The measurement spot size of small-spot reflectometers, ellipsometers, and similar instruments can be reduced by placing an optical fiber along the optical path of the instrument, such as between an illumination source and a sample or the sample and a detector. The angular range of the probe beam can be adjusted to be less than a natural numerical aperture of the optical fiber. A multimode fiber can be used, which can have a controllable amount of bend or coil, such that rays entering the fiber at larger angles of incidence are attenuated more than rays entering at shallow angles of incidence. Light passing through the fiber can be selectively attenuated and partially mixed to reduce the presence of secondary maxima falling outside the measurement spot. Minimizing these secondary maxima can improve the amount of light measured by the detector that is reflected from inside the measurement spot.