OBJECTIVE The lens in any optical system—such as a telescope, microscope, or photographic camera— which forms or projects an image either for examination by means of an eyepiece or for reception by a sensitised surface. The objectives of ordinary telescopes and microscopes are usually corrected for visual use only, and require u small allowance to be made when used for photography, so as to bring the surface of the plate into the position of sharpest focus. Supple mentary lenses for effecting this correction are sometimes fitted to telescopic objectives, and compensating eyepieces, or oculars, to micro scopic objectives. Many objectives of both classes are now made especially for photography.
The projecting lens of an optical lantern or cinematograph is usually called the objective.
The focal lengths of the microscope objectives in general use range from 3 in. to i- in., but lenses of both lower and higher powers are manu factured. These figures refer to the equivalent focal lengths and not to the distance of the lens from the object when the latter is in focus.
Objectives of less than in. focus are almost invariably immersion lenses ; that is, a medium, generally oil, fills the space between the front lens of the objective and the cover slip over the object ; by using in this way a medium of higher refractive index than air, a higher numerical aperture, and consequently increased definition, can be obtained. In some older types of lenses water was the immersion medium, but cedar oil is now almost universally used, as it is of practically the same refractive index as crown glass. The depth of focus given by microscope objectives, which is of more import ance for photographic than visual work, is extremely limited, especially with high powers. The penetration or depth of focus of a lens is approximately of its focal length ; thus the penetration of an objective of in. focal length would be about in. In photo micrography the depth of focus would be con siderably less than this, but stops can be placed in low-power microscope objectives to increase the depth of focus, and when thick sections or specimens are photographed, this plan is often necessary.
In microscopy the magnifying power of a lens refers to the number of diameters by which the image given by the objective at a distance of io in. from the back lens magnifies the object ; this is easily estimated when the focal length is known, as an objective of i in. focal length gives a magnification of ten diameters at a distance of io in. from the lens ; therefore, a * in. objective would magnify 6o diameters at the same distance. These figures represent the magnifying power of the objectives without the eyepiece, and are termed the initial power of the lens. When an eyepiece is used, the initial power of the objective multiplied by the magni fying power of the eyepiece, gives the total magnification of the lenses.
The resolving power of an objective—that is, the number of lines per inch which the lens will separate or resolve—does not depend upon its magnification, but upon the numerical aperture (which see), and a lens with a high N.A. will give better definition than a lens of higher magnifying power with a lower N.A.
Objectives with high N.A. have a small working distance between object and lens, and consequently less depth of focus. In photo micrography a good working distance and depth of definition are often extremely useful, and on this account an objective of high N.A. is not always desirable.
The best type of objective is known as the apochromat, which is expensive, and in practical work but little superior to first-class achromats, especially when colour filters are used. All modern objectives are fitted with the R.M.S. screw, that is, the worm at the back of the lens which is screwed into the body tube always has the same number of threads per inch, and any objective can be used with any microscope