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It sacrifices an even quality distribution across its focus range for compactness. That doesn't mean that it is a bad lens in real life. Even at shorter focus distances, the center quality is usually good enough if you stay at least a bit away from its minimum focus distance and the mediocre borders don't really matter all that much here anyway. Near infinity focus, the sharpness is actually very high across the field (stopped down). Lateral CAs aren't impressive.