There are a number of factors that can cause the above assumptions to be false or partially incorrect:
• particles and turbulence in the air between the object and the camera bend the light ray,
• an imperfect camera lens distorts the path of the light ray,
• the film or imaging sensor (a CCD in a video camera perhaps) does not image the light ray perfectly (it tends to blur it or to shift it),
• we do not know precisely where the imaging surface (film or CCD) was positioned relative to the camera at the time of the exposure, and
• the camera characteristics, such as focal length and lens distortion change from photograph to photograph thereby interfering with the repeatability of the light ray measurement.
The following sections address the five points above and discuss the implications on camera choice.