P HOTOOTIAPII IC OPTICS.
If a room be completely darkened, and if there be made in the shutters of the window a small hole so bevelled away on each side that it shall not be in the form of a tube, we shall have on the wall opposite an inverted image of any brightly-lighted object which may be over against the window.
Here we see opposite the shutter with a hole in it a tree. From every point in the tree there passes in a straight line through the hole and against the wall oppo site a beam of light. We show in our sketch three such beams, and all three go to make up an image of the tree on the wall.
We should here have a very perfect camera obscura but for one circumstance. The hole in the shutter must have some definite size. The consequence is that the rays reflected from a point of the tree do not come to gether at a point on the wall, but spread over a disc a little larger than the hole in the shutter. If we make the the hole very small, so as to get sharp definitions, we let through so little light that the image is carcely visible at all.
What we want in this ease is a lens. The effect of plac ing a lens in the hole will he that, although the opening is of large size, the rays of light will nevertheless meet at a point instead of forming a disc. This we illustrate here.
The lens which we show here is the simplest possible, and is one which, although by its means photographs of a kind might be taken, yet is not by any means a perfect instrument. Still, however complicated a photo graphic lens may be, its sole object is to produce the effect which we have shown here,—that is to say, to bring a set of rays of light which are either diverging or parallel to meet at a point. Photographic lenses are never made of one piece of glass only, but always of two at least, various valuable qualities being thereby obtained.
We must here explain certain terms which are con tinually used with relation to lenses.
Combinations of a lens.—When a lens consists of several distinct pieces of glass, although these appear each to be only one piece, they are, as a matter of fact, built up of two or even three pieces. Each of these built-up struc
tures is called a "combination," and we speak of the front and back combinations of a lens, meaning those which are nearest and farthest from the object to be photo graphed.
Aperture is the opening of a lens which admits light. When a stop is used, it is the opening of this. Where no stop is used, it is the opening of the smallest of the combinations of the lens, or, if these be all of the same size, the front one.
Focal length is the distance between the lens and the ground glass where the image is sharply focussed. It is measured from the lens in the case of a single combina tion one, from the stop or diaphragm in a double com bination one. It is common to speak of focus instead of focal length. When we say that the focus of a lens is so many inches, it will be understood that we mean the focal length. Unless it is stated to he otherwise, it is to be understood that the focal length of a lens is measured when a distant object is focussed.
Flatness and roundness of field.—In the last diagram which we gave, we showed the various rays of light as if they all met on the flat plane of the wall. As a matter of fact, with the lens which we showed, they would not, but would all meet at points equally distant from the lens, so that a sharp image could only be got by the use of a spherical screen, could such be had. We illustrate this here. The field is said to be round. Certain of the complications found in photographic lenses are in troduced with a view to get rid of this rotuidness of field, and to give a comparativelyflatfie/d.
Distortion is produced at times by certain kinds of lenses. That known as the single achromatic, or simply the single lens (see page 81), is the only one which gives this defect to any marked extent. If such a lens be used to photograph any object which is made up of straight lines which are near to its edge, these lines will not appear quite straight in the negative, but will be slightly curved. Thus a square object will appear somewhat like the fol lowing cut.