This section is from the book "The Boy Mechanic Vol. 2 1000 Things for Boys to Do", by Popular Mechanics Co.. Also available from Amazon: The Boy Mechanic, Vol2: 1000 Things for Boys to Do.
It is not necessary to purchase an expensive telephoto lens for a box or hand camera if the owner has a pair of opera or field glasses. First focus the glasses on the distant object to be photographed and then set the camera. One of the glasses is placed directly in line with and in front of the camera lens, as shown in the sketch. If the camera is of the focusing type, it is focused in the ordinary manner. Box and other cameras are set as usual.
Ill: A Field Glass Placed in Front of a Camera Lens will Increase the Diameter of the Photograph
The glasses should be well supported in front of the camera lens, as any slight move will be quite perceptible on the ground glass. As the light rays are largely reduced in passing through the field glass and camera lens, it is necessary to give a much longer exposure. This can only be determined by trying it out, as lenses have different speeds. - Contributed by Charles Leonard, St. John, Can.