The Photogrammetric Record
The Alpa 12 Metric, a medium-format digital camera system, is presented along with an accuracy assessment in accordance with VDI/VDE 2634 (2002). The camera combines the advantages of a modular medium-format camera system with high accuracy in object space. Camera calibration revealed a maximum length measurement error of 0·043 mm for a lens with mechanically fixed focusing distance and 0·063 mm for a lens where the focusing ring was taped during image acquisition. Only in the latter case did the maximum length measuring error improve, yielding 0·050 mm when calibrated with image-variant parameters of interior orientation. A bias towards positive length measurement errors indicated that the set-up for calibration could be improved and that even better accuracy in object space may be accomplished. The Alpa 12 camera system provides several unique features including lenses with fixed displacement of the principal point for terrestrial and other special applications, rigid tilt adaptors for Scheimpflug correction, shim kits for the fine adjustment of flange focal distance as well as the possibility of mounting a monochrome digital medium-format back, sensitive to visible and near infrared light. The camera is well suited for terrestrial applications requiring high accuracy and/or large pixel counts.