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Of all methods of particle size analysis determination only microscopy and subsequent image analysis enables accurate, geometrically determined size parameters to be measured. For this reason image analysis is a convenient referee or umpire method against which to compare other techniques. In this study, a series of seven poly(vinyl) chloride (PVC) resins were characterized by optical microscopy and image analysis, sieves, HIAC and Coulter counter. It was not found possible to perform the image analysis measurements without operator intervention with a light pen to separate agglomerates. The PVC particles were far too irregular to enable this task to be performed by erosion and dialation techniques. The errors at 90% confidence associated with the image analysis study were: calibration ± 1.7%, counting statistics ± 1% (max) with a focus and algorithms error which ranged from ½‐3% depending upon the quantity being measured. These numbers gave a total error of ± 2% (ca. one pixel) for equivalent circular diameter (ECD) and ± 4% (ca. two pixels) for object breadth. In general the agreement between the various techniques was quite good suggesting that the object breadth (OB) as measured by sieves was nearly identical to equivalent spherical diameter (ECD) as measured by the HIAC and Coulter counter for the samples investigates. A study of the image analysis data alone confirmed that OB and ECD were nearly identical with ECD running about 6.1 m̈m above OB. Mass resin samples, which are prone to electrostatic charging effects, tended to give slightly higher results in those methods which measured the resin in the dry state (HIAC and sieves). Finally, it should be suggested that many of the classical concepts of resolution and depth of focus which are used to place limits upon the performance of the optical microscope do not necessarily apply to particle size determination by image analysis. If the chord through the particle which describes its diameter can be considered an approximation to a square wave then it is the fundamental frequency of the Fourier components of that square wave that contains the size information. This should be true for microscopic objects which are absorbing of light and non‐retarding in nature. Once this fundamental frequency can be identified then sizing accuracy is dependent chiefly on the signal to noise ratio of the system.