The Optical Magic Lantern Journal (July 1890)

13 The Optical Magic Lantern Journal and Photographic Enlarger. enlargement, and to estimate how near it may be | considered to be comparable with a print from an original negative of the same size, it may be worth while to examine into the philosophy of the question, and see what really takes place when, instead of placing the small negative in contact with the paper, itis placed at a distance from it, and the divergence of the rays of light is used to obtain a larger picture. With the photographic details of the process it is not | now proposed to deal, but the optical side of the question will bear examination, and some points connected with it may be found interesting, If a negative is perfectly sharp and distinct in every | portion, and the definition of objects at various distances equally good, it is surprising to observe the extent to which it will bear enlarzement. Of course, to say that itis fer/ectly sharp would be toadmit that it would bear an indefinite amount of magnification without any loss of definition, but using the word perfect in its ordinary acceptation as referring to a sharpness quite possible of ready attainment, the amount of enlargement which may be permitted is very great. On the contrary. a softness and lack of microscopic definition, which in no way detracts from the appearance of a print made from the original negative, and indeed in the opinion of many is more artistic and effective, unfits it almost entirely for any enlargement without producing a blurriness and indistinctness which is almost painful. A negative to be used for enlarging should, if possible, be taken with that object in view, and by the use of possible means rendered as sharp, clear. and welldefined as possible A small stop in the lens, and a clearing reagent in the developer, together with ample ‘time both‘in the field and in the dark room, are necessities It must not be forgotten that the picture, in any case, is simply a matter of the distribution of light, and that-in enlarging an image already made by one impression of light, upon a sensitive surface, we are calling upon a second instalment of light to do the same work. This action is not greatly different from the use of a relay in telegraphing, where a feeble current passing along the main line simply throws-a local battery in and out of circuit, and the greater ' portion of the work is.done by the strength of the local battery, while the feeble impulses along the main line must merely indicate the action. The difference which may be seen in the brilliancy of a lantern-slide when thrown upon the screen by a good or a poor light, may serve to indicate the influence which this secondary illumination has upon the enlarged image, and it is an important element in the theory of enlarging. If we take two negatives of the same subject. one quite small and the other large, and then make an enlargement from the small one to the same size as the large direct picture, they will be comparable in several ways with interesting results, Unless the focal length of the lens with which the large negative is made is just as much greater than the small lens as the difference in the size of the pictures, the depth and definition in the shadows of the small negative will be much the better, and this advantage will be retained in the enlargement. and the r reinforcement of the second instalment of light during the enlargement will briny out depth and effect from the small negative which the larze one cannot possess. Again, this effect is shown again and again by the impression which is made by a good lantern-slide upon the screen, an effect which is due not only to the actual size of the picture, but also, and very largely, to the proportional increase in the focal distance at which the picture is viewed. This question of the relation between the focal length of the lens with which a picture is taken, and : the focal distance at which it is see7z,is a matter which has hardly received the attention which it deserves, and itis of especial importance in connection with this matter of enlargements. As a matter of fact, there is but ove focal length which is correct for each picture, and that distance is the distance at which the finished picture is to be held from the eye. The object : Of a picture is to produce upon the eye the same effect as the original subject. When the photograph is taken, the rays of light from the object pass through the lens, and form the image upon the ground glass at a definite distance behind the lens. If now we make a transparency from the negative, and stand in the same position as the camera, and hold the transparency before the eve, moving it toand ’ fro, until it just coincides with the actual view beyond, it will be found that this coincidence just occurs when the distance of the transparency from the eye is equal to the focal length of the lens with which the picture has beentaken. Ifthe picture is held at any other distance, either greater or less, it may be pleasing, but it will not be correct. The normal distance of distinct vision is about twelve inches, and this is the best focal length for lenses to be used in making photographs for illustration, or to be held in the hand for €xamination. For large pictures, to be examined from a distance, however, it would be most difficult to work with lenses of long enough focus to follow this law. and it is just here that the enlargement has the advantage, since the magnification of size is also equivalent to a corresponding multiplication of the focal length, and the correct point of view is removed to a corresponding distance. while the large direct picture, which has been taken from a focal distance proportionally not so great, fails to look so natural or so effective. HENRY HARRISON SUPPLEE. 50: Apparatus at the City Exhibition. DURING the short period in which this exhibition was held. at Drapers’ Hall, a large concourse of visitors availed themselves of the opportunity of seeing the rapid strides photography has made of late, and also of inspecting the various apparatus employed in connection therewith. Ross and Co.’s display consisted of a well-stocked case containing lenses and other photographic apparatus of which they make a speciality. Perken, Son, and Rayment exhibited their magazine hand camera. their Opéiveus lenses, lanterns, cameras, shutters, and complete photographic sets being arranged in an attractive manner. A fine display of chemicals occupied the stand of Mawson and Swan. Here also were to be tound plates,