Aperture/shutter conversions

[P3D <Linnstaedt@xxxxxxx>]

Friends,
This past week I shot up a roll of Kodachrome 64 around Dallas, TX (Dealey
Plaza, HardRock Cafe, etc.).  To use the exposure table, I had to make
conversions for the shutter and aperature.  Thought I'd pass along for others
to use and/or refine.

My Realist 3.5 has shutter speeds 150, 100, 50, etc. while the table on the
film carton lists for t = 1/125.  Sooo, I developed the following formula,
based on the formula that doubling the f-stop value produces a 2 stop (4x)
decrease in exposure.

t' = shutter given in table.  a' = aperture in table.
t'' = shutter desired.  a'' = aperture desired.

(( t' /  t''  x a' ) x a') ^ -.5  OR  ( t' / t'' x a' ^2 ) ^ -.5
That is, the square root of, the shutter ratio times the aperture (f stop)
squared.

If t'' = 1/150, then, to shoot ISO 64 in full sun requires:

  Given:   t' = 125 and a' = 16
 ( 125/150 x 16^2 ) ^ -.5 = (5/6 x 256) ^ -.5  =  (213.33) ^ -.5  =  ~ 14.6

If t'' = 1/100, then... full sun requires:
(5/4 x 256) ^ -.5 = ~ 17.9

If you're picky, you can calculate a conversion table.  Or, you can simply
open 1/5 stop (for t=1/150) or close 1/4 stop (for t=1/100), from the values
given for t=1/125.  Of course, with the itsy-bitsy dial on a Realist, I can't
be real precise.  But thank goodness, I don't have to be.

Before I get jumped by E6 (Sensia) lovers, I used KR64 for the archival
advantage.  And other, more subjective reasons.

Regards,
Robert Linnstaedt


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