At moderate distances of about 15 feet (~5 meters), there is positive field curvature: focus at the edges lies closer than at the center. This is seen near infinity also, where it results in diminished sharpness away from the center that requires at least f/4 to compensate for. At close range, the field flattens, making the Sigma a better choice for short to moderate distances.
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Sigma 50/1.4 @ f/1.4, field curvature
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