This section is from the book "Complete Self-Instructing Library Of Practical Photography", by J. B. Schriever. Also available from Amazon: Complete Self-Instructing Library Of Practical Photography.
Speed. The best method of classifying lenses and the one actually adopted in practice, is based upon their power of transmitting a greater or less amount of light to the sensitive plate. This power of a lens is called its luminosity or speed, or, as will be explained later, its aperture.
866. The groups in which lenses are arranged are called series, and consist of a number of lenses of different focal lengths, but all of the same luminosity or speed (aperture). A lens of large luminosity (high speed) will produce a fully exposed picture in less time than a lens of less luminosity or speed, and this is a matter of great importance in instantaneous work, or in a case where the light is poor. Every photographic lens is provided with a mechanical device, by which the quantity of light passing through it to the plate (its speed) can be regulated. This device is called the diaphragm, and is of metal construction having an opening or aperture. The usual form is the iris diaphragm, consisting of a set of thin movable blades capable of forming various sized circular openings. The iris diaphragm is almost universally used, the only exception being in photo-mechanical work where a large variety of openings is not essential, and where for special reasons it is also desirable to use various shaped openings other than circular. In this case the diaphragm consists of separate metal plates, with fixed opening, and is introduced in the lens through a slot in the lens mount, and known as Waterhouse stops.
867. The ratio between the effective aperture and the focal length is known as the "relative aperture," or more commonly as the "speed" or "rapidity" of the lens. When speaking of the speed of a lens the greatest speed is always meant - that speed which corresponds to the largest opening of the diaphragm.
868. In a great many lenses, reducing or increasing the size of the diaphragm opening introduces a slight change in the focus and makes it necessary to focus the picture with the stop with which it is to be taken. It may be in place to mention here, that our lenses have no variation in focus, whatever the diaphragm used. Focus may be taken at full opening, with the light at its brightest, and the lens then stopped down to the desired opening, giving the greater depth of focus.
Symmetrical Lenses. Lenses are frequently classified as symmetrical and non-symmetrical. The symmetrical, or double lens, consists of two equal halves, symmetrically arranged with a diaphragm between them, each half of which can be used by itself as a complete lens. In the non-symmetrical lenses the separate parts cannot be used independently, but must be used in combination as a whole. Of our different lenses the Collinears are the best type of symmetrical lenses. Each combination or half of a Collinear lens can be employed as an individual or complete lens by itself. It can, furthermore, be united with half of any other Collinear lens of different focus, in the same series. The combination of these two halves will produce a perfect lens of a still different focus. In this manner so called sets of lenses possessing a great variety of focal lengths are formed.
870. Voigtländer Euryscopes, likewise, are symmetrical lenses.