IN order to take a photograph we use a lens which forms an image of the object we want to photograph upon the film. The simplest lens which we could use would be a small hole. Suppose that we take a sheet of cardboard and make a hole in it with a pin, and then, in a darkened room, hold the cardboard between a sheet of white paper and an electric lamp; we shall see on the paper an image of the lamp filament.
The diagram shows how this image is produced. A ray of light from each portion of the filament passes through the pinhole and forms a spot of light on the paper, and all these spots joining together form the image of the filament.
If we take the lens out of a camera and replace it by a thin piece of metal pierced with a hole made by a needle (a No. 10 sewing needle is about right, and the edges of the hole must be beveled off so that they are sharp), then we can take excellent photographs by giving sufficient exposure.
If the pinhole is about six inches from the film then an exposure of about one minute for an outdoor picture on film will be required. It is necessary, of course, to make a well fitting cap for the lens aperture so that no light will get in except through the pinhole, and also to make a cover for the pinhole to act as a shutter for exposing.
But if a pinhole were the only means of forming an image it is very improbable that photography would ever have been developed, since the exposures are so long in consequence of the small amount of light which can pass through the pinhole.
Fig. 11. How an Image is Produced.
In order to get more light we could try making the pinhole larger, but the effect of this is to make the image very indistinct, and even the smallest efficient pinhole can not give as sharp an image as a good lens.
Suppose we have a small pinhole forming an image of a star, as shown in Fig. 12.
If we make the hole larger, we shall get a round, spreading beam of light and no longer get a sharp image. (Fig. 13.)
What we need, if we are to use the large hole is, some means of bending the light so that all the light reaching the hole from the star is joined again in a sharp image of the star on the screen, as shown in Fig. 14.
If a ray of light falls on a piece of glass so that it is not perpendicular to it, it will be bent. There is an interesting experiment which shows this very well. Take a thick block of glass and place it so that it touches a pin (which is marked B in Fig. 15) and stick another pin (A) in the board. Now look through the glass and stick a pin (D) between your eye and the glass, and in the same line of sight as A and B, and lastly another pin
(C) touching the glass and in the same line of sight as the other three.
Take away the glass and join up the pinholes with pencil lines. You will find that the line DC is parallel to the line AB but is not in the same line; that is, the ray of light marked by the line AB was bent when it entered the glass and then bent back again when it left it, so we can bend light by means of glass.
Fig. 12. Pinhole Image of a Star.
Fig. 13. Effect of Large Pinhole.
Fig. 14. Need of Means to Bend Light.
If we take a triangular piece of glass (called a prism) we can bend a ray when it enters the glass and also more still when it leaves the glass. (Fig. 17.)
And a lens is really two prisms stuck together base to base (Fig. 18). So that if we put a lens in the hole with which we want to form an image, we can do what we wish to and make all the rays from the star come together again in the image of the star. And this is the purpose of our camera lenses, to form an image sharper than that given by the smallest pinhole and yet much brighter than any pinhole would give.
Should we place a pinhole, instead of a lens, in the front-board of our camera, we could use the same size of pinhole for making all sizes of pictures, because the image formed by a pinhole is always of the same sharpness, whether the pinhole is far from the film or close to it. If we want a large picture we must, of course, use a large camera with a long bellows, so the pinhole will be a long way from the film, while if we want a small picture we shall only need a small camera with a short bellows, so the pinhole will be near the film. But if, instead of a pinhole, we use a lens, we shall find that the lens must be placed at a certain distance from the film (depending upon its focal length and its distance from the object photographed) in order to obtain a sharp picture. If it is placed at any other distance from the film the picture will be all blurred. The reason for this is that a photographic lens bends the rays of light that pass through it so that all the light rays from a star, for instance, will meet again to form an image of the star. By placing a sheet of cardboard at the position where the rays of light meet, the image of the star will be sharp, but if we put the card either nearer to or farther from the lens, the image will be blurred into a circle of light. The distance at which the lens must be placed from the film to give a sharp image represents the "focal length" of the lens.
Fig. 15. Deflecting a Ray of Light.
Fig. 16. Path of Deflected Ray.
Fig. 17. Prism Bending a Ray.