An aerial photograph which is captured while camera axis is present in a true vertical position is termed as vertical photograph. However, most of the aerial photographs are acquired while camera is in a tilted position i.e., from 1 to 3 degrees. Hence, these photographs are also called as tilted aerial photographs.
Following figure represents the geometry of true vertical photograph, where the axis is making 0 degree tilt at the optical center of the camera.
In the above image one can clearly see, that camera axis is intersecting the photograph at point o. This point o is called principal point of the photograph, which is normally represented with letter p.
Nadir point on the photograph is obtained by projecting a vertical line (plumb line) from exposure station onto the photograph. It is represented with letter n. Isocenter is the point, which bisects the line joining principal point and nadir point in a vertical photograph. It is represented with letter i.
In case of true vertical photograph, line of gravity extended towards the photograph from optical center (exposure station) will coincide with the camera axis. Hence, principal point and nadir point and isocenter all are located at same point in case of vertical photograph.
In case of tilted photograph the geometry can be observed as shown in the image below.
In both cases as we move away from the principal point scale starts decreasing in case of flat terrain. As a result, for a given pixel large area coverage is seen at the corners of the photograph.