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    Re: shooting indoors
    From: George Huxtable
    Date: 2010 Mar 24, 17:45 -0000

    I will interpose my comments into Brad's posting-
    | Sliding glass doors are typically of double pane construction, with
    multiple sheets of glass between
    | the inside and the outside.
     |Then we will need to consider the parallelism of each sheet of glass to
    | the others.
    No; no effect.
    |Not all double pane windows are air filled, some are gas filled to
    increase the insulation
    | value.  We would need to consider the index of refraction for that gas,
    should it exist, compared to
    | the nominal value for the atmosphere.
    | The stiffness of the glass also comes into play, independent of the age
    of the glass.  As the extent
    | of the sheet of glass gets larger, the propensity of the glass to bend
    and displace under self and wind
    | loading will be increased.  How significant this is will be a function of
    the thickness of the glass, the
    | angles to vertical it is installed and the like.  Just because it is
    installed in a house, you cannot assume
    | that the walls are perfectly vertical. Non-verticality will cause sag and
    deflection, which will vary with
    | location within that pane of glass.  Is the frame that the glass is
    installed in perfectly flat and planar?
    | If not, then that glass will have bends in it, to comply with the greater
    stiffness of the frame.
    Negligible effect.
    | Assuming perfect parallelism with a singular sheet of glass (or shade),
    when we place that glass in the
    | optical path, it will cause a shift (displacement) of the beam as a
    function of the thickness of the glass.
    | First the beam will shift in angle as it enters the glass, then when the
    beam encounters the opposite and
    | parallel surface, the original angle of the beam will be recovered.  The
    parallel shift of the beam is a
    | function of the thickness of the glass.
    |To the observer, will that result in a slight angular error in
    | I believe the answer to that to be yes, albeit small.
    | How significant is all of this?  I do believe that the dip short to the
    opposite lakeshore will be a
    | far larger effect than any prismatic effect.
    contact George Huxtable, at  george@hux.me.uk
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.

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