The Dictionary Of Photography

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

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Focal Length
when draw another line upon the paper also along the side of the camera; remove the camera, and with a flat rule continue these lines till they cut one another, so as to form an angle, across which draw a line, so as to form a triangle, which line must be exactly the same length as the distance between the two pencil marks on the focussing screen. Find the centre of this base accurately, and connect the junction or apex of the angular lines with the centre of the base. This line will then be the true equivalent focus of the lens. The lines aa', bb\ are those traced on the paper along the sides of the camera, extended until they meet at c. d e is the base, being the exact distance apart of the two pencil marks ; /"its centre, and cf the true equivalent focus of the lens. The same method can be employed to determine the angle of view (see Angle of View).
(2)  Method by Focussing a Given Object the Same Size. With long-extension cameras a very common method is to focus on any object till the image is exactly the same size as the object, then measure the distance between the object and ground-glass and divide by four, when the result will be the equivalent focus. A convenient object to use is a foot rule. Example : - On focus-sing a foot rule, till the image or part of it measures exactly the same, we find the distance between object and ground-glass to be in. Then the focal length of lens used. For single lenses this method is sufficiently accurate, but subject to error in other cases.
(3)  Schroeder's Method. This can only be used by possessors of a telescope. First of all take a piece of cardboard - a mount will do - and punch two holes in it some little distance apart; place this card flat on the ground side of the focussing screen, and with a lead pencil blacken the ground-glass through the holes. Now go into a darkened room and set up the card, and place behind it a lamp or candle; erect the camera, and rack the lens in or out till the bright spots of light coming through the holes in the card fall exactly on the black spots on the ground-glass, and mark accurately the position of the ground-glass on the base-board of the camera, as shown in fig. 64, in which a is the card with the two holes, p the lens - here represented by a single double-convex lens - and b the focussing screen, marked by the pencil with the two black dots, on which are focussed the bright spots of light as shown by the dotted lines. A small
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