30 jun 2007 kl. 21.49 skrev ADavidhazy:
As I shared earlier I did some "reverse engineering" and seem to
have come
up with a solution that corroborates the EV to FC or Lux tables
found on
several Gossen light meters or in the Sekonic instruction manuals.
But I
am not sure if I used correct thinking. The relationship I have is
based
on some reading from an exchange in German on a discussion group.
The number of Foot-Candles falling on a scene is equal to:
FC = antilog of < (EV x log2) - log (ASA / C) >
where: EV measured at some specific ASA film speed
ASA is the film speed
C is a constant, for Gossen it seems to be 24.5 and for
Sekonic 23.25
Wow, this is getting COMPLICATED... :-)
Let´s start with basics: For a given f/stop and film sensitivity,
and disregarding reciprocity errors, correct exposure time is
inversely proportional to scene illuminance, right?
If true, the second part of Ansel´s formula has the correct
mathematical form: "Then, the luminance (in candles per square foot)
of a surface read by a correctly calibrated exposure meter is just
the reciprocal of the shutter speed for the above f/stop and film
speed".
What´s left is to find the proportionality factor, and that can be
made to be equal to one by choosing the f/stop and ASA in a suitable
manner.
That´s what the first part of Ansel´s formula claims to do: "First
compute the f/stop value corresponding to the square root of the film
speed in ASA (e.g., for ASA 125, you get f/11)".
I can´t say whether AA did get it right numerically, but again, at
least his solution has the correct mathematical form: since you´ll
have to stop down one step if you multiply the ASA value by 2, and
one f/stop step is 1.4, the square root of 2, AA will yield the same
basic meter setting starting with any ASA value you want, just like
it should.
I guess most of you still have an old exposure meter with circular
computing discs lying somewhere. Get it out and play around with the
scales; you´ll soon see that the relationship between ASA and f/stop
is exactly like AA says.
So what he´s doing in the first part of the formula is just to find a
certain point on the meter scale. Then, for anything you meter, the
inverse of the shutter speed read at that point is (claimed to be)
the illuminance value in foot-candles.
Makes sense? Yes, the light measuring part of any reflected exposure
meter is just an illuminance meter, but witthout a proper footcandle
scale. What AA does here is just giving a method to recontruct such
a scale.
Phew! Now it´s time for some good Scots malt.....
Slainthe mhor!
Per Öfverbeck
http://ofverbeck.se
"In a world without walls or fences, who needs Windows or Gates?"
