The coarse adjustment knob on a microscope moves the stage or objective lens up and down in large, quick steps to bring the specimen into rough focus. It is usually used first, at low magnification, before switching to the fine adjustment knob for sharper detail.

What it does

  • Helps you find the specimen quickly.
  • Brings the image into approximate focus.
  • Makes bigger focusing changes than the fine knob.

When to use it

  • Use it at low power, such as when first viewing a slide.
  • Avoid using it at high magnification, where small movements matter more and it can risk hitting the slide.

Simple rule

Turn the coarse knob first to get close, then use the fine knob to sharpen the image.

TL;DR: The coarse adjustment knob is for fast, rough focusing on a microscope.