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MTF is shown below at f/2.5 and f/5.6.
Wide open at f/2.5, performance rolls off rapidly, suggesting a moderate field curvature. In the geometric plane of focus, the outer zones will not look so great.
Stopping down to f/5.6 reveals a strong astigmatism into the corners and a mid-zone wave that is some kind of field curvature. This behavior means that there will be uneven sharpness across the frame on planar subjects, such as a distant landscape, building, group of people, etc.
Calculated MTF curves including diffraction. Not measured from real lenses. Sagittal slit orientation drawn with continuous line and tangential with dashed. White light. Spatial frequencies 10, 20 and 40 lp/mm.
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