[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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