Hi Bill.I suppose i am making this a bit more
complicated that it should be,and i have read the
replies by you and Aaron,its the ISO thing that
still has me puzzled.If the red #25 only corrects for 1 stop
should i 'pad' the ISO to 100,even though i'm not "metering"
and tell the lab such.Or better yet,what are you setting
the H1a ?? on and what are you developing for.
As far as keeping track of exposures etc,i will,for future ref etc.
Dave
---- Begin Original Message ----
From: "Bill D. Casselberry" <[EMAIL PROTECTED]>
Sent: Thu, 14 Mar 2002 07:12:22 -0800
To: [EMAIL PROTECTED]
Subject: Re:photographing w/ IR - General Info
�Bill(snipped) wrote:
If you set at 50asa, the #25 will drop you down to
6asa, overexposing heavily. You would need to "pad"
the ASA level to retain the desired 50asa - but!
the #25 has only a one stop effect in the IR range,
as the non-filtered EV recommendation is f16@1/125th
... a quote from the info sheet:
�"Exact speed recommendations are not possible because the
ratio of infrared to visible radiation is variable and be-
cause photoelectric meters are calibrated only for visible
radiaiton."
Again, f11@1/125th takes the #25 filter into account
and eliminates any need for further metering. Meters
are "tuned" to the normal visible light wavelengths,
anyway, so it's a matter of calibrating your meter
to estimate an IR exposure from what it reads for
visible light. This is a nuisance and a waste of film.
Plus, the IR component isn't constant relative to visible
due to atmospheric conditions and such, so you end up w/
"seat of the pants" practical experience as your best
guide, anyway.
aside: I goofed - my IR pass filter mentioned in a previous
post is a #87, not #89 as I typed. It requires one more stop
than the #25 red.
Well, if you are curious, I suppose you could make
measurements, take notes and compare the meter readings
to your results from the simpler manual approach. You will
the *know* that either they are the same or are of no
relative value and can proceed from there w/ further IR work.
Pentax User
Stouffville Ontario Canada
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