Objective lens

The microscope lens nearest the specimen that gathers light and forms the primary image. Its optical design sets key limits on magnification, resolution, and working distance.

Numerical aperture: A dimensionless measure of an optical system’s ability to accept light and resolve fine detail. It governs the objective’s light-gathering power and achievable resolution.
Compound microscope: A microscope that uses an objective and an eyepiece or camera system to produce a magnified image. The objective forms the primary image that the rest of this instrument relays or magnifies.
Bright-field microscopy: A microscopy method that forms contrast mainly from light transmitted through a specimen against a bright background. Standard objectives are commonly used to collect transmitted light in this basic imaging mode.
Achromatic objective: A microscope objective corrected for chromatic aberration at two wavelengths and spherical aberration at one wavelength. It offers basic color correction, unlike more extensively corrected objective designs.
Oil immersion: A microscopy technique that places immersion oil between the specimen cover glass and an oil-compatible objective. Oil reduces refraction at the cover glass, allowing higher numerical aperture.
Eyepiece: The microscope optical component through which an observer views the image formed by the objective. It magnifies or relays the primary image created by the objective.
Fluorescence microscopy: A microscopy method that detects light emitted by fluorescent molecules after optical excitation. Objectives must transmit excitation and emission wavelengths while collecting weak fluorescence.
Apochromatic objective: A microscope objective corrected for chromatic aberration at three or more wavelengths and spherical aberration at two wavelengths. Its broader color correction suits demanding imaging but typically costs more.
Working distance: The distance between an objective’s front lens and the specimen when the specimen is in focus. Objective designs trade this clearance against magnification and numerical aperture.
Tube lens: A lens that focuses light from an infinity-corrected microscope objective to form an intermediate image. Infinity-corrected objectives require a tube lens to complete image formation.

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