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The depth-of-field (DOF) of an imaging system is closely related to the pupil function that describes the characteristics of the pupil such as size and shape. Shallow DOF is especially problematic in microscopic optical imaging systems because they have relatively large aperture size for high optical resolution. In this paper, a new DOF extension method is proposed. This is to acquire an image by integrating the incoming light while changing the size of annular pupil. By dividing the pupil into annular form, the effect of the defocus aberration is reduced. Theoretical background is explained and simulation and experimental results are shown for verification. The results show that the DOF can effectively be extended by this method.