In some fields, such as forensics or archaeology, it is quite common to need measurements from unknown photographs. These unknown photographs can come from archives, historical files, police, bystanders etc. In these cases, there is little, or nothing known about the camera that took the photographs.
PhotoModeler normally requires information on the camera (focal length, principal point and format size) before photographs taken with it can be used. The Inverse Camera feature of PhotoModeler overcomes this problem.
To use Inverse Camera some information about the scene or object is required, such as known 3D points or constraints (see the Working with Imports and Coordinate Systems section for more detail).
You may be asking, "If I know the 3D positions of the points already why would I need PhotoModeler?". What PhotoModeler solves is the case of knowing some points or shapes in the photographs but not the information required. For example, you might need to get the relative position of two cars that appear in a few photographs from an unknown camera. You know the dimensions of one of the cars but not the dimensions of the other car nor their relative position. This is a case Inverse Camera can give you the answers you want.
Inverse Camera is carried out by using Constraints. The most common form is done with Control Points. Single photo Axes Constrained projects can also perform Inverse Camera to a certain extent. To get a photograph to process its Inverse Camera information you set the Inverse Camera Flags in the Photo Properties.