Originally Posted by Leon
Leon you ask a valid question. The answer is that you apply the factors sequentially. In other words, let's say that you were shooting your macro shot with a filter having a filter factor of 1 1/2 stops and that in this instance you are exposing your object through a 210 mm lens at F32 and you find that your bellows extension (from lens nodal point to film plane is 11 inches) the film in this instance is ISO 100. You meter the scene and the meter indicates an exposure of 1/4 second at that film speed and aperture without allowances for bellows extension or filter factor. When we take into consideration the filter factor your new exposure is 3/4 second (one stop is 1/2--the additional 1/2 stop is 1/4 second) at F32. We then apply the bellows extension factor of one stop (infinity focus is 8 1/4 inches and the bellows is 11 inches this is one stop) and your corrected exposure is 1 1/2 second. You are now into reciprocity consideration territory (over 1 second exposure). Typically this would now become a 3 second exposure at F32 with all of the factors included. When developing this sheet of film you will want to decrease development by 10% appr. to compensate for the increased contrast that the longer exposure for reciprocity considerations create. Hope that this explanation is helpful to your future calculations. Good luck.