Proceedings of the First Park Naturalists’ Training Conference, November 1 to 30, 1929



By George A. Grant

It had been known for many years that certain substances containing silver were blackened when exposed to light for some time. Later on it was found that when some of those substances were exposed for a very short time, so short in fact that no blackening could be noticed, nevertheless a change took place which caused them to darken when treated with certain chemicals. The whole art of photography is based on these observations. If we should prepare one of these silver compounds, mix it with gelatine so as to form a thick paste, spread it on a glass plate or film today, what we would have would be practically the ordinary dry plate or film.

A camera must have three elements: A light proof box, a lens to project the image, and a plate holder. We will take up the lens later. But assuming that an image has been projected through the lens from the reflected lights on an object outside the camera and thrown on the sensitive plate, this is what happens. The parts of the image containing the strongest light will make the greatest impression on the sensitive silver plate or film. The weaker lights will make a weaker impression, and so on down to the shadows which will hardly make any impression at all. If we examine the plate after this exposure is made, there will be no apparent change in it. But if we put the plate in a photographic developer we find that in a short time the image as described before will slowly make its appearance. The parts affected by light have been reduced to metallic silver. But it is not yet a negative. If we examine the film after development we will notice that the shadows or weaker parts of the image still possess a creamy-like appearance like the whole plate had before it was developed. This we must remove by fixing, which is done by immersing the plate in a solution of hypo, or more properly, thiosulphate of soda, which dissolves out the unaffected silver salts that gave the creamy appearance to the plate. The plate is now in all respects, a negative, except that all traces of hypo must be removed by washing. It is then hung up to dry.

This image does not necessarily have to be projected by a lens. If we could imagine a pinhole about 1/1500 of an inch in diameter that would admit plenty of light as well, we would have a perfect lens of any focal length, and a covering power nearly up to 180 degrees. Incidentally, pin holes up to 1/20 of an inch are sometimes used with very pleasing results.

We all know how light rays can be bent through a prism. The ordinary double convex lens is nothing but a series of an infinite number of such prisms. If the surfaces are cut to perfect spheres, the light rays will cross, or come to a focus, a certain distance from the lens, depending on the radius of the curved surfaces and the refractive index of the glass used. The trouble with this lens is that all of the light rays will not cross or come to a focus at the same point. The blues will not converge at the same place with the reds. The main fault with this lens is color or chromatic abberation, although it still suffers from many others.

This fault is removed by the so-called achromatic or meniscus achromatic lens found on the cheaper cameras by combining two different lenses, and cementing them together. This lens has very little speed or light admitting power. And besides this, it will project a square shaped image in a barrel or pincushion form on the plate depending on whether the diaphragm is in front or behind it. This is known as curvilinear distortion. It can be removed by combining two lenses of this type opposing each other with the diaphragm between, which gives us the so-called rectilinear or rapid rectilinear lens. This lens is hard to surpass as an all round pictorial lens, and is usually supplied in speeds up to F8. It still has its faults, technically. It has not the needle definition of the anastigmat. It has a saucer shaped field, i.e., objects in focus in the center of the plate will not be in focus at the edges or corners; and it sometimes has astigmatism. Also, its covering power is limited.