A Close-Range Photogrammetric Solution Working with Zoomed Images from Digital Cameras 2012-01-0612
Close-range photogrammetry (CRP) is traditionally based on a network captured with the camera lens at a fixed focal length. A zoom lens is not desirable without solving the intrinsic camera parameters for varying focal length and lens distortion. When using a zoom lens camera, multiple focal lengths can be used if the camera is calibrated for each varying focal length, but most consumer grade lenses are not designed to accurately return to (or stay at) mid-range focal lengths. Similarly, using close-range photogrammetric software systems to accurately recover three-dimensional (XYZ) data from Point and Shoot (PAS) digital cameras has been problematic when the images were not intended for CRP. PAS cameras are automatically refocused and easily zoomed so the focal length and lens distortion are typically unknown for CRP mensuration purposes. In such circumstances, traditional CRP analysis can be both laborious and difficult without the correct camera parameters. Previously a CRP network involving imagery with unknown focal lengths, required identifying control points to back-calculate the camera's inner orientation and camera aim for each image. This paper describes a new tool capable of accurately and efficiently determining the focal length, principal point offsets and lens distortion for zoomed imagery of consumer grade digital cameras throughout the full telephoto zoom range. This new computational algorithm, called Zoom-Dependent (Z-D) calibration, ensures all zoomed images are usable for mensuration within a traditionally accomplished photogrammetric network. The Z-D process is described, the requirements of a suitable network are also described, and the Z-D results demonstrated on small scale (vehicle) and large scale (pavement) networks. Two Digital Single Lens Reflex (DSLR) cameras and several PAS cameras are compared to a control network captured with an off-the-shelf consumer grade DSLR camera that had been metrically calibrated.