To focus a microscope using the high power lens, you must first achieve focus under a lower power objective, then refine the focus using only the fine adjustment knob.
Here's a step-by-step breakdown:
-
Start with Low Power: Begin by placing your specimen slide on the microscope stage and securing it. Rotate the objective turret to select the lowest power objective lens (usually 4x or 10x).
-
Initial Focusing: Use the coarse adjustment knob to bring the stage up (or the objective lens down) until the specimen is roughly in focus. Then, use the coarse and fine adjustment knobs to get the sharpest possible image at this low power.
-
Center the Specimen: Carefully center the specific area of the specimen you want to view under high power in the field of view. This is crucial because the field of view decreases significantly with higher magnification.
-
Switch to High Power: Rotate the objective turret to the high power lens (typically 40x). Do not adjust the coarse focus knob at this stage.
-
Fine Adjustment Only: The image will likely be close to being in focus. Use only the fine adjustment knob to bring the specimen into sharp, clear focus. Because the depth of field is very shallow at high power, even small adjustments to the fine focus can make a significant difference.
Important Considerations:
- Parfocal Microscopes: Most modern microscopes are parfocal, meaning that when you switch between objective lenses, the specimen remains approximately in focus. This minimizes the amount of refocusing required.
- Do Not Use Coarse Focus: Using the coarse focus knob at high power can easily cause the objective lens to hit the slide, potentially damaging both the lens and the slide.
- Oil Immersion: For even higher magnification (e.g., 100x), an oil immersion objective is used. This requires placing a drop of immersion oil between the objective lens and the slide. Follow specific instructions for using oil immersion.
- Cleanliness: Always ensure your lenses are clean. Use lens paper and appropriate cleaning solutions to remove any dust or oil.
In summary, focusing under high power involves careful preparation at lower magnification and then very precise adjustment with the fine focus knob.
