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If a flat mirror, the correct distance is the distance from the camera to the object. If a non-flat mirror or surface, it's more complicated. In the case of a very convex surface, like that of a drop of liquid, the focal length of the drop is so short that almost anything behind it will be in focus when viewing through it (refraction) and almost anything reflected in it will be in focus. This is true for the same reason that very short focal lenses have large DoF; it's an extreme case of the same phenomenon. |
