The Dictionary Of Photography

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

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Fixed Focus
Mr. Cheyney gives the following table, tested by various rectili-near, wide-angle, and single landscape lenses : -
Fractions of an inch are unnecessary. On reading through the above table many readers may at once exclaim, "If this is right, Messrs. Blank & Blank, lens makers or lens sellers, are wrong when they state that all objects beyond so many feet will be in focus with their lens; their distances are not nearly so much as the above table. Take, for instance, one special hand-camera: this has a 5-in. focus lens, and works at//8, and yet the vendor states that everything beyond 15ft. is in focus: by the above table everything beyond 32 ft. is in focus." To reconcile these seeming paradoxes is not difficult, as it will be noted that Mr. Cheyney takes -j1 of an inch as the greatest permissible error; if, however, we take £§« of an inch as the permissible error we get totally different results, as seen by working out his examples: -