Bob wrote > Speaking of conundrums, can anybody resolve this one ? If you do a a > series of time exposures say 2secs , 3 secs ,4secs then the 2second > exposure is logically one less stop than the 4 second exposure > (reciprocity ignored ), and the three second exposure is a half stop > between the two.If you want to increase the 4 sec exposure by half a stop > , then it should be 6 secs . However , by the same logic , if I want to > increase the exposure of the 3 sec exposure by a half stop , I end up > with an exposure of 4.5 seconds . Similarly , if I do the same with my 6 > second exposure I end up with 9seconds , but one stop over 4 secs is 8 > seconds ! > I get this one all the time in the darkroom (yes... i occasionally use this primitive technique.... please bear with me...). If I change the grade of paper I am using, I often have to change the exposure for purely aesthetic reasons (the 'weight' of the image changes as the contrast changes). Often these changes are in the order of 1/4 - 1/3 of a stop.... and I have gone as far to make a spreadsheet for my Psion to do the working out for me because 1/4 stop up is not the same as 1/4 stop down... though for practical purposes it is almost as good as.
Halving and doubling it may be, but linear it aint. Probably a natural log (if memory about film density curves and DIN numbers serves). Giles Stokoe photographer/photojournalist. See some images at http://www.stokoe.co.uk =============================================================== GO TO http://www.prodig.org for ~ GUIDELINES ~ un/SUBSCRIBING ~ ITEMS for SALE
