NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: shooting indoors
From: George Huxtable
Date: 2010 Mar 24, 17:45 -0000
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. Yes |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. Negligible. | 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. Yes |To the observer, will that result in a slight angular error in observation? | I believe the answer to that to be yes, albeit small. Zero. | 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. Agree George. 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.