The Sigma-supplied MTF chart indicates a lens with very good contrast wide open, indicating well-controlled aberrations. It’s not clear if MTF is computed or measured. Zeiss measures MTF and doesn’t ship a lens unless it meets specifications—computed MTF means that’s the best possible result, and real lenses might not perform as well.
The red lines are at 10 line pairs/mm and the green lines are at 30 line pairs/mm. The Sigma description states that the MTF chart is wide open (f/1.4), which is an outstanding MTF curve for an f/1.4 lens, and field shots do tend to confirm it. Compared to the Zeiss ZF 50/1.4 Planar MTF chart, the Sigma maintains notably higher contrast over much of the field at 10 lp/mm, a remarkable performance. Beyond that it’s hard to be sure; Sigma’s results may be computed (not measured) and are at 10/30 lp/mm vs 10/20/40 lp/mm for the Zeiss. Interpolating, fine detail MTF is probably a bit better with the Sigma at f/1.4.
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