeah I know the guy had some controversy a few years back because of some wax that came off of burning fireplace logs that were used in a pan to light some formations in Arches National Park late at night but after spending time with this guy (over a week) and then meeting him again a few years later and talking with him I know that the incident was not one of maliciousness but just of not thinking at the time.
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