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Thomas R. Dallmeyer pada tahun 1892 mengembangkan pamflet ayahnya, John Henry Dallmeyer yang diterbitkan pada tahun 1874 "On the Choice and Use of Photographic Lenses" (tampaknya juga mengutip karya J.H.D. "On the Use of Diaphragms or Stops") mengatakan:
"Thus every point in an object out of focus is represented in the picture by a disc, or circle of confusion, the size of which is proportionate to the aperture in relation to the focus of the lens employed. If a point in the object is 1/100 of an inch out of focus, it will be represented by a circle of confusion measuring but 1/100 part of the aperture of the lens."
Pada tahun 1889, Edward John Wall pada bukunya "A Dictionary of Photography for the Amateur and Professional Photographer" mengatakan: