I was reading my own post and noticed a mistake when converting inches to millimeters. I used a factor of 24.5 when in fact an inch is equal to 25.4 millimeters. The time to move across I talked about, therefore, should be:
Time to move across film: (5 x 25.4) / 120 = 1.058333 seconds Not a big difference, but if NASA failed a mission to Mars because a conversion error, I don't want you to miss your exposure due to my conversion mistake :-) Guillermo > Let me think in "written voice" here: > > Since the film will be stationary, if we draw a very thin line on the film > (parallel to the shutter's slit), when the slit is moving accross the film it > will see that thin line passing by, the amount of time it takes that line to go > from one end of the slit to the other will be the exposure time. Exposure > time, therefore should be given by: > > Exposure time = slit width / speed of shutter moving across the film > > for instance: If you want an exposure time of 1/60 seconds and you have a slit > 2mm wide, you FP shutter should move this fast accross the film: > > FP shutter speed = Slit width / Exposure time > FP shutter speed = 2 / (1/60) = 120 mm/second > If the film is 4x5 (landscape), the FP shutter should take this long to move > accross the 5" length of the film: > > Time to move accross film: (5 x 24.5) / 120 = 1.0208333 seconds > > Thinking over !! > > Guillermo
