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

I give up.

 Please check my website: http://www.bobkiss.com/

"Live as if you are going to die tomorrow.  Learn as if you are going to
live forever".  Mahatma Gandhi

-----Original Message-----
From: pure-silver-bounce@xxxxxxxxxxxxx
[mailto:pure-silver-bounce@xxxxxxxxxxxxx]On Behalf Of BertS
Sent: Tuesday, October 03, 2006 11:03 AM
To: pure-silver@xxxxxxxxxxxxx
Subject: [pure-silver] Re: estimating filter factor: gray card?

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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