The fine focus knob is used with the 10x and 40x objectives because, at these higher magnifications, only tiny adjustments are needed to sharpen the image and large movements from the coarse knob would throw the specimen out of focus or risk damage.

Core reason

At 10x and especially 40x, the depth of field is very small, so even a slight movement of the stage changes focus a lot.

After you first focus the slide at low power (usually 4x) using the coarse knob, the specimen is already close to being in focus, so only small, precise movements are needed.

Why not the coarse knob?

  • The coarse knob moves the stage a relatively large distance with each turn, which can easily move the specimen completely past the focal plane at 10x and 40x.
  • With higher objectives closer to the slide, large movements increase the risk of hitting or cracking the slide or contacting the objective lens.

What the fine focus does

  • The fine focus knob makes very small, controlled movements of the stage, allowing careful ā€œfine-tuningā€ of sharpness at higher magnifications.
  • This precise control is ideal once the image is roughly focused at 4x, so you switch to 10x or 40x and adjust only with fine focus to get a clear, detailed view.

TL;DR: Use the fine focus knob with 10x and 40x because the slide is already roughly focused at 4x, and higher magnifications need tiny, precise adjustmentsā€”coarse focus moves too much and can blur or damage the setup.

Information gathered from public forums or data available on the internet and portrayed here.