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It is generally acknowledged that it takes at least three phase-stepped speckle patterns to obtain quantitative phase information. However, if only a phase change has to be determined, a two-bucket approach can be followed, under certain conditions. The background of the two-bucket algorithm, and the requirements with respect to the use of it, are explained. Examples of modeled and measured phase stepped speckle intensity data, showing numerical instability when conditions are not met are presented. It is also shown that two phase-stepped speckle pattern pairs, taken before and after an event can be sufficient to determine phase changes invoked by the event.