Is two stops more exposure the same as multiplying the time by four?greenspun.com : LUSENET : Large format photography : One Thread
Sorry maths isn't my strong point. If, say, ten seconds exposure needs an increase of two stops (to compensate for reciprocity law failure), I could leave the aperture alone and expose for 40 seconds instead, right? Of would I need to factor in an additional loss of speed for the extra time and make the exposure time even longer? Yours, mathematically challenged, Yaakov Asher Sinclair
-- Yaakov Asher Sinclair (firstname.lastname@example.org), July 20, 2000
Yes, generally 2 stops is equal to a factor of 4 in exposure time...this is called reciprocity... BUT reciprocity works for short exposure times, for example, from 1/4 sec to 1 sec... if you are really talking 40 secs, you need to investigate the reciprocity failure characteristics of the film you are using.
-- Glenn C. Kroeger (email@example.com), July 20, 2000.
Just to reinforce what Glenn said: In your situation (calculating exposure compensation where RECIPROCITY FAILURE is an issue), opening the lens 2 stops is NOT (!!!) the same as increasing exposure from 10 seconds to 40 seconds.
-- Andreas Carl (firstname.lastname@example.org), July 20, 2000.