The coarse adjustment knob should never be used on high power objective lenses because it moves the microscope stage or lens rapidly and over a larger distance, which can cause the objective lens to crash into the slide. This can damage both the slide and the lens. High power objectives have very little working distance, so even small movements can cause collisions. Instead, the fine adjustment knob should be used with high power lenses, as it allows for very small, precise focusing adjustments without risking damage. Additionally, coarse focusing at high magnification can cause loss of focus or difficulty in achieving detailed focus on the specimen. Starting focus should be done at low power with the coarse knob to prevent these issues.

Key Reasons to Avoid Coarse Adjustment on High Power

• Risk of damaging the slide and objective lens by collision due to large, fast movements of coarse knob.

• Loss of fine focus control required for high magnification detail.

• High power objectives have less working distance; small adjustments are crucial.

• Fine adjustment knob allows delicate, precise focusing essential at high magnifications.

Thus, the coarse adjustment knob is reserved for initial focusing at low power, and the fine adjustment knob is exclusively used for focusing at higher magnifications to protect the equipment and specimen and to achieve clear, detailed images.