70mm vs. 120/220

[jim Henderson <74023.723@xxxxxxxxxxxxxx>]

Willem-Jan wrote on July 22:

 >> BTW2: I have been pondering about the difference between 120/220 and
70mm....since 70mm cartridges use similar felt traps (do they at
 all?) as 35mm, they also require dark loading. Given that, I can't
 see how an IR-tight 120/220 back would be any different than a 70mm
 back. The argument of the IR-leaking paper backing only holds when
 exposing the rolls to daylight, and one should be able to prevent
 that. So the question why no HIE/IE in 120/220 exists remains....<<

 70mm film's width is actually 70mm, as compared with 62mm for 120/220.  The
actual image area is 57mm for 70mm and 120/220.  That extra 8mm in width is
taken up by the sprocket holes, and it effectively prevents one from loading
long roll 70mm onto plastic 120 cores.

 The paper backing used for 120 film is transparent to IR and would fog the
film if used to back a core of IR.  There is no backing at all on 220 film,
and the results there would truly be disasterous.

 BTW 70mm is available in 150-ft rolls for approx. $172(US).  That breaks down
to about $0.34/shot.  the metal magazines are still sold by Kodak in packages
of 8 at $8.00 each.  One magazine holds about 65 exposures.  Problems with 70mm
backs are numerous, one being the felt lining in the camera magazine itself
must be in good shape, otherwise light leaks will occur.

 >>... I would also
 like to mention the fact that one can load HIE under fluorescent
 light. I am still not sure why, since HIE doesn't show a sensitivity
 gap in the green spectrum, but apparently this works. Perhaps the
 effect of the film carrier acting as a fiber optic/and or the leaking
 felt trap does not occur in any other spectrum than IR.
 Perhaps other monochromatic/non-IR light sources would do as well.
 Any comments? <<

 There is very little infrared radiation emitted from fluorescent lighting,
because they are a non-continuous light source; however, there is enough IR to
fog 4x5 film processed beneath fluorescent lighting fixtures even after they
have been turned off.  The tubes continue to emit very minute amounts which
over a period in excess of 7 or 8 minutes can adversely affect the film.

 Jim Henderson


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Topic No. 3