The American annual of photography . s mountit is frequently a mere approximation, since objectives aremanufactured nowadays by the hundred at a time, and pre-cise identity is impossible. A wise photographer, therefore,will make a point of ascertaining the exact focus for himselfat the first opportunity. There have been many different ways proposed for findingfocal length, but the majority involve first focusing for in-finity or a far-distant object, which is not always con-venient, as in the case of workers who only have their even-ings available and are restricted to indoor operations. Those
The American annual of photography . s mountit is frequently a mere approximation, since objectives aremanufactured nowadays by the hundred at a time, and pre-cise identity is impossible. A wise photographer, therefore,will make a point of ascertaining the exact focus for himselfat the first opportunity. There have been many different ways proposed for findingfocal length, but the majority involve first focusing for in-finity or a far-distant object, which is not always con-venient, as in the case of workers who only have their even-ings available and are restricted to indoor operations. Thosemethods, too, in which an object is focused full-size are un-suitable with cameras having but a short extension. Here is a method, believed to be new, though obviousenough in principle, which dispenses with any need of focus-ing for infinity and has also the merit of not requiring a longextension and of being readily checked. Its only drawbackis that it is a little tedious. Take a foot rule and set it up as a copy. Then, placing 210. 211 the camera so that the rule is as central as possible on thescreen, focus it (or any portion of it) sharply with the fullaperture of the lens to exactly half-size, and measure care-fully the distance between the lens-board and the rule. Itshould be seen that the back and front of the camera are ver-tical, as well as parallel with the rule. Next, focus sharplyas before till the rule is exactly one-third size, again measurethe distance between lens-board and rule, and subtract the firstdistance from the second, when the remainder will give thefocal length. To make sure of correctness, focus a third timetill the rule is precisely one-fourth its proper size, and oncemore measure the distance between rule and lens-board. Sub- mm
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Keywords: ., bookcentury1900, bookdecade1910, bookpublishernewyorktennantandw
