An optical system includes a spectrograph having a concave diffraction grating and a detector. An aperture is selectively positioned by an associated actuator or positioning mechanism either into, or out of, an optical path of the input light beam downstream of a sample and prior to entering the spectrograph. A slit plate having a plurality of different size entrance slits is positioned downstream of the aperture and movable by an associated actuator or positioning mechanism to position one of the plurality of entrance slits in the optical path of the input light beam. A controller coupled to the detector and the actuators is configured to control the actuators to selectively position the aperture and the slit plate to provide a selectable resolution of the spectrograph. The aperture setting and slit plate setting may be determined from a lookup table in response to a request for finer or coarser spectral resolution.