Focal Length |
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(4)
Stolze's Method. This, or the lollowing method, No." 5, will be found the most convenient in practice. Dr. Stolze described his method in Photographische Nachrichten, vol. ii., p. 164, 1890, as follows : - "One first focusses on a very distant object, or, so to speak, on infinity, and marks this position of the focussing screen on the base-board of the camera. Then the camera is directed at a very near upright object of known, or easily measurable, size - best on a foot rule - so that the image falls on the focussing screen ; this is sharply focussed, and this position of the screen again marked on the base-board ; a negative is taken of the object in this position, and after development the size of the image of the photographed object is carefully measured, and one thus determines how much smaller the former is than the latter. If one multiplies the difference between the two foci with this number, the result will be the focal length." Possibly an example will make this a little clearer : We have focussed on a very distant object, and marked the base-board, and have also focussed on a foot rule and taken a negative, and find that the image of the foot rule measures 10 in., the distance between the two marks on the base-board is 19 in., then^{u} The size of the image should be as large as possible - that is to say, the amount of reduction should be as little as possible, because the greater the reduction the less accurate the result.
(5)
Proportional Method. This method is extremely con-venient when working with somewhat short-focus cameras and long-focus lenses. The camera is placed upon a table, when convenient, or if such is not handy, on the ordinary tripod. We support on a wall, shelf, or convenient place a foot rule which is plainly marked in inches. On the ground-glass of the camera a space of two inches is accurately marked as near the centre as possible: it is merely necessary to mark off the two inches, not to divide it out into parts. The camera is now moved backwards or forwards till the image of the foot rule of twelve inches just falls on the two-inch space we have marked out. The coincidence of the image on the lines should be examined with an eye-piece or compound focusser, to see that they do coincide, then the distance between the foot rule and the focussing should be care-33o |
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