EXCERPT page containing first few paragraphs. 2022-10-02 00:18:11
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MTF is shown below at f/2.5 and f/5.6.
Wide open at f/2.5, performance is very high, albeit with odd local dips of astigmatism and likely field curvature of modest degree.
Peformance rises into the corners at f/5.6 but overall is no better and even a little worse than at f/2.5. It is hard to say for sure what is going on since the 38/2.5 and 55/2.5 do not show this drop at f/5.6. It might be diffraction, but if so, then the 38mm and 55mm should show a similar drop.
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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