A GOOD photographic lens, as was explained above, consists of several single lenses of varying refractive and dispersive powers. We must now consider why it is necessary to use several single lenses to obtain a perfect image. A perfect image cannot be formed by a single lens, since such a lens does not cause all the light rays coming from a given point on the object to pass through a single focal point. This defect in the action of a single lens is called Aberration, and can be considered under two headings, viz. (i) Chromatic Aberration, (ii) Spherical Aberration. The image formed has additional defects which are considered under the various headings below.
This defect is caused by the splitting up of a ray of white light into its components when it is refracted. It has already been illustrated in Fig. 7. When the axial cone of white light AB, Fig. 15, is incident on the lens LL', it is obvious that the various coloured components of the ray are brought to different foci. These foci are shown at exaggerated distances apart in the figure, V being the point where the violet rays cut the axis, B the point where the blue rays cut, and so on. If a focussing screen is placed at R it will be in the focal plane of the red rays, so that there will be a sharply defined red image. This image has, however, violet, blue, etc., images superposed, and these images are blurred. Now, in the early days of photography, the blue rays were the rays which caused chemical action in the photographic plate, and the red and orange rays caused no chemical action at all. If, therefore, a sensitive plate was placed at v, a sharply defined image should have been formed on it. The rays which affect the eye are the yellow, orange, etc., so that, when focussing, the screen would be placed at O. It is, therefore, necessary to shift the screen through the distance ov, such distance being easily calculated. This distance is known as the correction for chemical focus. In these days of panchromatic plates all the rays would cause chemical action, so that the resulting image would be blurred.
This defect is due to the shape of the single lens; the surfaces of a lens are spherical, it is therefore called Spherical Aberration. In Fig. 16 a cylinder of rays, aba'b', co-axial with the principal axis, is shown. The rays AB, a'b' are brought to a focus at F2, but the rays CD, C'D' are brought to a focus at f'. At the position F3, between F' and F2, the circles of confusion will have a minimum size. If all the rays parallel to the principal axis are considered, it is possible to find a point F where the circles of confusion due to all the rays have the minimum effect. This is the point where the focussing screen should be placed. The amount of Spherical Aberration present is measured by the distance between the foci for marginal and axial rays, i.e. F'F2 in Fig. 16. For a positive lens, as in this figure, it is considered positive, and for a negative lens, negative. Chromatic Aberration must not be overlooked as a factor when dealing with Spherical Aberration.
We must now consider the effect of rays passing obliquely through the central portion of a single lens, and also the effect of those passing obliquely through its marginal portions. The effect of these rays is best considered separately. In Fig. 17 a small stop has been placed in front of the lens and the oblique excentric rays are excluded. Now it can be shown that the focal length of a lens for centric pencils varies with the obliquity of the incident pencil; as the obliquity increases, the focal length decreases. If we consider a plane object, AB, Fig. 17, at right angles to the principal axis, the rays from the point C will be focussed at c (where the circle of confusion is least), and the rays from B at b which is nearer the lens. The focal surface is therefore curved.
We will once more refer to Fig. 17 and consider rays from the middle point, D, of AC. These rays are brought to a focus at d, but d is not midway between ac. With the stop in the position shown the distance of d from c is greater than its distance from a. The image is therefore distorted.
This error is caused by the rays passing through the marginal portions of the lens, and only occurs when the lens is used with a fairly large stop. The nature of this defect is shown in Fig. 18 in which is shown part of a pencil of rays after refraction. The refracted rays pass through two mutually perpendicular lines ad, cd, known respectively as the First and Second Focal Lines. Between these lines at some position F, a section through the pencil is approximately circular. This section is called the Circle of Least Confusion, and is the position for the focussing screen. The circles of least confusion for a series of pencils lie on a curved surface, and this must also be considered with the curvature of field caused by oblique centric pencils. Astigmatism is measured by the distance between the lines ab, cd.
The various defects of a single lens having been indicated, we will see how they can be remedied.