Re: more IR questions

["Willem-Jan Markerink" <w.j.markerink@xxxxx>]

[Originally PhotoForum, but copied to the IR-list as well, since it
might be of interest to other people:]

On  6 Oct 96 at 1:11, phillip geller wrote:

> I am looking for info on IR filters.
> Current work involves the use of Konica 750 IR.
> An 87 filter cuts out way too much light to be useful with
> this film because of it's reduced speed and sensitivity (or
> lack of it.) What I need to know:
> Is there a deep red Wratten filter that one can see
> through well enough to see the effect on foliage?

Yes and no.
Yes, a filter color which gives a good start of the Wood effect does 
exist. 
Yes, it once was a Wratten specification as well, #88.
No, it is not listed in the Kodak Wratten program anymore (very few IR 
colors are currently!)
And no, you can't use it this way in front of a lens....this color is 
on the breach of being opaque, only with a good shaded bare eye and a 
bright sunny day you can see anything at all.

The full range of modern Wratten colors is this:
25
29
70
89B
88A
87
87C
87B
87A

The unofficial #88 is an intermediate between the deep red 89B and 
the opaque #88A (only faint contours of buildings and trees are 
visible, no color impression at all).

How I found out about these? Half by looking at various odd filter
colors world wide, from various manufacturers, and half with a
faulty batch of 87C, which in hindsight appears to be an #88
equivalent (didn't use a spectrophotoscope, but I own both the
lighter #89B and darker #88A, and found the once official
intermediate #88 in a Wratten catalogue from 1928.

All this said: the darkest color convenient on a lens is #89B, albeit 
with teles and working in the shade (weddings!) I would tend to #70 
for ease of composition and focus. But with wide angles and 
hyperfocal distance #89B is fine. (note that one can use AF even 
through an opaque IR filter!)

If you want to learn more about the various IR filters, see my 
Wratten listing and conversion table to Schott, B+W, Heliopan and 
Hoya on my homepage:

http://www.a1.nl/phomepag/markerink/mainpage.htm

PS: the best preview of the Wood effect is obtained by using a cheap
night vision scope (1st Gen is fine) together with an opaque IR
filter (and a lens with aperture to keep the scope from burning in).
You'd be amazed about the effect!
I once also had the opportunity to test a 2nd Gen scope with an #87A 
equivalent....as I recall, the effect was even more stunning, but 
part of it could be related to the extreme bright sunny day. Hope to 
test it with a 3rd Gen scope once as well....


--
Bye,

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                     _/_/_/  



      The desire to understand 
is sometimes far less intelligent than
     the inability to understand


<w.j.markerink@xxxxx>
[note: 'a-one' & 'en-el'!]

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End of INFRARED-PHOTOGRAPHY Digest 86
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