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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