Is two stops more exposure the same as multiplying the time by four?

[Yaakov Asher Sinclair]

Sorry maths isn't my strong point. If, say, ten seconds exposure needs an incre ase 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 fac tor in an additional loss of speed for the extra time and make the exposure time even longer? Yours, mathematically challenged, Yaakov Asher Sinclair

[Glenn Kroeger]

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.

[shaman]

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.