After manufacturing and prior to use, aerial cameras are carefully calibrated to determine precise values for a number of constraints, These constraints generally referred to, as the elements of interior orientation, are needed so that accurate data can be determined from pantographs.
The elements of interior orientation that can be determined through camera calibration are :
Equivalent focal length. The focal length, which is effective, nears the centre of camera lens.
Calibrated focal length (often called the “camera constraint” ). The focal length, which produces an overall mean distribution of radial lens distortion.
Average radial lens distortion. Distortion in image portion perpendicular to radial lines from the principle point.
Tangential lens distortion. Distortion in image portion perpendicular to radial lines from the principle point. (It is normally very small and except for the most precise work can usually be neglected)
Principle point location. Coordinates of principal point given with respect to x and y fiducial axis. ( Although it is the intent in camera manufacture to place the fiducial mark so that lines between opposite pairs intersect at the principal point there is nearly always some small deviation from this ideal condition.)
Distances between opposite fiducial marks (often given by coordinates of the fiducial marks)
Angle of intersection of the fiducial lines (should be 90 degrees + 0r – 1 inch.
Flatness of focal plane [should not deviate by more than + or -0.0005 in (0.01 mm) from a plane].
In addition to the determination of the above elements of interior orientation, resolution (the sharpness or crispness with which a camera can produce an image) is also commonly obtained as a part of camera calibration.
Resolution or resolving power of a lens is the ability of the lens to show derail.
One common method measuring lens resolution is to count the number of line pairs (black lines separated by white lines spaces of equal thickness which can be clearly distinguished within a width of one mm in an image produced by the lens. Good resolution is important in Photogrammetry because photo images must be sharp and clearly defined for precise measurements and accurate interpretative work. Photographic resolution is not just a function of the camera lens, however, but also depends on film quality and processing.