----- 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.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.
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
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