The first is "spherical aberration," by which is meant the property possessed by Lenses which are segments of spheres, of refracting rays of light unequally at different parts of their surfaces. The following diagram shows this:—
Observe that the dotted lines which fall upon the circumference of the Lens are brought to a focus at a point nearer to the Lens than those passing through the centre; in other words, the outside of the Lens refracts light the most powerfully. This causes a degree of confusion and indistinctness in the image, from various rays crossing, and interfering with, each other.
Spherical aberration may be avoided by increasing the convexity of the centre part of the Lens, so as to add to its refracting power at that particular point. The surface is then no longer a segment of a sphere, but of an ellipse, and refracts light more equally. The difficulty of grinding Lenses to an elliptical form however is so great, that the spherical Lens is still used, the aberration being corrected in other ways.
A second cause interfering with the distinctness of the outer portions of the image in the Camera is the obliquity of some rays proceeding from the object; in consequence of which the image has a curved form, with the concavity inwards, as may be seen by referring to the figure given at [page 53]. The following diagram is meant to explain curvature of the image.
The centre line running at right angles to the general direction of the Lens is the axis; an imaginary line, on which the Lens may be said to rotate as a wheel turns on its axle. The lines A A represent rays of light falling parallel to the axis; and the dotted lines, others which have an oblique direction; B and C show the points at which the two foci are formed. Observe that these points, although equidistant from the centre of the Lens, do not fall in the same vertical plane, and therefore they cannot both be received distinct upon the ground glass of the Camera, which would occupy the position of the perpendicular double line in the diagram. Hence it is that with most lenses, when the centre of the field has been focussed, the glass must be shifted forwards a little to define the outside sharply.
The Use of Stops in Lenses.—Curvature of the image and indistinctness of outline from spherical aberration are both remedied to a great extent by fixing in front of the Lens a diaphragm having a small central aperture. The diagram gives a sectional view of a Lens with a "stop" attached; the exact position it should occupy with reference to the Lens is a point of importance, and influences the flatness of the field.
By using a diaphragm the quantity of light admitted into the Camera is diminished in proportion to the size of the aperture. The image is therefore less brilliant, and a longer exposure of the sensitive plate is required. In other respects however the result is improved; the spherical aberration is lessened by cutting off the outside of the Lens, and a portion of the oblique rays being intercepted, the focus of the remainder is lengthened out, and the image is rendered flatter, and improved in distinctness. Hence also, when a small stop is affixed to a Lens, a variety of objects, situated at different distances, are all in focus at once; whereas, with the full aperture of the Lens, objects near at hand cannot be rendered distinct upon the ground glass at the same time with distant objects, or vice versâ.