It is well known that if the size of an object be ascertained, the
distance of a lens from that object, and the size of the image depicted
in a camera by that lens, a very simple calculation will give the
focus of the lens. In compound lenses the matter is complicated by the
relative foci of its constituents and their distance apart; but these
items, in an ordinary photographic objective, would so slightly affect
the result that for all practical purposes they may be ignored.
What we propose to do--what we have indeed done--is to make two of these
terms constant in connection with a diagram, here given, so that a mere
inspection may indicate, with its aid, the focus of a lens. All that is
required in making use of it is to plant the camera perfectly upright,
and place in front of it, at exactly fifteen feet from the center of the
lens, a two foot rule, also perfectly upright and with its center
the same height from the floor as the lens, and then, after focusing
accurately with as large a diaphragm as will give sharpness, to note the
size of the image and refer it to the diagram. The focus of the lens
employed will be marked under the line corresponding to the size of the
image of the rule on the ground glass.
As our object is to minimize time and trouble to the utmost, we may make
a suggestion or two as to carrying out the measuring. It will be obvious
that any object exactly two feet in length, rightly placed, will answer
quite as well as a "two-foot," which we selected as being about as
common a standard of length and as likely to be handy for use as
any. The pattern in a wall paper, a mark in a brick wall, a studio
background, or a couple of drawing pins pressed into a door, so long as
two feet exactly are indicated, will answer equally well.
And, further, as to the actual mode of measuring the image on the
ground glass (we may say that there is not the slightest need to take
a negative), it will perhaps be found the readiest method to turn the
glass the ground side outward, when two pencil marks may be made with
complete accuracy to register the length of the image, which can then be
compared with the diagram. Whatever plan is adopted, if the distance be
measured exactly between lens and rule, the result will give the focus
with exactitude sufficient for any practical purpose.--_Br. Jour. of
Photo_.
[Illustration]
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