Urban legends on the horizon?

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

>From the newsgroup rec.photo.equipment.35mm

Mr Rosen is a helpful contributor in Usenet, but isn't shy with
theoretical IR problems, I think he is also the person responsible for
the 'film window fogging' warning in the IR FAQ, although he
only did so out of mere precaution (I asked him).
Would be nice if anyone in this group could deny the below mentioned 
problems with the EOS-1 before they grow into a new urban legend. 
Theoretical they could exist, but I more than strongly doubt it.


xxxxxxxxx
FROM: David Rosen <golem@xxxxxxxxxxxxxxxxx>
SUBJECT: Re: Shooting infared with EOS 1n & 1n RS possible??
DATE: 22 Mar 1998 16:35:02 GMT
ORGANIZATION: Logical Net

SCox222 <scox222@xxxxxxx> wrote:
> Heard EOS 1n systems have some sort of infared DX code reading mechanism, so
> you can't use any infared film with these cameras because of high sensitivity
> of this film...
> Any knowledge out there on this subject?
> Scott

      I can't find any signs of IR sensors
      in the DX zone of my 1n [RS version].
      If it worries you, cover the area with
      black tape.  DX is useless with a film
      that has no rated speed, anyway !

      RE: IR exposure:  You may find that
      exposure and filter setups that work
      in a 1n [non-RS], or in any "normal"
      camera, will not transfer to the RS
      merely by a 2/3 EV adjustment.  The
      pellicle may have a rather different
      ratio of passthru/reflectance for IR
      than for the visible spectrum.  Once
      you determine what the IR loss for
      the pellicle is, it would remain the
      same if your filter is passing only 
      IR [87 series and other "black" IR
      filters].  If your filter passes a
      blend of IR and visible light, then
      the loss at the pellicle would vary
      as the blend varies [assuming any
      noticeable difference of IR pass vs
      visible spectrum pass attributable 
      to the pellicle].

      What is in question here is the
      "cold mirror" principle and whether 
      the EOS pellicle are cold mirrors
      to any important degree.

      Finally, one more possible headache
      for using IR in an RS [or RT].  If 
      using a "black" filter, exposure is
      set for basically IR only, with no
      visible spectum exposure.  IR film
      is more sensitive to visible than IR,
      but the filter blocks visible ....
      except for all the visible spectrum 
      light that sneaks into the eyepiece
      and through the screen to the dark
      box where the shutter is dosing out
      an IR-only exposure to the film, or
      at least that would be what's going
      on of the screen weren't glowing 
      with visible spectrum leakage from
      the eyepiece !

      Screen leakage is a tiny leak when
      compared to a full visible spectrum
      image [for normal film] projecting
      filmward from the rear of the lens.
      However, compared to the IR image
      delivered through a black filter,
      the screen leakage is signiifcant,
      especially since IR film is more
      sensitive to the visible spectrum
      than to IR.  Your mileage may vary.
    
      Regards,
      - dr



- -- 
Bye,

Willem-Jan Markerink


      The desire to understand 
is sometimes far less intelligent than
     the inability to understand


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