RE: Stupid Questions

[Theo Benson <TBenson@xxxxxxxxxxxxx>]

> Would two medium red filters add up to a dark red
> filter?  I'm thinking it doesn't because it would not change the
> wavelength
> of the light that got through, just the amount.
	[Theo Benson]  You're partly right.  No filter is absolutely sharp.
Each "leaks" some light at other than the desired wavelength(s).  The leaked
light is generally small, and is made smaller by putting another identical
filter in line.  You don't change the wavelength(s) passed -- only the
efficiency with which other wavelengths are stopped.
> But, for the purpose of IR
> effects on film, will 2 filters block the low wavelengths more and let the
> same amount of IR light through?  
	[Theo Benson]  It should eliminate more of the lower wavelengths,
but since no filter is perfect, it will also cut some of the IR, too.
> Could I do this with a ND filter or a
> polarizer. Kind of a poor man's 87?
	[Theo Benson]  The polarizer will cut some of the blue, but the ND
won't help!  It's called a neutral density (ND) filter because it's supposed
to cut light regardless of its wavelength, thus not imparting a color shift.
It will only make things darker.  I don't have knowledge of how ND's act in
IR, though.  They may be a little squirrely (after all, everything else is
in IR).  Any ideas, George?

	I once tried an experiment using a red and blue filter.  IR film is
sensitive to both, and my thought was that if I cut out some of the near-red
(with the blue filter) and the blue (with a red filter) then I'd have more
IR and less visible.  It seemed to work, but I had too much vignetting due
to my lens/filter adapters.  You might want to try this out (like an 80A and
a #25).  Let us know what you find out!

	Best regards,
	Theo
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