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 pixels.
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 in Pro 5.2.1 or the Preferences and ini file in earlier versions (contact us at firstname.lastname@example.org for further information). 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 Global 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 us at email@example.com for further guidance.