ENLARGING.
It may be said that, other things being equal, the larger a photograph is the finer it is, at any rate within limits. The apparatus for taking very large pictures is, however, exceedingly cumbersome, and the plates necessary are very expensive. Moreover, certain optical difficulties are met with. It is exceedingly difficult, indeed impossible, to make a large lens equal to a small one even in its properties of definition and flatness of field. But it is in a want of depth of focus that large lenses show them selves most defective. When we get beyond such lengths as fourteen and twenty inches we find that, to get the foreground and distance both anything like in focus, we have to use excessively small stops, so that the exposures are prolonged and certain effects become impossible to attain. For example, instantaneous views are most diffi cult to do with a lens of beyond about 20-inch focal length.
For all this the photographer need not despair of getting prints as large as he likes of any subject which may be taken on even the smallest plate. He may have recourse to the process of enlarging. Recently this process has been made much more easy by the introduction of gelatine bromide paper, which is simply paper coated with a gelatine emulsion specially prepared for the pur pose.
First, as to the negatives to be used for enlarging. These require, of course, to be very sharp. For this reason focussing must be performed with great accuracy, a focussing magnifier being used in every case. A nega tive which will give a perfect silver print will always give a good enlargement. The thing to be specially avoided is hardness. We must therefore neither under expose nor over-develop. A negative which will give a silver print somewhat too soft will generally give an excellent enlargement, especially if the shadows be very clear.
Various designs of apparatus have been made for en larging, the most convenient of which is " and Co.'s Enlarging Apparatus," which we illustrate here. We believe that we shall most readily make the object of this clear by saying that it is simply an improved magic lantern, and that indeed it may be used as such. When it is used for enlarging, the negative takes the place of the magic lantern slide, and the enlarged image is thrown on to the sensitive film. No lens is shown
in the engraving, but any photographic lens of suitable focus will do well. A portrait lens, or one of the rapid type of about six or seven inches equivalent focus, will be found the best.
We illustrate also a cheaper form of apparatus designed specially for amateurs. It is not quite so convenient or perfect as the more elaborate appliance, but will be found to give excellent results in use.
An easel is made to go with the apparatus. This has an adjusting screw, so that its distance from the camera may be varied. The sensitive paper is fixed to this.
We shall now describe the precise method of using the apparatus. It may be used in any room which can be darkened, or rather in which there is only non actinic light.
The lamp of the apparatus is lighted. Opposite the lantern, and so that the disc of light shines full upon it, is placed the easel at a distance of a few feet, care being taken to ensure its being at right angles to the axis of the lens. A piece of white drawing-paper is fixed to it with drawing-pins. If the disc of light appears evenly illuminated we may proceed. If not, the lamp is moved farther from or nearer to the condenser (the large lens which will he found between the lamp and the groove for the negative) until an even illumination is got. The negative is now placed in its groove. We must deter mine of what size the enlargement is to be. We adjust the distance between the negative and the lens till the image is fairly sharp on the drawing-paper which is pinned to the easel. We now measure the image to see if it is the right size. If it is too large we move the easel towards the apparatus; if it is too small we move it away. We focus roughly once more, and measure again. When we have got the size we want we make a final very accurate adjustment by means of the screw of the easel till we get the image quite sharp. We may say that it is seldom that gelatine negatives will stand enlarging to more than six or eight diameters, as beyond that the texture of the emulsion itself becomes very evident. As, however, such a degree of enlargement will give even from a quarter-plate negative a print thirty-four inches by twenty-six inches, it will be seen that it is ample. An en largement of four or five diameters is generally sufficient.