PhotoModeler requires a description of the camera that was used to take the photographs. This description defines the focal length, pixel size, and lens distortion. This description is called a ‘camera calibration’.
There are several ways to create a camera calibration in PhotoModeler. One method is a pre-calibration – this is a separate project taken with special targets. This method is not often used. The other methods use the data from the project being executed – that is, no separate calibration steps. These other methods are: Inverse Camera, Auto-Calibration, and Field Calibration. For many projects using Coded Targets and/or SmartPoints, Auto-Calibration is the preferred choice. It provides the highest accuracy and avoids the extra pre-calibration step. Inverse Camera is used on single photo projects with a source of control points (known 3d points). Field Calibration is used to fine tune calibrations that come from Inverse Camera and Auto-Calibration.
Each different camera or each lens/camera combination needs its own description. This description includes data on the focal length, imaging scale, image center and lens distortion. PhotoModeler uses this information to build a proper geometrical relationship between points on the photograph and points in 3D space.
When should one pre-calibrate a camera with the separate Camera Calibration steps? One might pre-calibrate a camera when the projects being executed are not strong enough (meaning enough photos, enough overlap, good angles) to generate a good camera calibration. If good photos can be taken, then auto-calibration is preferred.
The camera descriptions can also be ‘approximate’, that is, user entered parameters. Creating an Approximate Camera requires entering in information about the camera and creating a camera file. Pre-calibrating a camera involves taking photos of a calibration pattern (supplied with the package) and running the Camera Calibration.
In addition, with PhotoModeler Premium one can estimate lens distortion with straight lines – see Lens Distortion Estimate.
See the section Working with Cameras for additional background.