Re: Digest submission

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

On 20 Aug 96 at 9:11, David L DuPuy wrote:

> Sorry if I've missed this question... having just subscribed to this 
> digest (which looks useful).

It sure is!
[note: even the listowner learns a lot, so it must be good....:-))]
 
> I'm debating whether to try a little 35mm vs. 120 IR (first time IR).  An 
> appropriate filter is the next question.  So...

Kodak only comes in 35mm, not in 120. Konica does, but does not give 
the same unwordly effects.
 
> (a) does an 87, 87C, or 89 filter produce more of an IR effect than 
> the conventional 25 filter?  I presume these filters have a slightly 
> longer wavelength cutoff, admitting less of the orange wavelengths?

The range from light to dark is:
25
29
70
89B
88A
87
87C
87B
87A

The real visible cut off is #87 IMO, all lighter filters can be
looked through with some effort. Note that Kodak recommends a 29 to
start with.
And yes, you do get a different picture, the darker the filter, the
more unwordly the results. 
Not easy to explain though, since the Wood effect of foliage
starts(peaks?) at 680nm, so one would not expect any more noticeable
gain by using darker filters than that. 89B has a 50% cut off at
695nm btw, 87C at 780nm [for more spec's and a conversion table see my 
homepage http://www.a1.nl/phomepag/markerink/mainpage.htm].
One possible explanation I found in Guenter Spitzings UV/IR book: his 
explanation for the contrast of HIE is that the sky gets more and 
more transmissive into the IR-spectrum. And shadow contrast is mainly 
related to sky reflections: on an overcast day, you don't have 
shadows because the light is so diffuse. Even *if* the sun would 
point its beams directly at us, the sky/cloud reflections would act 
as a shadow light. So even on a sunny clear day, some light is still 
reflected by the sky, appearing as blue to the eye [have a nice 
posting from sci.optics about 'why the sky is blue', will put that 
online soon!]. The more you enter the IR spectrum, the less light is 
reflected, making contrasts unwordly, almost as moonlit landscapes. 

 
> (b) I've found an 87 filter (Calumet) in 3x3 inch format, polyester, 
> which I'm guessing could be used in a Cokin filter holder for a 
> Hasselblad.  Anybody know of any reason this wouldn't work for a few 
> trial IR rolls?  The price is right:  $12.50  :-)  

Yep, this is the polyester foil made by Lee. Thin foil, so you need
a gel filter holder (or tape it to the lens of course, and even
handholding is easier as with true gelatine filters (gel's melt in
sweaty hands....:-)) 


> (c) Are exposures likely to be more tricky with these filters, 
> compared to a #25? 

The only tricky part is that most light meters (especially onboard
TTL meters) show some sort of insensitity to IR, and that the
relative amount of IR vs visible light can change a lot. If either of 
those problems would seize to exist, there wouldn't be a problem at all....

There are only two solutions: experience (estimate 10 films for 
experiments before you do anything serious) and bracketing....:-))

 
--
Bye,

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The difference 
between men and boys
is the price of their toys

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

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Topic No. 9