Towler, John. The Silver Sunbeam. Joseph H. Ladd, New York: 1864. Electronic edition prepared from facsimile edition of Morgan and Morgan, Inc., Hastings-on-Hudson, New York. Second printing, Feb. 1974. ISBN 871000-005-9

Chapter V.

THE camera obscures was the invention of Porta,* a Neapolitan; this instrument is, in fact, a miniature glass-house, a conjugate glass-house, which admits no light but that which passes through the lens. The ground-glass is the screen, which must be at right angles, and slide at right angles with the axis of the lens. The model, therefore, or sitter, must likewise be so arranged that the various component parts that have to appear in the picture shall be as much as possible in a plane perpendicular to the optical axis. In this case, it becomes the duty of the photographic artist, as soon as his model is gracefully and compactly arranged, to fix upon the point which is to be the center of the picture, as, for instance, the eye of the sitter, then to reconnoiter the ground, and examine the inclination of the different parts of the figure forming the visible surface, and to ascertain the direction of a line drawn from the eye at right angles to this surface; now bring the camera, raise it and incline it until the axis of the lens coincides with this previously determined direction. In this position, it will be possible to obtain a picture in which the different parts are almost equally in focus. Before you begin to obtain the focus on the ground-glass, fix the lens in its brass slide in the middle of its motion by the rack and pinion. Next move the bellows slide of the camera until the image on the glass is distinct, and clamp the slide; finally obtain a sharp focus by means of the thumb-screw on the pinion-wheel. With a quick motion backward and forward of the lens, the point of sharpest definition can easily be descried with the naked eye, as long as the image is much smaller than the object; but in copying photographs or engravings, where the picture is to be of equal size with the original, it is not easy to obtain the exact focus; in this case the microscope is called into requisition. The first thing to be done, where this difficulty exists, is to hunt about upon. the original photograph or engraving for some distinct landmark, as a very minute circle, or a couple of lines in apparent juxtaposition, or the opening in the letter a or o, or the extreme lines on the sides of a blade of grass; the space between these will become very manifest under the microscope, and by a sweep of the lens backward and forward, the boundary-lines can be designated when most sharp. It requires much practice to focus well in copying; hence it is that few photographers are good copyists. The microscope suitable for such purposes may be a common magnifying-glass, the front lens of one of the stereoscopic tubes, or a compound microscope of low power. An error in the focal distance of one sixteenth of an inch, in portraiture, is scarcely perceptible; whereas the same amount of error in copying will produce a total failure in the negative or positive. In taking a view, and in copying, it is frequently a plan to be recommended, to focus a point midway between the center of the picture and the outside. This is said to equalize the definition; it is essentially a means of dividing the error of spherical or chromatic aberration, where either exists. The eye of the sitter may regard some fixed point on a level with its direction; care must be taken that it is neither raised nor depressed nor in any way strained. By looking at some point on the camera, which is situated in the darkest part of the glass-house, the eyes will be able to remain quite at ease, even whilst steadfastly gazing at this point; if, however, the sight were directed to a point brilliantly lighted, the eyelids would involuntarily close, and the pupil contract, by which the picture would be impaired.

The photography of architecture and of landscapes requires absolutely that the camera be horizontal, and so does that of card-pictures, when the whole figure is comprehended, in order to avoid the pyramidal inclination of parts which in nature are parallel. This pyramidal distortion is the consequence of the obliquity of the rays as they are thus made to enter the lens, and for which obliquity the lens has not been corrected. On account of the large angle which a card-picture must necessarily comprehend, a long-focussed lens is preferred, much longer than is required for taking a portrait at the same distance. It is a frequent occurrence to those who occupy themselves with out-door photography not to be able to comprehend certain very desirable elevations within the compass allotted to the photograph without inclining the tube upward; but the tube must remain horizontal; therefore the only alternative remaining is to raise the camera upon a platform or to place it on a window-sill, on the roof of a house, on the branch of a tree, or on the spokes of two ladders, tied or hinged at the top, and with the feet drawn out so as to form a large base between them. Lenses with large aperture are exceedingly useful in such cases, as, for instance, in taking views of churches, public buildings, etc., from the opposite side of the street. The great desideratum has been to find a lens of short focus and large angle for such sort of work, which can not be performed with lenses of long focus and small aperture.

If the objects in the foreground of a view, as is the case with a stereograph, are to be the principal items of attention, the lens will have to be focussed either upon the central object or upon one intermediate between the center and the edge. In this case, unless the difference between the focus of parallel rays and the focus at an infinite distance be exceedingly small, almost all remote objects will be slightly out of focus, and the picture in the distant background will be defective. To counteract this effect, a much larger lens is employed, which is carried to some distance from the principal objects, until the picture be of the same size as was intended to be taken with the lens of shorter focus. The camera, too, in such a case, must be raised above the horizon, but focussed parallel to it. The scenery in close proximity can be thus excluded, and the distant view will be nearly equally well defined and in true perspective. A small view taken in this manner can be enlarged afterward either into a negative or positive, as may be required, by the method which is fully explained hereafter.

There are certain rules to be observed in field-photography in reference to the light, as in room-photography.

The first is, not to place the axis of the camera in the same straight line with the sun and the object. This means that a picture is not to be taken in the direction of the sun's rays, where the front and central objects are equally illumined, and consequently must be very flat in the photograph; it would be equally absurd to attempt a picture in the shade, whilst the sun is shining, as it were, into the camera through the lens.

An inclination of the axis of the camera with the direction of the sun's light, to the amount of forty-five degrees, will produce an agreeable contrast of light and shade. It is very possible and very probable that such an illumination from the unobscured rays will produce too strong a contrast, and thus give rise to a very hard picture. The best effects are attained when the sun is obscured by a white cloud; the lights and shades still exist with the addition of decided middle tints, giving the photograph the appearance of an artistic production.

With these recommendations in view, the photographer must visit the ground previously to his taking a picture, in order to ascertain at what time of the day the light falls upon it, or can fall upon it, so as to produce the best photographic illumination; this sort of proceeding distinguishes the artist from the operator, and gives the same distinction to his work. It may happen that the principal object in a landscape, which it is required to photograph, is so situated as not to receive the direct light of the sun, as is the case with many northern aspects. The artist, in such a case, will have to wait for a cloudy day, when the direct light of the sun can produce no real shadows, and when perhaps a white cloud in the north-cast or north-west may be found to make sufficient contrast.

Cameras for lenses of short focus can be roughly adjusted to focus by means of the bellows-slide, and afterward finely adjusted with the thumb-screw on the lens; but when the focus is long, the thumb-screw is useless, unless attached to a long lever, as was formerly used in the Lucernal microscope; in such cameras, the bellows-slide has a rough or quick motion, and a slow or fine motion by means of a thumb-screw in front of the operator or on the posterior part of the slide. Such cameras, too, by reason of their length, have to be supported on two camera-stands, in order to make them rigid.


* Porta, Giovanni Battiste Della, was born at Naples, in 1540.