Coarse and fine focus are two types of adjustments found on a microscope that are used to bring the specimen into clear focus and achieve a sharp, detailed image. They are typically controlled by separate knobs or mechanisms located near the microscope's main body or on its sides.
Coarse Focus: The coarse focus adjustment is the larger and more prominent of the two knobs. It is used for the initial focusing of the specimen. When you are trying to locate or bring a specimen into approximate focus, the coarse focus knob is used. This adjustment moves the stage (where the specimen is placed) up and down rapidly, allowing for large changes in the distance between the objective lens and the specimen. By using the coarse focus knob, you can quickly move the stage closer to or farther away from the objective lens to get the specimen into view.
Fine Focus: The fine focus adjustment is the smaller and more delicate knob located near the coarse focus knob. Once you have used the coarse focus to bring the specimen into view, the fine focus knob is used for precise focusing. This adjustment moves the stage up and down very slowly and in smaller increments. It allows you to make fine adjustments to the distance between the objective lens and the specimen, which is crucial for obtaining a sharp and detailed image.
The reason for having both coarse and fine focus adjustments is that a microscope's high magnification can create a very narrow depth of field. This means that only a small portion of the specimen is in sharp focus at any given time. The coarse focus helps you quickly bring the specimen into the general focal range, while the fine focus allows you to fine-tune the focus to capture specific details of interest.
When using a microscope, it's essential to start with the coarse focus to bring the specimen into view, and then switch to the fine focus to achieve the best clarity and detail. It's also essential to use the appropriate magnification (objective lens) for the level of detail you wish to observe.