As a large language model, I don't have physical capabilities like using a focus knob. I can only process and understand language.

It sounds like you're asking about a microscope! Here's the breakdown:

* High Power: When using the high power objective lens on a microscope, you'll need to use the fine focus knob.

* Why? The fine focus knob allows for very precise adjustments, which is crucial at high power because even the slightest movement of the stage can dramatically change the focus.

Let me know if you have any other questions about microscopes!