Objectives

A variety of microscope objectives are available. All objectives use lenses to focus light. Light is broken down into various wavelengths (colors) as it travels through a lens. The various wavelengths have different focal points. That means that red, green and blue appears to focus at different points. This is called chromatic aberration. Spherical aberrations are focal mismatches caused by the shape of the lens. Quality lenses are designed correct for chromatic and spherical aberration to bring the primary colors to a common focal point. These terms may help you determine the best objective for your application:

• Achromatic objectives–This objective brings red and blue light to a common focus, and is corrected for spherical aberrations for green. It is excellent for black and white viewing. If an objective is not labeled, it is achromatic.
• Fluorite or semi-apochromat objectives–These lenses are chromatically corrected for red and blue, and the green focus is also close. They are spherically corrected for blue and green. This objective is better suited for color viewing or recording than achromatic objectives.
• Apochromatic objective–This is the most expensive objective. It is chromatically adjusted for four colors (deep blue, blue, green and red) and spherically corrected for deep blue, blue and sometimes green. This is the best choice for color viewing. These have a higher numerical aperture (N.A.) than achromats or fluorites.
• Plan objective–These objectives produces a flat image across the field of view. The three objectives discussed above all produce a curved image. A plan-achromat, plan-fluorite or plan-apochromat are corrected.
• Infinity Correction–When measuring from the back end of the objective to the primary focal plane, many microscopes are limited to a specific distance (160mm). More expensive microscope use a different series of lenses, prisms and mirrors to allow for an "infinite" distance between those two points. This is called infinity correction.