Abstract: An imaging device (010, 10, 110) comprises a first optical system (020, 20, 120) at a distal end of the imaging device, a second optical system (080, 80, 180) towards the proximal end of the imaging device, and a sensor (074, 74, 174) at the proximal end of the imaging device. The first and second optical systems and the sensor are aligned along a common longitudinal axis. The first optical system is or comprises one or more reflective and/or refractive optical components (24, 124; 22, 122) symmetrically and/or coaxially arranged with respect to the longitudinal axis, and the second optical system comprises one or more reflective and/or refractive optical components (24, 124; 22, 122) for focussing incident light towards the sensor. A calibration system (200) and method for calibrating such an imaging device, and a method of processing image data obtained from such an imaging device are also provided.