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13 March 2015Addressing challenges of modulation transfer function measurement with fisheye lens cameras
Modulation transfer function (MTF) is a well defined and accepted method of measuring image sharpness. The slanted
edge test, as defined in ISO12233 is a standard method of calculating MTF, and is widely used for lens alignment and
auto-focus algorithm verification. However, there are a number of challenges which should be considered when
measuring MTF in cameras with fisheye lenses. Due to trade-offs related Petzval curvature, planarity of the optical
plane is difficult to achieve in fisheye lenses. It is therefore critical to have the ability to accurately measure sharpness
throughout the entire image, particularly for lens alignment. One challenge for fisheye lenses is that, because of the
radial distortion, the slanted edges will have different angles, depending on the location within the image and on the
distortion profile of the lens. Previous work in the literature indicates that MTF measurements are robust for angles
between 2 and 10 degrees. Outside of this range, MTF measurements become unreliable. Also, the slanted edge itself
will be curved by the lens distortion, causing further measurement problems. This study summarises the difficulties in
the use of MTF for sharpness measurement in fisheye lens cameras, and proposes mitigations and alternative methods.
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Brian M. Deegan, Patrick E. Denny, Vladimir Zlokolica, Barry Dever, Laura Russell, "Addressing challenges of modulation transfer function measurement with fisheye lens cameras," Proc. SPIE 9403, Image Sensors and Imaging Systems 2015, 94030F (13 March 2015); https://doi.org/10.1117/12.2083347