The Dictionary Of Photography

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

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Diaphragms
It has been stated above that the ratio aperture of diaphragms could be found by dividing the focal length by the diameter of the aperture. This is practically accurate, but not scientifically so. Where the diameter of either or both of the lens combina-tions is less than the fixed stop, the diameter of the combination is the actual aperture; where there is a fixed stop or a ring of metal in the interior of the lens mount, the diameter of this is the actual maximum aperture. These ratio apertures are strictly accurate when the lens is a single landscape lens ; but with all doublet lenses the condensation of the light by the front com-binations must be taken into account - especially when comparing, either for scientific or lens-testing purposes, the actual ratio apertures; practically we are testing the effective or working apertures of the lens. This is by no means a difficult operation, and one of two plans may be adopted. The first is to rack the
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