EXCERPT page containing first few paragraphs. 2021-06-22 10:40:36
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MTF (calculated*, not measured) is shown below at f/2.8 and f/5.6 for both infinity focus and at MOD (Minimum Object Distance).
Performance is strong at f/2.8, but falls off to the edges steadily; this is probably field curvature more than lens performance per se. Stopping down to f/5.6. improves performance strongly in central areas, but only modestly in the outer zones. The lack of much improvement might be due to a focus shift in those outer zones; field shots can verify that.
The falloff in performance to outer zones might be lens performance in its own right, but more likely it is a peripheral forward focus shift or similar which would mislead in the chart but show distinctly better results in real photography, that is, with a curved zone of peak sharpness (typical for many lenses). Combine that with focus shift and outer zones, and field shots are needed to verify the exact behavior.
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