Photography in the Studio and in the Field / A Practical Manual Designed as a Companion Alike to the Professional and the Amateur Photographer

by Edward M. Estabrooke

The design in forming lenses is to procure a medium through which the rays of light from any object may pass and converge to a corresponding point beyond; the manner in which the rays proceed through the lens, and then centre in a focal point, will depend on the form of the lens, its capacity for refraction and the distance of the object.

The double convex lens may be viewed as a portion cut out of the side of a sphere. Here, as in all cases of convexity, the focus of the parallel rays passing through the lens is at the centre of the sphere. (See fig. 3.)

FIG. 3.

A plano-convex has only half of the refractive power of the double convex; the parallel rays, falling on the convex side of the lens, would converge at the distance of the whole diameter of the sphere. Thus the focal point at which the rays of light converge is always regulated by the degree of curvature of the lens. Thus the double convex lens has the greatest power of converging the rays of light; the plano-convex has only half the power of the former. Both these lenses have also the power p89 of magnifying the image of an object seen through them in the same proportion.

The double concave and the plano-concave have the power of dispersing the rays of light and of diminishing the image of an object seen through them in the same proportion.

FIG. 4.

The meniscus lens has but a very slightly dispersive power, and the concavo-convex merely separates the parallel rays to the thickness of the lens and sends them on parallel as they entered.

All these lenses, having something of the prism in their shape, have the power to a greater or lesser extent of decomposing the light that passes through them. This is called chromatic aberration, because the colored rays do not all converge to the same focus; thus the image seen through them is surrounded by a fringe or border of color.