Re: IR Speed -- WWW info

[v.bromfield@xxxxxxxxxx (Vaughan Bromfield)]

At 9:10 PM 18/7/96, Robert Long wrote:

>read in the Kodak instructions, which specify that you should set a
>separate meter to ISO 80 if you're using no filter or to ISO 50 if
>you're using a Wratten 25.  Or am I somehow misreading these
>instructions?  I've been following the instructions as I understand
>them and getting negatives that while obviously generously exposed
>(and sometimes infected with more halo than I'd like) are weak enough
>in the shadow areas that I'd hesitate to set my SLRs for ISO 400, as
>W-J suggests, seconding the above.
>
>BL

=46olks

Here is an extrat of an IR web article that got me started in IR, and it
makes sense to me. The URLs for all three articles (I haven't read the
third yet) are below, with the extracts from the first article on exposure
following.

If you can find the time I suggest you read at least the first two of the
articles below, they are well written and quite information rich.

----------

The URLs are:

IR part 1
http://www.apogeephoto.com/mag1-1/mag1-IR.html

IR part 2:
http://www.apogeephoto.com/mag1-2/mag1-2ir.shtml

IR part 3
http://www.apogeephoto.com/mag1-4/mag1-4mfir.shtml

----

Infrared Photography
Debunking Myths
by Michael Fulks

(IR part 1 above)

"... My first roll of Kodak Infrared Film was a disaster. At least I know
that now. I loaded the film in total darkness, set my camera's through the
lens meter to ISO 80 as was suggested in the literature, put on a red 25
filter, and quickly proceeded to blow the whole roll. I developed it in D76
and when it dried I had a strip of film with curiously clear film base and
extremely dense image areas. ... After my initial elation, however, doubt
began to seep in. Surely this can't be right. It should not take two
minutes exposure on my enlarger to get a viewable print. ... I loaded up
again, and decided to do a calibration test. I chose a fairly
representative landscape and shot a series of frames from ISO 80 to 2000.
After I developed the roll, I looked at the strips and this time there were
actually frames that looked like real negatives.

Next I went to do a contact sheet. As I would a normal film I found a time
where the film base first appeared as black on the test strip, and then
proceeded to do a contact sheet based on that time. If you haven't done
this before, it is based on the theory that a black in your scene should
equal or approximate the film base. If a black in the scene is denser, then
the film is overexposed. I knew the sky  should be black, so I used that as
my black. The result was that ISO from 400 to 1600 fell within the ball
park. I printed these as regular 4X5 prints and then evaluated the damage.

Now I was close. All that was lacking was the contrast I thought it needed.
So the next roll I developed 33% longer, and this time I had very
acceptable images, in terms of contrast range, grain and sharpness. I have
since experimented with other developers all aimed at getting the best
tonal range and sharpness. I am now using D-19 for Kodak's film which
believe it or not has given me great tonal range and sharp crisp grain. (We
will talk about Konica film next time.) [see part two... Vaughan]

Why was there such a difference? Kodak, after all, said use an ISO of 80,
but I was getting pretty prints at 800! At first I thought about altitude.
We are at 8200+ feet above sea level. During midday, we use the sunny 22
rule. That is, if your film is rated at ISO 100, then in bright sun you
should set your shutter at 100-125 and your aperture at 22, not 16 as you
might have learned. But that would only account for part of the difference.
What was the rest?

At about this time I bought my first hand-held incident meter. Anxious to
shoot some more film I set it to ISO 400 and went out and shot a roll. When
I developed it, I was in for a shock. The film was almost clear. It was
grossly underexposed. I checked out my equipment but nothing was wrong. So
I shot another roll exactly the same, and it was still underexposed. What
was wrong?

And then it occurred to me. Light meters are calibrated for visible light.
I knew that. When I put on the red filter it increased my exposure by 2
stops. This was beginning to make sense. Maybe to infrared light the red
filter is transparent. Was the red filter fooling the meter into
overexposing 2 stops when it didn't need it?

Subsequent tests supported this. In open light, a  hand-held meter set at
ISO100 did produce the right exposure, while it was 400 through the lens.
That mystery was solved, and it proved to be the same when I began to use
Konica. (ISO 12-25 with handheld meter, 50-100 thru the lens)

Did I solve the problem once and for all? No. It is not always possible to
accurately predict from the amount of visible light as measured by the
meters what was available in the IR range. It is always necessary to do
some bracketing. Also according to John Wilkerson in his articles that
appeared Darkroom & Creative Camera Techniques on the subject, both films
react differently from ordinary film to over exposure. He found that as
exposure increases tonal values in the zone VIII to IX are pushed into the
zone X area much faster then ordinary B/W film, thereby increasing apparent
contrast, but causing a loss of detail in zone VIII and above. Thus while a
=BD stop plus or minus of bracketing might be used as a further tool in
producing a particular look, overexposure generally should be avoided.

-----------

Sorry for the long post, but I hope this helps to answer your questions.

Vaughan

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| Vaughan Bromfield                  |                         |
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