The compound microscope uses two primary mechanisms to bring a specimen into focus: the coarse adjustment knob and the fine adjustment knob. The coarse adjustment knob is engineered for rapid, large-scale movements of the microscope stage or objective lenses. This speed allows a user to quickly locate the specimen and bring it into a general, rough focus. This initial adjustment sets the stage for more precise focusing.
Initial Focusing Under Low Power
The coarse adjustment knob is only used when the lowest power objective lens is in position, typically 4x or 10x magnification. This practice ensures a wide working distance, which is the physical gap between the objective lens and the slide. To begin focusing, the user first rotates the knob to bring the stage as close to the objective lens as possible. They must carefully look from the side to prevent any contact.
Once the stage is positioned near the lens, the user looks through the eyepiece and slowly turns the coarse knob to increase the distance between the slide and the objective. This gradual separation brings the specimen into a preliminary focus. The large movement increments quickly move the specimen through the focal plane, allowing the user to spot the specimen and stop turning the moment a blurry image appears. This coarse adjustment brings the specimen into the general vicinity of the sharpest focus.
Hazards of Using the Coarse Knob at High Magnification
The coarse adjustment knob must be avoided entirely once the magnification increases beyond the lowest power objective. This restriction is due to the dramatic decrease in the working distance when switching to a higher power objective, such as 40x or 100x. At these higher magnifications, the objective lens is positioned extremely close to the glass slide, leaving a minimal gap.
Using the coarse adjustment knob at this point causes the stage to move too quickly and too far, leading to a high risk of mechanical damage. The rapid, large-scale movement can cause the objective lens to physically collide with the slide, an event known as a “scope crash.” This collision can result in the glass slide cracking or shattering, which destroys the specimen and creates a safety hazard.
The objective lens itself can also be damaged, potentially scratching the delicate glass surface. The high-power lenses are particularly vulnerable because they extend closer to the specimen. Because the coarse knob is designed for large movements, it is impossible to make the minute adjustments required at high magnification without risking an impactful collision.
Transitioning to Fine Adjustment
Once the specimen is visible and roughly focused using the coarse adjustment knob under low power, the user stops touching the coarse knob for the remainder of the viewing session. The specimen should be centered in the field of view before the user rotates the nosepiece to a higher power objective. Because most modern compound microscopes are parfocal, the image will remain nearly in focus when switching to the next objective, such as the 40x. The fine adjustment knob then takes over the focusing duty, providing the necessary small, precise movements to achieve ultimate clarity. This precise control moves the stage only a fraction of a millimeter per rotation, which is necessary to bring the specimen into a perfectly sharp focus at higher magnifications.