A lens group for an endoscope or microscope, comprising one or more lens elements, each of uniform refractive index, adapted to: i) focus, with high wavefront aberration correction, driving or excitation light received from an exit tip of an optical waveguide (such as an optical fibre) located substantially against a proximal surface of the lens group to a point observational field with narrow point spread function beyond a distal surface of the lens group (such as outside an optical window located distally relative to the distal surface)； and ii) transmit, with high wavefront aberration correction, fluorescence or reflected return light received by the distal surface from the point observational field (and its neighbourhood defined by the fluorescence wavelength point spread function) back to the exit tip of the optical waveguide at the fluorescence wavelength.
