Re: why using #87 and #87C filters?

[Joshua_Putnam <josh@xxxxxxxxxxxx>]

Jaap Los <imagefotografie@xxxxxx> writes:

>1. Sharpness is less.

The first roll of HIE that I shot with a Lee 87 between the rails
of my OM-1 wasn't nearly as well focused as the rolls I had shot
using a no.25 in front of the lens.  

After a few experiments I realized that it was because I was
used to focusing through the lens with the no.25, where the
visible light is red only, and gives a focus close to that of
IR.  With the filter between the rails, my visible focus was
further from the IR focus, since I was seeing all the blue in
the spectrum, too.  Once I remembered to adjust the visible
focus to the IR focus mark on the lens, the problem went away,
and my shots with the between-the-rails filter were as sharp as
the shots with a standard filter in front of the lens.

>2. Contrast has completely gone. 
>3. Parts in the shade do not reflect IR so they appear almost blanc on
>the negative. Shots made at overcast sky are useless.

These two sound like perhaps you should try a bit slower
exposure index -- I get good results with an 87 even on fairly
cloudy days, bracketing down from EI 25 or so.

- -- 

 Josh@xxxxxxxxxxxx  is  Joshua Putnam / P.O. Box 13220 / Burton, WA 98013
                       "My other bike is a car."                   
                     http://www.wolfenet.com/~josh/
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