ENLARGING The operation of enlarging consists simply in taking a print from a negative by means of a lens instead of by placing a paper in contact with the surface of the negative in the usual manner. The essential parts of the apparatus required are a holder for the negative from which the enlargement is being made ; a camera body or similar arrangement of which the nega tive holder forms one end, a lens being fitted at the other ; and a board or easel for holding the paper on which the enlargement is being made. The positions of the easel and the lens should both be adjustable, so that enlargements of any desired size may be made. The illus tration explains the arrangement fully. N is the negative, r, the lens, and P the paper. The type of lens used for making an enlargement is not important, beyond the fact that it ought to cover the plate with crisp definition from corner to corner with a fairly large aperture. If it is necessary to use a small stop to obtain good definition, the exposures required may be inconveniently long. Also, a lens of long focus should be avoided, especially if very large pic tures are desired from small plates, as the apparatus would have to be very long. If the focus of the lens is known, the apparatus may be set up approximately in position for any degree of enlargement by measurement, leaving only the fine focusing ; it will then be easy to ascertain what degree of enlargement forms the limit of the apparatus.
The distance A from the centre of the lens to the sensitive paper must be equal to the focus of the lens multiplied by the degree of enlarge ment, with the focus of the lens added to the result. The distance B from the centre of the lens to the negative must be the distance A divided by the degree of enlargement. The distance A bears the same proportion to B as the enlargement bears to the original negative. An example will render this dear. A lens of 6 in. focus is being used to enlarge a 4 in. by 3 in. picture up to 12 in. by 9 in. The degree of enlargement is three times linear. Three times 6 in. is r8 in., plus 6 in. brings the distance A up to 24 in. ; and this distance divided by 3, the degree of enlargement, gives 8 in. as the dis tance B. These general principles apply equally to all methods of enlarging. Details of making enlargements on paper direct, or by means of enlarged negatives, are given in later articles.
It is useful to know the relative exposures for different degrees of enlargement, when all the conditions are equal. Beginning with the pro duction of a print the same size as the original as a basis, and calling the exposure for this r, the relative exposures for other degrees of enlargement, using the same stop throughout, will be :—