Identifier: americanannualof1892newy (find matches)
Title: The American annual of photography
Year: 1892 (1890s)
Authors:
Subjects: Photography
Publisher: New York : Tennant and Ward
Contributing Library: Harold B. Lee Library
Digitizing Sponsor: Brigham Young University

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304 THE AMERICAN ANNUAL OF PHOTOGRAPHY, EQUATIONS RELATING TO FOCI The following simple optical formulas and calculations, worked out by Mr. J. A. C. Branfill, for the British Journal Almanac, will prove useful in many branches of photography, especially where several lenses of varying foci are in constant use for a variety of purposes: p = Principal focus. .F = Greater conjugate focus. / = Lesser conjugate focus. r = Ratio of any dimension in original to the same dimensions in copy (In case of reduction), or vice versa (in case of enlargement).a = Diameter of aperture to lens. p x = Exposure required, assuming that x = 1 when a = —• 4 p = f r(F+f) (r+l)a +F = p(r + l) = rf (i±Z\ _ F+f V r )~r+1 I) (r + l)» F+f =pX = p(2 + r + r F—p p F P f~P f 16 a* N. B.—For ordinary landscape work, where r is greater than 20, x may be takenp2 as 16 as Note.—In case the above may not be clear to some photographers, the followingrules may be better understood : To find the principal focus

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