The invention relates to a measurement apparatus, and method, for analysis of particles in a liquid sample, the apparatus comprising: an image acquiring device, an image analyser, a holder arranged to hold a sample retaining device, and a light refractor positioned between said image acquiring device and said holder; whereby at least one of the image acquiring device, the light refractor and the holder is movable by means of a piezoelectric motor, thereby moving a focus plane within the sample retaining device when it is held by the holder whereby the image acquiring device is adapted to acquire a plurality of images of said sample at different focus planes within the sample retaining device; wherein the image analyser is arranged to analyse at least one acquired image for identifying which of the particles are imaged in focus, and analysing those particles which have been identified as being imaged in focus.