Paul D. Farrar wrote:
> At 03:29 PM 9/29/98 -0700, Ted wrote:
> > I previously asked a generic question regarding the means of
> >calculating
> >proper flash exposures. The responses I got were quite helpful
> >(thanks John
> >Doris, William).
> >
> > But, now I'm having a problem because my flash unit's
> >(Vivitar 283)
> >exposure calculation chart doesn't list the film speed I need to
> >shoot at. It
> >only goes up to ISO 400.
>
> You can figure out the rule from the dial. Since each auto setting provides
> a fixed amount of light, regardless of ISO, each doubling of the ISO
> requires you to close the aperture by 1 additional stop.
>
> >
> > Here is precisely what I'm attempting to do - - - I'm trying
> >to do a
> >full roll of triple exposures. The actual film speed is ISO
> >200. For all 3
> >exposures to be equally emphasized the normal calculation would
> >be
> >[3(exposures) x 200(actual ISO)] = 600. However, I wish to
> >emphasize the last
> >(3rd) exposure. Thus, I've already shot the first 2 exposures at
> >ISO 640 -
> >this was done with existing light, so no flash problems.
> >However, as I wish
> >to set the 3rd exposure at ISO 500, I'm unable to determine the
> >proper F-
> >stop/distance-to-subject for the aformentioned reason. Any
> >suggestions on how
> >to derive the proper settings?
>
> Multiply the ISO by the sum of the relative weights, then divide by the
> weight for each exposure. The weights are linear, so doubling each doubling
> of the weight means increasing exposure by one stop. You have already
> figured out the values for 3 equal weights (say 1,1, and 1 -- the absolute
> value doesn't matter, just the relative sizes).
>
> (ISO*sum-weights)/(individual-weight) = (200*3)/1 = 600
>
> Three exposures at ISO 600.
>
> If one is given twice the weight, sum = 1+1+2 = 4
>
> (200*4)/1 = 800 for the two least exposures. Note that each is 1/4 of a full
> exposure.
>
> (200*4)/2 = 400 for the third, which is 1/2 of a full exposure.
>
> You may want to expose more than this if the subject moves, maybe as much as
> full exposure if there is no overlap of images.
>
> Paul
> >
> >
> >Ted.
> >
> >
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> >
> >
>
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Thanks Paul. I realize some of my questions may be rudimentary and
a bore to some 'photo-snobs.'
Best,
Ted.
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