4 February 2013 Focus finding using scale invariant patterns
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Abstract
We present a new method for accurately determining the best focus position of a camera lens in the context of image quality evaluation and modulation transfer function (MTF) measurement. Our method makes use of the “live preview” function of digital cameras to image a test chart containing spatially and rotationally invariant alignment patterns. The patterns can be located to sub-pixel accuracy even under defocus using the technique of blur-invariant phase correlation, which leads to an absolute measure of focus position, independent of any backlash in the lens mechanism. We describe an efficient closed feedback loop algorithm which makes use of this to drive the lens rapidly to best focus. This method achieves the peak focus position to within a single step of the focus drive motor, typically allowing the peak focus MTF to be measured to within 1.4% RMS. The mean time taken to find the peak focus position and drive the focus motor back to that position ready for a comprehensive test exposure is 11.7 seconds, with maximum time 26 seconds, across a variety of lenses of varying focal lengths.
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David Morgan-Mar, Matthew R. Arnison, "Focus finding using scale invariant patterns", Proc. SPIE 8660, Digital Photography IX, 86600O (4 February 2013); doi: 10.1117/12.2001717; https://doi.org/10.1117/12.2001717
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