Optical system for performing scattering and absorbance assays in clinical diagnostics, the optical system comprising - a light source for emitting collimated light parallel to an optical axis in an optical path, - a sample holding unit comprising at least one sample holding position holding an optical cuvette, - an optical detector for measuring light (50', 67) transmitted through a sample being located in the optical path in the optical cuvette, - an adjustable light angle selector, characterized in that, the light angle selector is located in the optical path between the optical cuvette and the optical detector, and comprises i. a diaphragm with variable aperture, the diaphragm being configured to vary the aperture according to the assay type and the desired sensitivity to scattering, the smaller the aperture the higher the sensitivity to scattering; or ii. an LCD with switchable pixels, each pixel being configured to be switched off such as to block light or to be switched on such as to allow light to pass through according to the assay type and the desired sensitivity to scattering, the smaller the number of pixels in the center allowing light to reach the detector the higher the sensitivity to scattering; or iii. a group of lenses (80, 81, 82) and a driver for varying the relative distance between the lenses (80, 81, 82) as a zoom objective with variable focal length (f), the objective being configured to vary the focal length (f) according to the assay type and the desired sensitivity to scattering, the longer the focal length (f) the higher the sensitivity to scattering; wherein for a scattering assay said light angle selector is adapted to be adjusted such as to allow only light transmitted through the sample and diverging from the optical axis with an angle (α) smaller than 5 degrees to reach the detector, and wherein for an absorbance assay said light angle selector is adapted to be adjusted such as to allow light transmitted through the sample and diverging from the optical axis with an angle (α) smaller than at least 60 degrees to reach the detector. - a light source for emitting collimated light parallel to an optical axis in an optical path, - a sample holding unit comprising at least one sample holding position holding an optical cuvette, - an optical detector for measuring light (50', 67) transmitted through a sample being located in the optical path in the optical cuvette, - an adjustable light angle selector, i. a diaphragm with variable aperture, the diaphragm being configured to vary the aperture according to the assay type and the desired sensitivity to scattering, the smaller the aperture the higher the sensitivity to scattering; or ii. an LCD with switchable pixels, each pixel being configured to be switched off such as to block light or to be switched on such as to allow light to pass through according to the assay type and the desired sensitivity to scattering, the smaller the number of pixels in the center allowing light to reach the detector the higher the sensitivity to scattering; or iii. a group of lenses (80, 81, 82) and a driver for varying the relative distance between the lenses (80, 81, 82) as a zoom objective with variable focal length (f), the objective being configured to vary the focal length (f) according to the assay type and the desired sensitivity to scattering, the longer the focal length (f) the higher the sensitivity to scattering;