EXCERPT page containing first few paragraphs. 2021-09-28 02:59:30
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Performance is optimized for infinity focus as can be seen at each aperture for infinity focus versus close focus: at close range, the performance drops off substantially in the mid and outer zones as well as developing substantial astigmatism.
Curiously, MTF for infinity focus at center rises from f/5.6 to f/8, suggesting a slight focus shift (or possible some other optical shortcoming); use of f/8 ought to suffer versus f/5.6 due to diffraction. Yet the effect is for coarse, medium and fine structures, suggesting some other reduction in optical aberrations that comes with stopping down. It is an odd behavior that your author does not recall seeing before! Still, the Forest Trees at Dusk series supports the idea that f/8 is the optimal aperture. At close range the behavior is as expected: f/8 contrast declines versus f/5.6.
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