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.
On The Selection Of Lenses. When it is desired to use a photographic lens for more than one kind of work, the selection of the most suitable objective becomes a matter of careful thought. As to the so-called "all-around lens," so much desired by the great majority of amateurs, only instruments of moderate rapidity come into consideration. This is due to the fact that lenses of a rapidity exceeding f.6.3 cease to be useful for wide-angle views, even when worked with small diaphragm openings.
779. The longer the focal length of a lens the more decided are its special characteristics. Consequently, long focus lenses lack adaptability to a variety of purposes - whatever their rapidity may be. A lens for all-round photographic work should not exceed 8 to 10 inches in focal length, which limits the size of the camera to a 6 1/2 x 8 1/2 outfit. Applying these considerations to actual practice we find that an outfit suitable to fill the greatest possible number of uses should preferably consist of a camera 5 x 7 inches, or 6 1/2 x 8 1/2 inches, with an f.6.8 lens, either of 8 1/4 or 9 1/2 inches focus. When long range work is a factor of importance, it would be preferable to select a convertible Pantar of corresponding focal length, as the single members of these objectives are particularly suitable for long range views. When the requirements go beyond the possibilities of such equipment, they are no longer to be considered as part of a variety of purposes, but lead to the use of special instruments.
781. In portraiture great rapidity of the lens is, of course, an important point. We, therefore, select the fast lenses for this work, such as an f.4.5 or f.5.5. The covering power of these lenses is so great that it needs no special consideration; but attention must be given to the choice of the most suitable focal length. The size of the portrait desired should be the controlling factor in the selection of a portrait lens, and practice has proven that the most pleasing results are obtained by the use of a lens, the focus of which is twice the length of the largest "bust" which it is to make. For instance, on an 8 x 10 plate one can properly make a "bust" measuring 7 inches from the top of the head to the chest. A lens of 2 x 7 inches=14 inches focus, will do this at a comfortable distance, insuring good perspective. In short operating rooms it is not always possible to follow the above rule, but, wherever possible, it is very desirable to adhere to it. The degree of sharpness of definition is entirely in the control of the operator, and can be changed from the most critical sharpness to the softest diffusion.
782. For commercial work the principal condition is a fine perspective effect. This can be secured by using a lens of great focal length, so that the camera can be set up at considerable distance from the object. Owing to the great depth of focus required in such case, and therefore the compulsory use of small diaphragms, it is not desirable to employ extra fast lenses for such work. An f.6.8 lens is perfectly suitable, as it permits of a sufficiently large aperture for easy focusing, and will produce the maximum possible depth of focus when stopped down to the smaller diaphragms.