A particle characterization apparatus is disclosed comprising: a sample cell for holding a sample, a light source for producing a light beam for illuminating the sample in the sample cell, thereby producing scattered light by the interaction of the light beam with the sample; a focussing lens for focussing the light beam within the sample; and a detector for detecting the backscattered light along a detection optical path that intersects the focussed light beam within the sample. The intersection of the light beam and the detection optical path in the sample define a detection region. The apparatus comprises an optical arrangement for varying the volume of the detection region.