Zoom-dependent camera calibration in digital close-range photogrammetry

One of the well-known constraints applying to the adoption of consumer-grade digital cameras for photogrammetric measurement is the requirement to record imagery at fixed zoom and focus settings. The camera is then calibrated for the lens setting employed. This requirement arises because calibration parameters vary significantly with zoom/focus setting. In this paper, a zoom-dependent calibration process is proposed whereby the image coordinate correction model for interior orientation and lens distortion is expressed as a function of the focal length written to the EXIF header of the image file. The proposed approach frees the practitioner from the requirement to utilize fixed zoom/focus settings for the images forming the photogrammetric network. Following a review of the behavior of camera calibration parameters with vaiying zoom settings, an account of the newly developed zoom-dependent calibration model is presented. Experimental results of its application to four digital cameras are analysed. These show that the proposed approach is suited to numerous applications of medium-accuracy, digital, close-range photogrammetry.