An instrument for biological analysis, comprising: an excitation source; an optical sensor configured to receive emissions from biological samples in response to the excitation source; an optical sensor circuit board; an excitation optical system disposed along an excitation optical path; an emission optical system disposed along an emission optical path an imaging unit comprising: a first surface, an opposing second surface and a threaded housing together forming a cavity that contains the optical sensor; a sensor lens; a lens case (449, 450) at least partially enclosing the sensor lens and having a threaded portion that engages with a threaded portion of the threaded housing; and a focusing mechanism comprising a focusing gear arranged to engage gear teeth provided to an outer surface of the lens case, rotation of the focusing gear rotating the lens case to cause the sensor lens to move along an optical axis of the sensor lens by means of the threads in the lens case and the threaded housing; wherein the first surface comprises a surface of the sensor lens; wherein the second surface comprises the optical sensor circuit board; and wherein the focusing mechanism is configured to be accessible from outside an optical enclosure enclosing the optical paths for adjusting a focus of the sensor lens. an excitation source; an optical sensor configured to receive emissions from biological samples in response to the excitation source; an optical sensor circuit board; an excitation optical system disposed along an excitation optical path; an emission optical system disposed along an emission optical path an imaging unit comprising: a first surface, an opposing second surface and a threaded housing together forming a cavity that contains the optical sensor; a sensor lens; a lens case (449, 450) at least partially enclosing the sensor lens and having a threaded portion that engages with a threaded portion of the threaded housing; and a focusing mechanism comprising a focusing gear arranged to engage gear teeth provided to an outer surface of the lens case, rotation of the focusing gear rotating the lens case to cause the sensor lens to move along an optical axis of the sensor lens by means of the threads in the lens case and the threaded housing; wherein the first surface comprises a surface of the sensor lens; wherein the second surface comprises the optical sensor circuit board; and wherein the focusing mechanism is configured to be accessible from outside an optical enclosure enclosing the optical paths for adjusting a focus of the sensor lens. a first surface, an opposing second surface and a threaded housing together forming a cavity that contains the optical sensor; a sensor lens; a lens case (449, 450) at least partially enclosing the sensor lens and having a threaded portion that engages with a threaded portion of the threaded housing; and a focusing mechanism comprising a focusing gear arranged to engage gear teeth provided to an outer surface of the lens case, rotation of the focusing gear rotating the lens case to cause the sensor lens to move along an optical axis of the sensor lens by means of the threads in the lens case and the threaded housing; wherein the first surface comprises a surface of the sensor lens; wherein the second surface comprises the optical sensor circuit board; and wherein the focusing mechanism is configured to be accessible from outside an optical enclosure enclosing the optical paths for adjusting a focus of the sensor lens.