Why should I not use the coarse focus knob for high-power objectives?

With the exception of stereo microscopes (which generally only have one focus knob), compound microscopes have a coarse and a fine focus. The coarse focus knob raises and lowers the stage quickly, the fine focus knob does this slowly. As a matter of fact, you will not see the movement of the stage. The coarse focus knob should only be used with the 4x low power objective in place. The fine focus know can be used with all objectives, including the 4x, but the effect will not be visible with this latter one (you will have to turn the knob many times, which is inconvenient).

Short introduction to focusing a compound microscope

1. Using the coarse focus, lower the stage. Be careful that you turn it into the correct direction. Usually this means that you have to turn the focus towards you.
2. Rotate the low power 4x into position.
3. Use the coarse focus to raise the stage all the way up until it blocks. The objective at the highest position, the 4x objective should of course not touch the slide.
4. Close the condenser and looking through the eyepiece lower the stage using the coarse focus until you see a clear image. Only rotate the coarse focus into one direction and stop when you see a clear image. If you are all the way down and still have not found a clear image, then you are either looking at a place of the slide that contains no specimen or the concentration is too low. Often dust, air bubbles etc. can help you find the focus.
5. Only when in focus, switch to the next higher objective and then only use the fine focus. Re-center the slide as well./li>
6. If you want to change the slide, you have to start again at point 1.

There are several reasons why you should use the coarse focus knob only with the 4x objective and not with the high power objectives.

Reason 1: protect the objectives from damage

If you focus the high power objectives with the coarse focus knob, then you risk slamming the objective into the slide, risking the damage of both. Some high power objectives are spring-loaded, with a front-part that retracts when touched by the slide. This is a protective mechanism.

Reason 2: danger of loss of focus

The coarse focus raises and lowers the stage to quickly, that it is inevitable that you lose the focus when you use high-power objectives. One purpose of focusing is to look at the different layers of a specimen. By turning the fine focus, you can “section” through the specimen. The coarse focus does not allow you to do this.

Reason 3: faster to find the focus

Last, it is simply faster to start with the 4x objective and then work your way up. The 4x objective also allows you to center the slide better, because it gives you a better overview. The 4x also has the greatest depth of field and this means that the whole specimen (top and bottom parts) are in focus. If you start with a high power objective and then start focusing, then you don’t know if you are focusing at the actual specimen or on the dust which is on the top of the cover glass.

Reason 4: Different specimen thickness causes loss of focus

If you look at a specimen under high power and then directly exchange the slide, you run the risk of losing focus again. The thickness of the mounting medium is not always the same and also the position of the specimen in the mounting medium can be different. Using the coarse (and even the fine focus) will take longer than starting again with the 4x objective and the coarse focus.

When you are (!) allowed to use coarse focus for high power

Some better microscopes have a focus lock lever. This lever blocks the stage at the pre-determined position. You set up the focus starting at 4xy and work your way up to 100x oil. You then engage the lever and the position will be remembered. You can then use the coarse focus to lower the stage, insert a new slide, and without changing the objective use the coarse focus (!) to raise it up again until the stage blocks (this position was remembered). This is an efficient method that allows you to quickly observe one slide at high magnification after another. Why not just change the slide by pulling it out horizontally? The reason is that if there is immersion oil on the slide, then objective should be lowered into the oil from the top and not from the side, to prevent smearing of the immersion oil all over the place.