This article will guide you through the steps for focusing a compound microscope. Since there is a wide range of models available from many manufacturers, there may be some differences in the appearance or operation of components and other details. The instruction manual for your particular microscope model should have the final say. However, if it is not available, you're unsure of how something works, or if you think you might be missing a step or two, I hope this will provide some guidance.

If you have a stereo microscope, refer to the article How to Focus and Parfocal Your Stereo Microscope for Great Viewing.

Use the illustration below while following the procedure.
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Procedure

1. Turn on the illuminator (A) and set it to the lowest intensity level to start.

2. Fully open the iris diaphragm of the substage condenser (B). If your model doesn't have a substage condenser (common in metallurgical microscopes), there might be an iris diaphragm somewhere above the nosepiece (C).

3. Rotate the nosepiece (C) so that the objective with the lowest magnification—usually the shortest one—is in working position.

4. Place a specimen slide on the stage (D) and secure it with stage clips or a finger assembly.**

5. Set the eyepiece diopter adjustments (H) to 0 and adjust the eyetubes to match your interpupillary distance.

6. Turn the coarse focus knob (E) to raise the stage as far as it will go without touching the objective.

7. While looking through the eyepieces (F), adjust the illuminator intensity (A) to a comfortable level of light.

8. While still looking through the eyepieces, turn the coarse focus knob so that the stage is moving down (away from the objective) until the specimen comes into focus. Use the fine focus knob (G) to sharpen focus.

9. Move the slide so the specimen is in the center of the field of view and sharpen the image further using the fine focus knob if necessary.

10. Adjust the condenser diaphragm (B) and/or illuminator intensity (A) as needed to obtain the clearest image.***

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Adjusting the Diopters
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To correct for any differences in vision between your two eyes, we will now adjust the diopter settings on the eyepiece(s). This ensures that the images in both eyepieces are simultaneously in focus.

If your microscope has a diopter adjustment ring (H) on only one of the eyepieces, then follow these steps:

1. Close the eye that would normally look through the eyepiece with the diopter ring and while looking through the other eyepiece, adjust the coarse or fine focus knob until you see a sharp image.

2. Now close the eye that would look through the eyepiece without the diopter ring and look through the eyepiece that has the diopter ring. Adjust the diopter ring (not the focus knob) clockwise or counterclockwise (+ or -) until you see a sharp image.

3. Take note of the diopter setting so you can return to it. If you share the scope with other users, they will probably be making their own adjustments.

If your microscope has a diopter adjustment ring on both of the eyepieces, then follow these steps:

1. With your left eye closed, look into the right eyepiece with your right eye. Rotate the right diopter ring clockwise or counterclockwise (+ or -) until you see a sharp image.

2. With your right eye closed, look into the left eyepiece with your left eye. Rotate the left diopter ring clockwise or counterclockwise (+ or -) until you see a sharp image.

3. Record the diopter settings so you can return to them if you share the scope with other users who may need to change them.

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Final Notes on Focusing
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At this point you should be able to switch to a higher magnification objective without having to adjust the fine focus knob more than a half turn or so to get a sharp image. Upon switching you might also need to tweak the illuminator intensity, condenser height, or condenser aperture to get a clear image.**** If you're not getting good contrast or even illumination throughout the field of view, try adjusting for Köhler illumination.

If you are unable to focus on the specimen after switching to a higher power objective, then repeat the steps in the procedure. When you switch from a low power scanning objective to a much higher magnification, say from 4x to 60x or 100x, you may find it useful to perform the procedure for each intermediate objective until you get to the one you want to use.*****

Also, if you're using an oil immersion objective, you'll want to put a drop of immersion oil on the slide before gradually moving it into position.

At this point you may want to parfocal your microscope.

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Footnotes

* Also called an "aperture" diaphragm.

** If possible, choose a specimen that has areas of high and low contrast.

*** Some condensers have markings which indicate the ideal position it should be in for each objective lens. These are set to match the numerical aperture of the objective you're using.
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**** In general, each time you switch to a new objective, you'll need to make a corresponding adjustment to the condenser's iris diaphragm to maximize image resolution and contrast. It may have markings that correspond with specific objective magnifications (essentially a graded scale that denotes the approximate size of the aperture diaphragm). If your condenser doesn't have this, then a good rule of thumb is to open the aperture as wide as possible while still achieving a good image with clear, sharp contrast. Too wide and you'll have glare and poor contrast, too narrow and you won't have good resolution and may possibly get a distorted image.

***** At the higher magnifications, e.g. 60x or 100x, you'll want to use only the fine focus knob for focusing. You'd be less likely to zoom past the specimen or damage the slide if you accidentally crash into it.

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