Which magnifications require focusing with the course knob according to the procedure?

• A. 4x and 10x
• B. 4x
• C. 10x
• D. 40x

Answer: (A)

Using the coarse adjustment knob lets you bring the slide into rough focus quickly, which is most practical at low magnifications. At 4x and 10x, you can move the stage a larger distance to find the general plane of focus, and the depth of field is relatively large, making this quick adjustment effective. When you switch to higher magnifications like 40x, the depth of field becomes very shallow, and small changes matter a lot; the procedure recommends using only the fine adjustment to sharpen the image without risking crashing the objective into the slide. So, the procedure calls for coarse focusing at the two low-power settings (4x and 10x) and reserve fine adjustment for the higher magnification.