Re: 4x5 halo (was: 4x5 camera suggestions

[eml@xxxxxxxxx]

> 
> On Fri, 14 Feb 1997 00:36:00 GMT, you wrote:
> 
> |I pondered about that some time ago....assuming it lacks the=20
> |anti-halo layer for the same spectral reasons as 35mm (and 70mm?=20
> 
> Spectral reasons?  You mean there's a *reason* it lacks an
> antihalation--aside from the "status quo at all cost" attitude of U.S.
> Government procurement policies?
> 
I have often speculated on why no antihalation layer was used, especially
when the terrible effects on resolution are considered.  Now, Kodak
may be ponderous, bureaucratic, and unresponsive, but they're not dumb.
There MUST be some reason!

Finally, it hit me!  The film is made primarily for government contract
use, and it has not been changed in years, even though halation wrecks
the film's resolution.  Unless, that is, the film is intended to be exposed
THROUGH THE BASE!  Exposing film through the base is not uncommon in some
specialized applicarions. Then an "antireflection" layer would be preferable!

Now, think about your basic photochemistry.  Remember that a sensitizing
dye ABSORBS the wavelengths of light to which it makes the film 
sensitive!  Result?  HIE, when used with a red filter and when loaded
"backwards" into the camera, with the emulsion against the pressure
plate, effectively HAS an antihalation layer!  That layer is the 
sensitizing-dye tinted emulsion, which supresses the REFLECTION
of the very light that it absorbs!

Now I know that there are those out there who are saying that Ed's
not playing with a full deck, but I submit that it's the only plausible
reason that I've been able to come up with in all the many years that I
have volubly cursed HIE. 

Any comments?

Ed Lukacs


-- 
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