Objectives are available in powers of 4X, 10X, 40X and 100X. These numbers represent the rate of magnification, with 4X being the least powerful and 100X being the most powerful. For example, a 4X objective magnifies objects at four times their actual size. The objective is used with a 10X eyepiece lens to produce the desired amount of magnification. For example, an objective with the power of 4X together with an eyepiece lens of 10X power provides magnification of 40 times the object's actual size.
Air objectives, the standard objectives used in a microscope, differ in their rate of magnification. The more powerful examples that contain more lenses can require careful handling because of their fragility. They may also be more difficult to focus, requiring extra adjustment. The two or more lenses within the objective work together to provide the strength of one powerful lens. Objectives with a high rate of magnification require many weak lenses that pool their strength to obtain the higher magnifying power. An objective can be spherical or aspheric. Aspheric objectives are manufactured to not be perfectly spherical, and this special shape serves to minimize artifact, called aberration, that occurs when using spherical objectives. For example, an aberration can lead to discoloration at the edges of the field being viewed. Some air objectives are achromatic, providing only shades of gray.
An immersion objective is used when it is necessary to use the highest magnifying power available. Immersion oil is used as an optical medium through which slides are viewed. The immersion oil is the only optical medium between the front lens of the objective and the specimen. If the slide is to be stored, the oil does not need to be removed, as another viewing will prompt another drop of immersion oil. Like standard objectives, immersion types are manufactured to be aspheric, and some lenses magnify in gray shades only.
The catadioptric objective, also called a mirror objective, is a newer type of microscope objective. The catadioptric objective uses one lens along with mirrors to separate incoming light and reflect it off the mirrors. Viewing the sample as reflected from the mirrors provides a more complete view and ensures that no aberration is present. This type of objective provides high optical resolution. For example, a catadioptric objective is used to view finely etched lithographic marks used in producing integrated circuits. Catadioptric objectives are insensitive to variations in the glass index, making them ideal for wide temperature ranges. These objectives are available at a reasonable cost and are small enough to fit onto a standard microscope.