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Using a Bright Field Light Microscope

Author(s): David R. Caprette, PhD

Focusing at High Magnification

At low magnifications (up to 100x or so total magnification), you should use the coarse focus control. Not only does it take too long to move a distance with the fine control, but the limit of travel with the fine focus may be less than with the coarse. Trying to focus past the limit of travel can damage a focusing mechanism.

When you bring in a high dry objective (a high power lens which is used without oil, usually a 35x or 40x lens) with the specimen in focus, the end of the objective will approach the specimen closely. It is unwise to use the coarse objective with such a lens, because it is too easy to ram the lens into the slide. In this case, use the fine control only.

Suppose you mount your slide upside-down. You will be able to focus at 40x total magnification, and again when you go to 100x magnification by swinging in the 10x objective. However, the thickness of the slide may exceed the depth of focus with the high dry objective (35x or 40x). If so, you won't be able to focus at all. If you don't pay attention, you probably will bump the slide with the end of the objective. Good high power lenses will telescope so as to buffer such shocks, but if you reach the limit, further movement will damage the slide and also may scratch the objective, and even the exit lens of the condenser. Such damage cannot be repaired.

Because high magnification lenses come so close to the specimen, to reduce the risk of a disaster, you might want to take your eyes from the eyepieces and instead watch the lens as you rotate it carefully into place. Until you are used to your microscope, you should check the position of the lens frequently while focusing, or (better) have someone else watch the objective and warn you if it contacts the slide.