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Lens Breathing
Even normal radially-symmetric prime lenses often exhibit focus breathing, which is a small change in the zoom (focal length) of the lens as the focus distance changes. This arises as a result of the motion of individual lens elements within the lens barrel, either directly, or indirectly via a change in the lens’s entrance pupil location (nodal point), such that the scene is effectively closer or further away.
In an anamorphic lens, the situation is more complex: in addition, the horizontal and vertical focal distances can change independently, again resulting from the different moving components and potential different changes in the horizontal and vertical entrance pupil locations—a changing anamorphic distance.
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