The lens or mirror in a microscope, telescope, camera or other optical instrument that receives the first light rays from the object being observed is an objective in optics. The objective is as well called the object lens, object glass and objective glass. 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. Microscope objectives are characterized by two parameters namely, magnification and numerical aperture. Cheap optics produces fuzzy and/or distorted images. One usually gets what they pay for in optics, and the extra money spent usually results in sharper, more accurate images and even with extreme magnification. The ability to discern fine details is called resolution. For image systems, it is expressed as a dimension; objects separated by more than a certain dimension will be imaged as separate objects. The Numerical Aperture is one way to describe the quality of a lens. Primarily due to Optical Aberrations, actual resolution will be less than what the calculations predict. Chromatic aberration is the inability of a lens to focus different colors of light onto the same spot. The shorter the wavelength of light is, the more it will be refracted by an optical surface. Spherical aberration occurs when the edges of a lens refract more light than the center. At the area where most of the optical rays focus together, an image forms a disc, the circle of minimum confusion. Curvature of field occurs when a lens focuses on round objects. Pin Cushion and Barrel distortion occurs because when an object moves off the optical axis, the focal distance to the lens is farther. This causes image magnification errors in either a pincushion or barrel distortion. The optical aberration increases relative to the cubed power of the NA. if you increase the diameter of a lens, the theoretical resolution increases, while aberrations erode the image quality. Every optical system has a finite resolution limits, the results will be useless. Depth of Focus is the length of the area the object of interest that stays in acceptable focus. The single most influential parameter determining the DOF of a lens system is its Numerical Aperture. The focal point is a vertical line, at the object of interest. The acceptable Depth of Focus is dependent on magnification. The Numerical Aperture of the lens is stopped down by use of an aperture ring. This decreases the angle of acceptance; the rays of light enter at a shallower angle, which causes the Depth of Focus to increase. Another attribute of a lens system is its Contrast. Resolution is worthless without contrast, the ratio between the dark and the light, the number of shades. There are several mechanisms that can be used to improve contrast. Most optical systems are bright field, which makes use of absorption contrast, the same mechanism as in normal human vision. Diffraction contrast is when light hitting the edge of the object of interest bends and is diffracted out of the optical path. This is the mechanism that enables Dark Field optical systems. Another contributor to the performance of a lens system is its Illumination System. The higher the magnification, the more light is required.