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True, BTW, as one last final question, do you see anything wrong with my "old schoolboys trick" of using the FR like a kid's magnifying glass, with the sun's image to measure the FL?
All of them gave a very clear image on the white cardboard of the cloudy sky/pinpoint sun when at hand-held focus, I then just measured the distance from Paper to rear of lens (glass) and assumed that to be the ~focal length. Is that OK?
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