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My theory is that when the subject is, say, 1/3 of the frame or less, we have a chance that the light rays coming into the objective lens are arriving from scattered light irradiating the leaves, water, or even atmosphere in the heat haze or similar conditions. These rays are much more complex to account for, so even if the PF-including lens deals well with normally irradiated objects (in the best controlled case, stuffed toys come to mind), the tolerances the designers worked with hold well and the PF lens performs extremely well.
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In contrast, non-PF traditional design is much simpler where the majority of work is done by refraction through the dispersion of the lens bulk, it is very well understood optics from long ago. These lenses like Sony, Canon, or Nikon exotics are not really exotics, they are quite standard (at least theoretically) so they do not suffer from these stray rays phenomena as much or even to any significant degree. So another example was a raptor in flight, very small in the frame, the atmospheric distortion creates stray rays around the subject/bird, all hitting the objective and potentially, according to my theory/hunch above, affecting the resulting photo more than is the case for the really quite standard 'exotics'.