It will be uncommon for the processing to fail and not complete. Some possible reasons for failure are: the automatic referencing stages mis-referenced some points, the camera parameters are set up incorrectly due to EXIF content fault, the target pattern changed shape, there are not enough photographs at the correct angles, or the calibration is over-parameterized. Occasionally calibrations have problems because of unusual or difficult lens-camera relationships. For example, cameras with long focal lengths are more prone to failure. These are some considerations for long focal length calibrations:
• Lenses with long focal lengths can be problematic. First, what is a "long" lens? A long lens has a narrow field of view and allows you to zoom-in on distance objects. A lens that is long, as far as the calibration is concerned, might have a focal length over 150mm on a 35mm camera. A digital camera with smaller format size would have a shorter focal length to be considered long. For example, a digital camera with an imaging chip that is 18mm by 12mm would have a long lens if its focal length was over 75mm long.
• Second, what sort of errors does one see when calibration fails with long lenses? For very long lenses the calibration often fails during second stage processing. For calibrations that succeed you might have large post-processing precision errors on the parameters or large correlation values (as seen in the Project Status Report) and marking residuals over 1.0 pixel.
• Third, why are long lenses hard to calibrate? As a lens gets longer, you move farther away from the calibration sheet during photography. This combination reduces the perspective and makes the light rays close to parallel. This reduces the stability of the mathematical solution and makes it especially difficult to solve for lens distortion parameters.
• Fourth, what can I do about it? Depending on the length of the lens there are a few different approaches. If the lens is not excessively long (unfortunately hard to determine this before hand), you retry the calibration and reduce the parameters that get solved. If the camera is properly set up and has square pixels we suggest reducing the solution to just solving for focal length. This is controlled by the Calibration Options Dialog. If it solves and errors are small you can try to add the solution of principal point, and then perhaps K1 lens distortion. Also ensure that a) there are good angles and roll angles for the camera station positions, and there is good frame fill for the targets.
• If the lens is quite long the above may not produce a result (or the result will have error values that are too high). The next step is to move away from the flat calibration sheet and go to a 3D field of targets. You will need to build this set up yourself. Coded targets make it easier but it can be done with normal targets too. Using the corner of a room, or a wall and some posts in front of the wall, form an N by N by N cubic area of targets. 25 to 75 targets will suffice. Take angled and rolled photos as you would with the flat sheet. This project will need to be marked and perhaps hand referenced (if coded targets are not used). Then Optimize with field calibration option is used to calibrate the camera. Even with a 3D field of targets you may still only be able to solve for focal length, and principal point. Use the error metrics (largest residual under 1.0 pixel, no correlated parameters over 95%, and no precision warnings) as indication the calibration is successful. Contact Technical Support for further guidance.
Note: The Multi-sheet Calibration method using a ‘3D’ pattern of targets (with some sheets at different depths and off-plane) can help with some long focal length lens calibrations.
Another reason for failure of Single Sheet calibration method is the automatic referencing error might occur with cameras with high levels of lens distortion - try increasing the number of photos each point must be referenced on (See Camera Calibrator section in Preferences - Processing and Cameras). An EXIF focal length error encoding may be gotten around by removing the EXIF information (load images into an imaging program re-save without the EXIF data). A grid that changes shape or insufficient number of photographs can be remedied by retaking the photographs of the grid.
Over-parameterization of the camera means that one or more of the camera parameters are not needed. See Choosing Camera Parameters to Calibrate for more information. Calibrations of cameras with long focal lengths often fail due to over-parameterization. If you allow PhotoModeler to automatically parameterize and the deviation and correlation checks are completed during Calibration, the most stable and accurate set of parameters will be solved (see Automatic Processing Parameter Assignment and Camera Calibration Options Dialog).