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From: kanner@tymix.UUCP (Herb Kanner)
Newsgroups: net.rec.photo
Subject: Re: filters
Message-ID: <598@tymix.UUCP>
Date: Wed, 18-Dec-85 21:53:50 EST
Article-I.D.: tymix.598
Posted: Wed Dec 18 21:53:50 1985
Date-Received: Fri, 20-Dec-85 05:14:29 EST
References: <307@tellab3.UUCP> <814@sfmag.UUCP>
Reply-To: kanner@tymix.UUCP (Herb Kanner)
Organization: Tymnet Inc., Cupertino CA
Lines: 38

In article <814@sfmag.UUCP> howard@sfmag.UUCP (H.M.Moskovitz) writes:
>> 
>> Since the subject has come up, what books are reccomended for learning
>> about filter usage?
>> 
>> Also, when using a filter with a through the lense meter, is it necessary
>> to adjust the exposure by the filter's f-stop rating?  Does altering the
>> spectrum of light being metered confuse the metering system?
>> 
>
>Since the TTL metering system measures light intensity as it hits
>the film plane, adding filters is compensated accordingly. Metering
>systems do not know anything about color so this has no effect.
>
>-- 
>
>
>---------------------------------------------------------------------
>					Howard Moskovitz
>					AT&T Info. Systems
>					attunix!howard

Not strictly true.  True for most practical purposes.  If the spectral
response of the meter differs sufficiently from that of the film, there
will be a net change in effective film exposure when the filter is used.
This can be checked if the film and/or filter manufacturer has supplied a
filter factor and if the camera has read-outs telling what it is doing.
See what the readings are before and after attaching the filter and see if
the reading change is consistent with the published filter factor.

I certainly remember that for certain really extreme filters, e.g. red,
meant to be used with B & W film, that the factor varied quite a bit from
one film to another.  In fact, you sure as hell wouldn't use a red filter
with orthochromatic film :-)
-- 
Herb Kanner
Tymnet, Inc.
...!hplabs!oliveb!tymix!kanner