[pure-silver] Re: estimating filter factor: gray card?

  • From: BertS <aasainz@xxxxxxxxxxxxx>
  • To: pure-silver@xxxxxxxxxxxxx
  • Date: Tue, 03 Oct 2006 11:03:29 -0400

BOB KISS wrote: 

The relationship between f stops and factor is NOT a square root.  As
demonstrated in my previous e-mail:   " the square root of a factor of 8 is
2.8 but we all know that a factor of 8 equals exactly 3 stops (2X2X2)."
The relationship is exponential factors of 2 NOT the square root which is
ONE SPECIFIC exponential factor of 2.
Therefore it DOES happen to correspond to the square root in some SPECIFIC
cases but not all.
                CHEERS!
                        BOB


The relationship between f-stops and filter factor is a square root. Just divide the metered f-stop by the square root of the filter factor and you will get the f-stop.

If you meter a scene and the meter indicates that f/16 is the correct exposure and then you put a filter with a filter factor of 8 then the correct f-stop is f/5.6. 5.6 is 16 divided by square root of 8 (2.8).

If you change the filter to one that has a filter factor of 4 then the correct exposure is f/8. 8 is 16 divided by square root of 4 (2).
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