The objectives are the most important component of an optical microscope because they determine the quality of images. There’re many types of objectives, achromatic, semi-plan, plan-achromatic and apochromatic are the most popular but some brands use their own description to identify best quality objectives like for Motic use EF Plan, etc…
The properties of each objective’s lens refer to aberrations or defects.
Chromatic aberration: color defect
Spherical aberration: shape defect on the image produced by the curve of lens.
Achromatic : the lens only correct the chromatic aberration, colors red and blue, but still give spherical aberration. It is the cheapest type lens most used in microscope construction.
Semi-plan : the lens corrects the chromatic aberration (red & blue) and spherical aberration. It is made of 2 lens, one concave and one convex.
E-Plan: deliver long working distances, high numerical aperture and flat images over the entire field of view with virtually no curvature of field. (E-Plan, Nikon nomenclature).
Plan-Achromatic : the lens corrects the chromatic aberration as well as provide excellent plan images without spherical aberration. It is usually made of 1 len without curve.
Plan-Apochromatic: is the best type of lens, corrects all chromatic aberrations: red, blue and green as well as spherical aberration, provides a plan image and wider visual field of view. It is used on lens of + 22mm field view or superior. (Zeiss has 25mm plan-apo)
Infinity Optical System: In modern and research microscopes the objective is designed to focuses an image to infinity. This permits to insert additional accessories between the stand and head of microscope like epi-fluorescence set.
Numerical Aperture (N.A.): This number is imprinted on the objective lens. It is a measure of the resolving power of the objective (how fine a detail can be seen). The condenser aperture diaphragm should be adjusted to the same value of the N.A. of the objective, to obtain the best results
LWD or ULWD: These abbreviations stand for “long working distance” or “ultra-long working distance”. These objectives are able to work with a large specimen-objective distance and are used for specific applications. Mainly on inverted microscopes but also other applications like metallurgical.