To get the best possible results from your stereo microscope, you'll want to ensure that it has been properly adjusted to your unique physical features. (If you have a compound microscope, refer to How to Focus Your Compound Microscope for Great Viewing.)

Why Parfocal?

We often begin using a microscope by simply focusing on the sample at whatever magnification it's set to, then switching back and forth between different magnifications and adjusting focus as needed. Then we'll notice that we constantly have to refocus the image, sometimes turning the knob through multiple full rotations just to get a clear picture.

Parfocaling sets up the microscope so that it remains in focus throughout the magnification range, requiring very little—if any—adjustment of the focus knob as you move between magnifications.

Procedure

These steps apply mainly to zoom models, but can be adapted to turret and fixed models for focusing. Most turret stereo microscopes are parfocal out of the box, although some may be non-parfocal to within the standard travel distance of the coarse focus knob.

Refer to the figure below while following the procedure.
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1. Place your specimen on the stage (A) and turn on the illuminator (B).

2. Turn the zoom knob (C) or objective changer (for turret models) to the lowest magnification and adjust the focus knob (D) until the specimen comes into focus. Set the eyepiece diopters (E) to zero.

3. Adjust the distance between the eyepieces (F) to match your interpupillary distance, at which point you should see a round, uniform field of view (one circle instead of two).

4. Adjust the focus knob (D) to a place near the middle of its focus range.

5. Turn the zoom knob (C) to the highest magnification and focus on the specimen using the focus knob (D) until you have a sharp image.

6. Now turn the zoom knob (C) to the lowest magnification. Do not adjust the focus knob (D) even if the image is out of focus.

7. If your microscope has a diopter ring on only one of the eyepieces, then follow these steps:

• Close the eye that would look through the eyepiece with the diopter ring and while looking through the other eyepiece with the other eye, adjust the focus knob (D) until you see a sharp image.
• Now close the eye that would look through the eyepiece without the diopter ring and look through the eyepiece with the diopter ring with the other eye. Adjust the diopter ring (not the focus knob) clockwise or counterclockwise (+ or -) until you see a sharp image.
• Take note of the diopter setting so you can return to it, if you share the scope with other users.

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

• With your left eye closed, look into the right eyepiece with your right eye. Rotate the right diopter clockwise or counterclockwise (+ or -) until you see a sharp image.
• With your right eye closed, look into the left eyepiece with your left eye. Rotate the left diopter clockwise or counterclockwise (+ or -) until you see a sharp image.
• Record the diopter settings so you can return to them if you share the scope with other users who may need to change them.

9. The microscope is now parfocal and the image should remain in focus throughout the full range of magnification.

The microscope will remain parfocal as long as you use it, but each new user will have to perform this procedure if they want to avail of its benefits. The reason is that each person has a different interpupillary distance, eyepoint (for example, if they wear glasses), dioptric adjustment, etc.

Furthermore, your eyes change over time, so occasional readjustment may be necessary.

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