The Dictionary Of Photography

A True Historic Record Of The Art & Practice Of Photography 100 Years Ago.

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Focal Length
The depth of focus behind = 37 in. (See Fixed Focus.)
The following simple optical formulae and calculations, worked out by Mr. J. A. C. Branfill, and published in 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 : - Let p = Principal focus.
F= Greater conjugate do.
/ = Lesser do. do.
D = F + /= distance of image from object.
r = Ratio of any dimension in original to the same dimension in copy (in case of reduction), or vice versa (in case of enlargement).
a = Effective diameter of diaphragm. U. S. No. = " Uniform System " No. of do.
x as Comparative exposure required.