> as lemmas

1867 T. Sutton & G. Dawson Dict. Photogr. 57 Circle of least confusion, the nearest approach to a focus of a pencil after oblique reflexion or refraction.

1903 C. Beck & H. Andrews Photogr. Lenses (ed. 2) iii. 58 When the screen reaches X the circle of illumination will be the smallest size attainable with the lens under consideration, the circle at this point is called the circle of least confusion.

1906 R. C. Bayley Compl. Photographer xii. 150 ‘Depth of focus’..depends upon the size of what is termed the ‘circle of confusion’.

1940 Chambers's Techn. Dict. 163/2 Circle of confusion, the minimum area of a focused bright point of light, the size of which determines the maximum definition possible with a given lens arrangement and stop.

1953 S. W. Amos & D. C. Birkinshaw Television Engin. i. ix. 173 Objects near the object plane give images which are only slightly blurred, points being reproduced as circles which are known as circles of confusion.

1958 H. J. Gray Dict. Physics 88/2 Circle of least confusion, circle through which all the rays over the aperture pass.

1961 A. L. M. Sowerby Dict. Photogr. (ed. 19) 120 It is usually assumed that a circle of confusion of ¹/ ₁₀₀ in. corresponds to the boundary between ‘sharp’ and ‘unsharp’ in a print held at the normal viewing distance of about 10 in. from the eye.

1961 Webster's 3rd New Internat. Dict. Eng. Lang. Circle of confusion, the indistinct circular patch formed by a lens representing the out-of-focus image of a single object point—called also blur circle.

1961 Webster's 3rd New Internat. Dict. Eng. Lang. Circle of least confusion (Physics), the minimum cross section of a symmetrical bundle of rays that have no common focus because of spherical aberration.