Microscope Optics

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Understanding the Microscope Objectives and Infinity Optics

What is a microscope objective lens?

A compound microscope consists of a group of lenses (called objective) which focuses a real image of the object inside the microscope. A second group of lenses (called eyepiece) magnifies this image and projects it on the retina of the eye. This compound optical system (objective – eyepiece) together with other mechanical components are the basics of a modern microscope. The overall resolution and image quality of a microscope is determined by the quality of the optics and the light source.

There are four types of objectives:

Achromatic – Built up out of one or two lenses and has about 60% of focused area across the center.

Semi plan – Can either be built up out of two lens element achromats or three or more element apochromats and has about 80% of focused area across the center. EPL on objectives stands for Eco Plan or economical plan and is comparable to semi plan.

Plan – Corrects better for color and spherical aberration than either the semi plan or the achromatic objective. Plan objectives have a flat field around the center of close to 100% of the image and give flatter fields and slightly higher resolving power (details). Plan objectives are supplied to middle and high-end laboratories.

Semi-APO – Additional lenses built-in to improve corrections even further, providing a superior image. While they give the best image, they also are the most expensive. Semi-APO objectives are used by high end users where extreme quality of images and details are required.

Although all four microscope objective types will correct for achromatic errors (color errors), there is no achromatic in the name of semi plan, plan, and semi-APO. The difference between the lenses is the focusing area which can be seen from the eyepieces when using the objective lenses. The difference at the low power lenses are smaller than at the higher 40/60/100x lenses

What is an infinity corrected optical system?

Microscopes with infinity corrected optical systems have infinity image distance. These systems are designed by placing a tube lens within the body tube between the objective and the eyepieces, resulting in an intermediate image. This optical system permits optical accessories such as illuminators to be placed into the light path between the objective and the tube lens (also known as the infinite space) with minimal effect on focus and increase the tube length to greater than 160 mm. Infinity corrected optics are used in research laboratory microscopes and industrial metallurgical microscopes.

What is infinity optics?

Infinity optics is the production of a flux parallel light rays after passing through the objective. This is different than the infinity space.

About the tube length

The tube length, or focal length, in infinity corrected optical systems ranges from 160 to 200 mm. The Royal Microscopical Society standardized microscope tube length at 160 mm during the nineteenth century. This was the standard until the 1980s when infinity corrected optics were introduced. To adjust for this change, manufacturers needed to place additional optical elements into the accessories to restore the 160 mm tube length, resulting in reduced light and increased magnification. Microscopes without infinity correct optics have a specified tube length of 160 mm.

Benefits of infinity space

When accessories are added into the optical path, parfocality between different objectives can still be maintained. Accessories can be designed to produce 1x magnification without altering alignment between the objective and tube lens. Optical accessories placed into the infinity space do not shift the location or the focal point of the image. This allows one to use a combination of optical techniques to compare specimens.