The depth-from-focus–defocus approach to 3D reconstruction is based on the fact that objects closer to or farther from the object in focus appear blurred, and the amount of blur increases with the distance from the object in focus. An important characteristic of any depth-from-defocus system is the depth reconstruction accuracy. Several 3D reconstruction algorithms have been proposed, and the influence of image noise and image spectrum on the system accuracy has been studied. However, so far the effect of optics on the accuracy has not been fully explored. Here, we derive an expression estimating the system accuracy as a function of its optical parameters. It turns out that optics plays a major role in the accuracy, and tenfold increase of the lens focal length, and the aperture can increase the overall accuracy by a factor of more than 1000. The derived expression allows one to review several results, revealing that the accuracy is defined primarily by the optics. We also provide guidelines for the design of new depth-from-defocus systems in compliance with predefined specifications by choosing the appropriate optics.
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