American Photography Volume 10 (Paperback)

This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1916 Excerpt: ...problems. These two formulas are: Size of Obj ect Distance of Object from Lens Size of Image Distance of Image from Lens and, let us assume that the degree of enlarge ment is 2, and that the distance from the negative to the paper in the enlarger is found, by measurement, to be 18 inches. In the first equation the size of the object is i; the size of the image is 2; the distance of the object is 18 less the distance of the distance of the image from the lens; the distance of the image from the lens may be called p'. We can then figure Distance of 0 from Lens Distance of / from Lens i Focal length of Lens By means of these two formulas, such problems as are above presented, can be solved, provided we have enough to start with. We assume that our friend has or wishes to buy an enlarger of the collapsible box type, and fit to it a lens that can be used with gaslight papers for rapid work. All that he need know about the enlarger to determine what lens is needed, is the distance from the negative to the paper and the degree of enlargement; that is, the ratio of the size of the image on the enlargement to the size of the image on the negative. This can be determined by a visit to the dealer, who probably cannot say what is the focal length of the lens in the enlarger, as the specifications do not mention the focal length of the lens. The ratio of the dimensions of the negative and enlargement cannot be used; the ratio of a single line in the negative to the image of that line on the enlargement must be taken, ror the sake of illustrating the formula, must If p' equals 12, it appears that p equal 6, since p equals 18-p'. Passing to the second equation, it is merely a matter ot substitution to find the focal length of the lens that will fit the case. Substituting ...