  Particle size measurement is crucial in various applications, be it sizing
droplets in inkjet printing or respiratory events, tracking particulate
ejection in hypersonic impacts, or detecting floating target markers in free
surface flows. Such systems are characterised by extracting quantitative
information like size, position, velocity and number density of the dispersed
particles, which is typically non-trivial. The existing methods like phase
Doppler or digital holography offer precise estimates at the expense of
complicated systems, demanding significant expertise. We present a novel
volumetric measurement approach for estimating the size and position of
dispersed spherical particles that utilises a unique 'Depth from Defocus' (DFD)
technique with a single camera. The calibration free sizing enables in-situ
examination of hard to measure systems, including naturally occurring phenomena
like pathogenic aerosols, pollen dispersion or raindrops. The efficacy of the
technique is demonstrated for diverse sparse dispersions, including dots, glass
beads, spray droplets, and pollen grains. The simple optical configuration and
semi-autonomous calibration procedure make the method readily deployable and
accessible, with a scope of applicability across vast research horizons.
