Re: [Leica] Filter Factors

[Henning Wulff <henningw@archiphoto.com>]

At 11:47 AM -0700 9/18/01, Mark Rabiner wrote:
>Shel Belinkoff wrote:
>>
>>  Everything I've read, including the information sheet packed
>>  with my new filters, indicates that the filter factor for a Medium
>>  Yellow filter (Y2) is 2X, or 1 stop.  So, while checking my gear
>>  this morning I decided to check the needed exposure compensation using
>>  two Pentax spotmeters, the digital Zone VI modified meter and the
>>  standard, but calibrated, Spotmeter V.  Both showed that the needed
>>  exposure compensation would be about 1/3 stop more with the filter
>>  than without.  I metered off a white truck, the overcast sky, and a
>>  building. Always about 1/3 stop. Off some other colors there was a
>>  slight
>>  difference in the readings between the Zone VI meter and the standard
>>  meter, which is to be expected, but in no circumstance did either of
>>  the meters indicate the need for an additional full stop with the Y2
>>  filter.
>>
>>  I suspect this discrepancy may be due to the sky being overcast and
>>  there being less blue light for the filter to absorb.
>>
>>  Since I'll be doing some work with hand-held meters and manual
>>  cameras, can we discuss this situation a bit.  One thought that
>>  crossed my mind was to make the exposure readings off a grey card,
>>  which I'll try later when I'm outside and away from the computer.  Any
>>  thoughts on that idea.
>>  --
>>  Shel Belinkoff
>>  mailto:belinkoff@earthlink.net
>>  --
>>  To unsubscribe, see http://mejac.palo-alto.ca.us/leica-users/unsub.html
>
>Instead of pointing it at the sky, Shel, try pointing at a grey wall so the
>color of the object you are metering is not an effect.
>An extreme example is if you meter green foliage or green anything 
>though a red filter.
>Well the red aborbs all the green so you'd be over exposing like a 
>sun of a gun.
>You've got to meter off a neutral non color Get the filter factor I'd think.
>
>
>Mark Rabiner
>
>Portland, Oregon
>USA
>
>http://www.rabiner.cncoffice.com/
>--
>To unsubscribe, see http://mejac.palo-alto.ca.us/leica-users/unsub.html

Since film hardly ever has the same spectral response as the light 
meter's cell, the best thing is to start with the stated filter 
factor and test. With the darker coloured filters, the meters usually 
will indicate a reading that results in underexposure.

For example, a 25 Red usually requires a filter factor of 3 stops 
with most panchromatic films, or 8x the exposure. Most meters only 
indicate a factor of 1.5 to 2 stops when measuring through them. So 
when I want to do quick and dirty metering with a red 25 on the lens, 
I set the film speed to half (ie., HP5+ to 320 instead of my usual 
640).

- -- 
    *            Henning J. Wulff
   /|\      Wulff Photography & Design
  /###\   mailto:henningw@archiphoto.com
  |[ ]|     http://www.archiphoto.com
- --
To unsubscribe, see http://mejac.palo-alto.ca.us/leica-users/unsub.html