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.
Unsymmetrical Lenses. Unsymmetrical lenses, the single parts of which cannot be used independently, are Voigtländer's Heliar, Dynar and Portrait Lenses IA and I. These cannot be divided into halves or parts, but must always be used as complete lenses exactly as furnished. Preference will naturally be given the symmetrical lenses, on account of this quality of symmetry and the resulting inter-changeability of lens halves.
Depth Of Focus. If an object at a certain distance from the camera is sharply focused with a lens possessing speed, for instance, a Collinear Series III., working at f.6.8, it will be noticed that objects, both those which lie in front of and those lying behind the object focused upon, are less sharply reproduced on the ground-glass. It must be born in mind that every optical instrument can only fulfill such conditions as are not contrary to the laws of nature. Contradictory demands are often made upon instruments by photographers. We will mention as an example of requirements which it is absolutely impossible to fulfill at the same time, the frequent requests to furnish a lens which while possessing great intensity of light or rapidity also possesses a wide flat field and great depth of focus. As a matter of fact, it is unavoidably necessary in increasing the capability of a lens in the direction of rapidity, to neglect other conditions, such as depth of field and covering power.
873. It is apparent that a lens at a large opening does not reproduce with perfect sharpness all the objects contained in a certain space, but only those lying in a certain plane. The more the diaphragm opening is reduced in size the deeper apparently becomes the range of sharpness and the more distinct the reproduction of the various objects at varying distances. By stopping down, therefore, the lens gains a greater depth of field.
874. If, without changing the position of the camera, the Collinear at full aperture of f.6.8 be replaced by a Heliar of the same focal length, but with aperture of f.4.5 ,the depth will be found to have become less. But if the Heliar lens be stopped down to f.6.8, it will be seen that the depth is exactly the same as that produced by the Collinear working at f.6.8. This experiment merely proves the truth of the following physical law:
875. Under equal conditions of focal length and distance from the objects, the depth of field of a lens depends upon its aperture, and does not depend upon the type of the lens; it is impossible to increase the depth of the lens without decreasing the diaphragm opening, which means decreasing its luminosity - its speed.
876. The question often arises, how far may an object be from the point of sharp focus and still be fairly sharp itself? In this respect it is possible to lay down certain rules which generally lead to satisfactory results.
877. Let us suppose that a lens is focused on a very small luminous object, the image of which on the ground-glass is a point. If we move the ground-glass either away from or toward the lens the point widens in either case to a small circle or disk. As the ground-glass must be set farther away from the lens when the object is nearer and closer to the lens, it follows that if we focus sharply on any one point the images of the nearer points, as well as farther points, are drawn out into small circles or disks. These are called the diffusion disks, and are the measure for the indistinctness which results from the shifting of the ground-glass out of the plane of sharp focus, and for the indistinctness of objects which are not at the distance focused upon. It being impossible to focus sharply on all points, at all distances, at one and the same time, the focus must be so chosen that it will produce the best general effect in each case.
878. This question of focusing and the extent of permissible indistinctness in depth need only be discussed for lenses of short focal length, such as those to hand and small tripod cameras. Practice teaches that with lenses having focal lengths of from 4 to 10 inches, the diameter of the diffusion disk may be 1-200 of an inch and the picture still appear sharp to the eye. This figure has been arrived at merely by experience, and covers the general average of cases. At times it may be increased and at others it may have to be reduced, according to requirements. In a great many cases it will be safe to admit of an enlargement of the diffusion disk even to 1-100 of an inch, without causing any objectionable unsharpness. Depth may, therefore, be defined as that distance on either side of a sharply focused object, within which the image of a point enlarges into a small disk of from 1-100 to 1-200 of an inch. The depth is greater in a backward direction than toward the lens, and with equal relative apertures the depth is greater in lenses of shorter focal lengths than with larger ones.
879. Theoretically, lack of sharpness in certain parts of the picture is a great drawback. In reality, however, if we eliminate special cases, such as architectural photography, photo-grammetrical, tele-photographs, etc., the lack of sharpness in depth is of no great importance in lenses of great rapidity. On the contrary, the very lack of sharpness is frequently an aid in the making of pleasing pictures, helping, as it does, to tone down the minute definition which is so objectionable in many photographs, and giving the atmospheric effect so necessary in the rendition of a pictorial subject. It may even be said that lenses of great rapidity are a valuable aid in the distribution of distinctness and indistinctness in a picture, and in imparting to it a feeling of depth, roundness and life, in contrast to the monotony of a photograph possessing extreme definition. Always use the largest possible aperture consistent with general effect - do not stop down unnecessarily.
880. According to what has just been said, the only proper way of testing a lens for sharpness of definition is to examine a picture of an object located in a plane, the latter being vertical to the axis of a lens. This will eliminate indistinctness of depth and permit of correct examination of the sharpness of the image focused upon. Do not draw the conclusion that the lens will not give definition because it fails to produce the subject, foreground and background sharp at the same time.