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All of those formulae assume a single thin lens. Most actual lenses are compound lenses with multiple lens elements. When those elements are all moved in unison by the same amount to change the focus distance, such as is often the case with many classis lens designs, the lens will more or less act according to expectations. But when different groups of ...
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The site you reference does not state that the effect of focus breathing is small. It states (emphasis mine),
The nominal focal length applies for the lens focused at infinity, but may be shorter for small distances.
For that Sony lens, indeed, it is experiencing focus breathing. Most internal focus lenses do — focus breathing is usually a side effect of ...
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