[pure-silver] Re: filter factor
- From: "BOB KISS" <bobkiss@xxxxxxxxxxxxx>
- To: <pure-silver@xxxxxxxxxxxxx>
- Date: Sat, 30 Sep 2006 13:04:51 -0400
One small correction to Richard's information. The square root of the
factor is not entirely accurate; you either:
1) Take the log base 2 of the factor or
2) You divide the log base 10 of the factor by .3
3) E.G. 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). Log base 10 of 8 = .90 divided
by .3 = 3
4) The square root is close and, while not exact, will work to within 1/4 of
a stop.
5) It all depends on how close you need to be.
CHEERS!
BOB
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 Richard Knoppow
Sent: Friday, September 29, 2006 4:57 PM
To: pure-silver@xxxxxxxxxxxxx
Subject: [pure-silver] Re: filter factor
----- Original Message -----
From: "Dave Valvo" <dvalvo@xxxxxxxxxxxxxxxx>
To: <pure-silver@xxxxxxxxxxxxx>
Sent: Friday, September 29, 2006 1:47 PM
Subject: [pure-silver] Re: filter factor
> The correct way to determine a filter factor is to
> photograph an 18% grey card with and with out the filter.
> Then measure the density of each image and determine the
> LogE difference from the D-logE curve you would get from
> the process because how you process is important. Now who
> is going to do that? Meter reading is Kentucky windage.
> Works for me as I get older.
>
> Dave
>
> ----- Original Message -----
> From: "Koch, Gerald" <gkoch02@xxxxxxxxxx>
> To: <pure-silver@xxxxxxxxxxxxx>
> Sent: Friday, September 29, 2006 3:56 PM
> Subject: [pure-silver] Re: filter factor
>
There is more to it than this: filter factors are based
on some assumptions about the light source and scene. Real
scenes may not fit the assumptions. For instance, a
medium-red filter (Wratten A or No.25) has a very large
filter factor but does not attenuate red light much. Suppose
one is photographing a predominantly red subject, an
exposure made using the filter factor (8 I think) may result
in considerable overexposure. The milder the filter the less
important this becomes. No.8 (K-2) filters do not attenuate
much of any color other than far blue so the filter factor
as given will probably fit most scenes.
Filter factors are nearly given as the ratio of increase
of exposure. This is directly translatable as exposure time
but the square root must be used to calculate stops. i.e., a
factor of 1.5 is one and one-half times increase in exposure
time but only about a quarter stop increase.
---
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx
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