How do coarse-focus and fine-focus controls differ on a microscope?

On a compound microscope, focusing is achieved by adjusting the distance between the objective lens and the specimen. While both coarse and fine controls are used to achieve a clear image, they differ in their precision and purpose:12

• Coarse-focus control: This mechanism is designed for rapid, large-scale vertical movement of the microscope's body tube or stage. It is primarily used to bring the specimen into the general field of view and approximate focus.
• Fine-focus control: As the magnification increases, the depth of focus decreases—sometimes to as little as 1 or 2 micrometres—requiring extremely stable and precise adjustments. The fine-focus control allows for these minute, incremental movements necessary to sharpen the image and resolve fine details that the coarse adjustment cannot achieve.1

Together, these focusing controls ensure that the optical system can maintain the necessary alignment and clarity required for high-magnification observation.12