An objective in optics is the lens or mirror in a microscope, telescope, camera or other optical instrument that receives the first light rays from the object being observed. Microscope objectives are typically designed to be parfocal, which means that when one changes from one lens to another on a microscope, the sample stays in focus. The former typically ranges from 5X to 100X while the latter ranges from 0.14 to 0.7, corresponding to focal lengths of about 40 to 2 mm, respectively. The objective is specially designed and has to be immersed in refractive index matching oil. This figure depicts a typical 60x plan apochromat objective, including common engravings that contain all of the specifications necessary to determine what the objective is designed for and the conditions necessary for proper use. Microscope manufacturers offer a wide range of objective designs to meet the performance needs of specialized imaging methods, to compensate for cover glass thickness variations, and to increase the effective working distance of the objective. Finite microscope objectives are designed to project a diffraction-limited image at a fixed plane, which is dictated by the microscope tube length and located at a pre-specified distance from the rear focal plane of the objective. Microscope objectives are usually designed to be used with a specific group of oculars and/or tube lenses strategically placed to assist in the removal of residual optical errors. Newer microscopes have objectives that are fully corrected and do not require additional corrections from the eyepieces or tube lenses. Most manufacturers have now transitioned to infinity-corrected objectives that project emerging rays in parallel bundles from every azimuth to infinity. These objectives require a tube lens in the light path to bring the image into focus at the intermediate image plane. Infinity-corrected and finite-tube length microscope objectives are not interchangeable and must be matched not only to a specific type of microscope, but often to a particular microscope from a single manufacturer. Objectives usually contain an inscription denoting the tube focal length correction as will be discussed. There is a wealth of information inscribed on the barrel of each objective, which can be broken down into several categories.  These include the linear magnification, numerical aperture value, optical corrections, microscope body tube length, the type of medium the objective is designed for, and other critical factors in deciding if the objective will perform as needed. A more detailed discussion of these properties is provided below and in links to other pages dealing with specific issues. Manufacturer is the name of the objective manufacturer is almost always included on the objective. Linear Magnification is 60x, although the manufacturers produce objectives ranging in linear magnification from 0.5x to 250x with many sizes in between. Optical Corrections are usually listed as Achro and Achromat, as Fl, Fluar, Fluor, Neofluar, or Fluotar for better spherical and chromatic corrections, and as Apo for the highest degree of correction for spherical and chromatic aberrations. Field curvature corrections are abbreviated Plan, Pl, EF, Achroplan, Plan Apo, or Plano. Other common abbreviations are ICS and UIS, N and NPL, Ultrafluar, and CF and CFI (chrome-free; chrome-free infinity). The objective in the illustration is a plan apochromat that enjoys the highest degree of optical correction.