Early Work In Photography
A Text-book For Beginners

Faels about Lenses. 53 This is simply because light rays from the candle proceed in all directions, and while the rays proceeding through the centre of the pin-hole are making a definite image, those passing through the upper part of the hole are making an- other, while those passing through the bottom and the sides of the hole are all making their own images and the result is confusion. Fig. 12. As the hole is further enlarged, the image constantly becomes brigliter, and constantly more indistinct until it is simply a circular patch of light, with no resemblance to the shape of the candle flame. This would, perhaps, be more easily and clearly understood if instead of the one large hole, four or five-pin holes are made at about a quarter inch distance from a common centre. The blurring of the image will be seen easily; and the large hole acts as an in- finite number of smaller ones. If we return to our original small pin-hole and take the camera out of doors, we shall find that an image of any fairly lighted natural object toward which the pin-hole is directed will fall (in an inverted position) upon the ground glass. If we cover our heads and the back of the camera with the focussing cloth we shall see that the image, though faintly illuminated, is clear and sharp, and if we expose a sensitive plate to this image, an impression is made which can be developed into a negative. But the amount of light passing through the pin-hole is so small that it must act for a long time (i.e. we must give a long “exposure”) in order to make a strong impression. But in photography we must face the fact that we wish to make our exposures as short as possible, especially when the objects to be photo- graphed are in motion (say express trains or unruly children). This brings us to the problem which led to the use of lenses in photography—the desire to use a big hole, in order to pass plenty of light, combined with the necessity of keeping the image " sharp " and well defined. The lack of sharpness when the rays passed through the large hole was seen to be due to the want of coincidence between