13. Atmospheric Pressure and Refraction 
Pressure changes will affect the focus of a camera. As the atmospheric pressure is 
reduced, the apparent refractive indices of the glasses in the lens increase, so that the 
plane of best focus becomes closer to the lens. In a survey camera there is no attempt 
made to re-focus in flight, so that the image location is determined very largely by the 
geometry of the camera body. One would expect both the principal distance and 
the distortion to be little affected by even large changes in pressure, although the 
precision of measurement may be reduced because of loss of resolution. 
Atmospheric refraction, i.e. deviation of the light in passing through the 
atmosphere from the ground to the camera, is a well-known phenomenon, dealt 
with in detail by Ekelund [181 at our 1956 Congress. More recently Angus-Leppan [19) 
has reported that refraction is far from constant, varying with temperature gradient 
in the lower atmosphere, and he has suggested various methods of computing it from 
meteorological information. Barrow [20] has considered the form of correction to be 
applied in the presence of both Earth curvature and refraction. He has deduced 
equations, convenient for digital computation, for the evaluation of both effects 
and shows that for great heights one cannot separately correct for curvature and 
refraction since these are mixed terms. The refraction correction is small, perhaps 
10 p is a usual figure, but it is quite beyond the control of the photographer, so that 
uncertainties in its magnitude may represent one limit in possible accuracy. Of 
course it becomes much more serious in ultra wide-angle photography. 
There are further possibilities of atmospheric refraction in the airstream close 
to the aircraft, and in the air between the camera and the aircraft window. As air 
craft go faster, so the disturbance in the air around the fuselage increases, turbulence 
sets in and towards Mach 1 shock waves begin to appear and may cover the window. 
Calculation shows that these effects are likely to be small, and will appear first as a 
deterioration of image quality. Nielson [21] has given a very interesting account of 
these and other problems likely to be met in survey photography from supersonic 
aircraft. Only military aircraft are likely to be affected in the near future, but if the 
day does come when supersonic aircraft are in everyday use for survey, then we will 
apparently have to contend with image displacements of 20 p or more from refraction 
alone. 
The use of hot air between the camera lens and the window to reduce misting 
can affect both image quality and location. The hot air produces “shimmering” 
effects which are most spectacular if the image is viewed by a microscope and can 
certainly lead to movements of some few microns in the focal plane. For this reason 
it has often been suggested that the hot air should be switched off during the actual 
photographic run. 
Camera windows are a common source of location errors. Since they are part 
of the optical system they need to be made to the same high standard as the camera 
filter. This is difficult because they can be very large pieces of glass and even if 
these surfaces are polished flat, inhomogeneity in the glass itself can occur. More 
over, there are no simple methods of testing these large windows. Interferometry is 
perhaps the best method, but the fringe pattern is not always easy to interpret and 
equipment of the right size is fairly expensive. Measurement of deviations on a 
calibrating table is as laborious as the calibration of a complete camera and is quite 
likely to miss small areas of poor quality. Warm air must be blown over the windows 
to prevent condensation on the inner surface, so that quite considerable temperature 
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