If a surgeon or technician measures a patient's pupils but they are not "straight ahead" of the patient and and they do not have their own gaze lined up with the patient's gaze, the pupil measurement will be incorrect because it will be affected by parallax. Parallax always makes things look smaller. So parallax will lead the doctor or technician to take a measurement which they feel is correct, and in fact it is correct from their position and direction of gaze. However it is NOT correct because the patient is gazing "straight out" through his or her pupil, which is larger than the pupil seen by the doctor. Parallax is one reason why pupil measurements might be too small; it is probably a relatively minor (but non-trivial) element of the overall problem of incorrect pupil measurement.
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