What Is the Coarse Focus on a Microscope?

By Neal Litherland

The microscope is one of the most common instruments currently in use for optical enhancement. While advanced microscopes work through the interpretation of radiation, simpler microscopes use a lens or combination of lenses to bring smaller objects into a larger focus. One of the key components of any microscope is the coarse focus.

Purpose

The coarse focus on a microscope is used to view the subject more clearly. While the larger of the two focuses, the coarse focus could be compared to a hammer and chisel, whereas the fine focus would be a smaller file. The coarse focus brings the subject into view, and the fine focus brings the subject into sharper detail.

Knobs

The coarse focus is adjusted by turning a knob on the side of the microscope. There are most often two knobs, a larger and a smaller one. The larger one is for the coarse adjustment and the smaller one is for the fine adjustment. The size difference is mostly so that a person won't have to look away from the viewer in order to tell which knob she is turning.

Stage Movement

One method of coarse focus is for the knob to move the stage closer or farther away from the microscope's lens. The stage is the area on which the specimen or sample is held, usually on a slide. Many microscopes that use this method have a safety measure built in so that the stage will not press against the lens, but this should be checked for each individual microscope.

Lens Movement

Other varieties of microscopes have a completely stationary stage. This means that when the coarse adjustment knob is turned, the top of the microscope moves down and brings the lens closer to the stage. Once again, there should be some measure that stops the lens from physically touching the stage or specimen when the coarse adjustment knob is turned too far, but this is different for each microscope.

Fine Adjustment

Once the coarse adjustment knob has brought the subject into the best focus that it can, the fine adjustment knob is turned. The fine adjustment knob focuses the lenses in smaller increments and is a much more delicate adjuster than the coarse knob. This knob is the last stage of focusing on the subject so that it may be examined closely.

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