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Why is it desirable that microscope objectives be parfocal

Most professional microscopes contain multiple objective lenses on a rotating nose piece to facilitate quick change in magnification of a slide sample. Parfocal lenses are those that are oriented in such a way that the specimen remains in focus as the objectives are rotated into place.

Microscope Components Involved In Magnification.

Microscopes tend to have two types of lenses: eyepieces and objective lenses. the eyepiece lens is the one in the eyepiece that the wearer looks through, and the objective lenses are the ones on the rotating nosepiece. Images are focused using the focus controls. A typical lens set includes a 10x eyepiece lens, or one that magnifies the image ten times, and objective lenses ranging from 4x to 100x, for a total magnification range of 40x to 1000x.

Basic Use Of Microscope

A specimen is prepared on a slide and placed on the stage, the horizontal platform directly below the objective lenses. light is projected up through the stage and onto the target to illuminate the display. The coarse and fine focus knobs are used to bring the image to the sharpest possible view.

Parfocality

When viewing a specimen through a microscope, it is easier to start viewing the slide at a lower magnification to get a broader view. this helps the operator determine where to center the slide to select a higher magnification. for this reason, it is ideal that the objectives be oriented in such a way that the slide remains in focus when changing to a higher or lower magnification level.

Definition

“Parfocal” is a term that literally means “side-to-side focus.” all objective lenses in a parfocal microscope have focal points that are all in the same plane. Parfocality is not limited to microscopes; It can also refer to lenses used in telescopes or photography.

Determining Parfocality

To determine if a microscope has parfocal objectives, a slide must be brought into focus using the highest magnification settings. the operator must then switch to a lower magnification objective to check the sharpness of focus on the slide. if the slide stays in focus with very little adjustment, the objectives are parfocal; conversely, if the slide is not in focus, the objectives are not parfocal. it is important to start with the most powerful lens because it has the narrowest range of focus.

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