The effect of a focused laser beam on optical particle sizing by dynamic light scattering is analyzed. The first-order electric-field autocorrelation, as determined by numerical integration and by asymptotic expansion, is distorted compared with the plane wave autocorrelation. The distortion is strongest at forward-scattering angles. Although the focused beam autocorrelation may be modeled, considerations of accuracy and resolution of a particle-size distribution obtained by inverting correlation data suggest that, when a focused beam is used, backscatter, which most closely resembles the plane wave result, is preferred.