When using the 100X objective, which focusing control should you primarily use?

• A. Only use fine knob on 100X
• B. Coarse knob only
• C. Use both knobs on 100X
• D. Never use fine knob on 100X

Answer: (A)

Focusing at high magnification requires precise, small movements. The 100X objective has a very shallow depth of field, so even a tiny shift can bring the image in or out of sharp focus. The fine adjustment knob provides those tiny, controlled changes, allowing you to achieve a crisp image without risking contact between the objective and the slide. The coarse adjustment moves the stage in larger steps and can easily push the lens into the slide or slide out of proper focus at this power, which can damage the slide or the lens. For best results, switch to high power only after the rough focus is already achieved at lower magnifications, then use only the fine knob to fine-tune the focus at 100X.