Never use the high-power objective lens (typically the 40x and 100x/oil immersion objectives) with the coarse adjustment knob.

Quick Scoop

When using a compound light microscope, the coarse adjustment knob is only safe with the scanning (4x) and low-power (10x) objectives, because the working distance between the lens and the slide is relatively large. At higher magnifications, that distance becomes very small, so even a slight turn of the coarse knob can drive the lens into the slide and crack the glass or damage the lens.

So, which lens is a “never”?

  • High-power objective lens (around 40x) should never be used with the coarse adjustment knob.
  • Oil immersion objective lens (around 100x), when present, is also treated as high power and must only be focused with the fine adjustment knob.

Safe focusing sequence

  1. Start with the scanning or low-power lens and use the coarse adjustment to locate and roughly focus the specimen.
  1. Switch to high power (40x and above) and then use only the fine adjustment knob to sharpen the image.

In forum-style teaching discussions and lab manuals, this is often summarized as:
“Coarse on low power only; fine on high power only.”

Answer for your title question:
The objective lens you should never use in conjunction with the coarse adjustment knob is the high-power objective lens (including 40x and 100x/oil immersion).

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