Parfocality keeps the detail being observed in focus throughout the magnification range of a surgical microscope, without the need to refocus. This requires the optical system to be set correctly, including individual dioptric adjustment of the eyepieces, and the microscope and the object being observed to remain in the same positions. Below, we describe how to adjust and check parfocality, using the Leica M822 F40 ophthalmic microscope as an example.
Parfocality in practice
In the photograph, we have marked the point in the core of the orange on which focus was set. The video shows that this point remains in focus throughout, even as the magnification changes.
The importance of dioptric adjustment
Dioptric adjustment adapts the microscope eyepieces to the user's eyesight, separately for the right and left eyes. Accommodation, the eye's natural ability to adjust its focus, can allow an incorrect setting to go unnoticed. An image that appears sharp at one magnification may therefore no longer be sharp when the magnification changes.
Adjusting parfocality step by step
In this procedure, we assess focus through the microscope eyepieces, without using a camera. Prepare a flat object with clearly defined details, such as a sheet of paper with small print. Before releasing the microscope brakes, make sure the microscope is properly balanced.
Focus is checked in the same way when changing magnification on other surgical microscopes. However, the adjustment procedure itself may differ between models, so check the user manual for the particular microscope for the sequence of steps and the initial dioptric settings.
1. Adjust the eyepiece spacing to your interpupillary distance
Look through both eyepieces and adjust the distance between them to match your interpupillary distance. Adjust the spacing until you see a single, circular field of view.
2. Adjust the inclination of the binocular tube
If you are using a tiltable binocular tube, hold it with both hands and adjust its inclination so that you can look comfortably through both eyepieces.
3. Set the focusing mechanism to its starting position
Activate the Reset Focus function and wait for the mechanism to stop moving. On the Leica M822, this position leaves 1/3 of the travel range upwards and 2/3 downwards. This allows focus to be adjusted in both directions after the working distance has been set approximately.
4. Set the initial working distance
WD stands for working distance: the distance between the objective and the surface of the object being observed at which a sharp image is obtained.
Check the WD marking on the objective. On the Leica M822 F40, grasp both control handles and rotate one of them to release the brakes. Use the swing arm to position the microscope head at the appropriate distance from the object, then release the rotated handle to lock the position again.
For example, for WD 200 mm, set a distance of 20 cm — you can check it with a ruler.
You will set the precise focus in step 6 using the focusing mechanism. If the microscope head is too far from or too close to the object, the travel range of this mechanism may not be sufficient to obtain a sharp image.
5. Select the lowest magnification
Use the foot control to select the lowest magnification available on the microscope.
6. Focus at the lowest magnification
Look at the prepared object through both eyepieces. Use the focus adjustment buttons on the foot control to bring a selected detail, such as a letter on the sheet of paper, into focus. Leave the swing arm in its current position.
7. Select the highest magnification and fine-tune the focus
Use the foot control to select the highest magnification available on the microscope. While observing the same detail through both eyepieces, fine-tune the focus using the buttons on the foot control.
8. Return to the lowest magnification
Use the foot control to select the lowest magnification again. Do not adjust focus or change the position of the microscope or the object being observed at this point — retain the setting obtained in the previous step.
9. Set both eyepieces to +5 dioptres
Without looking through the eyepieces, set the dioptric adjustment rings on both eyepieces to +5 D. This is the starting position for adapting each eyepiece to your eyesight in the next step.
10. Adjust the dioptric setting separately for each eye
Look through one eyepiece with the other eye closed. Slowly turn the dioptric adjustment ring from +5 D towards −5 D. Stop when the detail being observed comes into focus — you do not need to reach −5 D. Repeat for the other eye.
At this point, adjust focus only with the eyepiece rings. Do not use the focus adjustment buttons on the foot control. The settings for the right and left eyes may differ.
11. Check focus at the highest magnification
Use the foot control to select the highest magnification. Look through both eyepieces and check whether the previously selected detail remains in focus. Do not adjust focus with the foot control at this stage — you are checking the result of the adjustment.
12. Adjust the eyecup extension
Rotate the eyecups to extend or retract them and set a comfortable distance between your eyes and the eyepieces. Choose a distance that allows you to see the entire field of view. Keep the previously set dioptric adjustment unchanged.
13. Check focus throughout the magnification range
While looking at the same detail through both eyepieces, use the foot control to change the magnification from the lowest to the highest and back again. The selected detail should remain in focus throughout, without any further focus adjustment. Keep the microscope and the object being observed in the same positions during this check.
If focus changes with magnification, check the dioptric adjustment of each eyepiece again.
Frequently asked questions
Can the same procedure be used on every surgical microscope?
Parfocality is checked in the same way: a selected detail is observed while the magnification changes. However, the sequence of adjustments and the initial dioptric settings may differ between models. The procedure described applies to the Leica M822 F40; for another microscope, consult its user manual.
Why is the image sharp at one magnification but not at another?
One possible cause is an eyepiece dioptric adjustment that does not match the user's eyesight. Accommodation can mask an incorrect setting at one magnification. That is why, after adjustment, we check the same detail throughout the magnification range.
Do you need to reach −5 D during adjustment?
No. In this procedure, +5 D is the starting setting, and −5 D indicates the direction in which to turn the ring. Stop adjusting when the detail being observed comes into focus. The dioptric setting is adjusted separately for each eye, and the resulting values may differ.
What should you do if focus still changes with magnification after adjustment?
Check the dioptric adjustment of each eyepiece again. Keep the microscope and the object being observed in the same positions during the check, and look at the same detail. Do not adjust focus with the foot control during this check — you are checking the result of the adjustment.
Sources
The adjustment procedure was prepared on the basis of the Leica M822 user manual, with additional practical explanations. The explanation of the effect of accommodation is based on materials concerning observation through microscope eyepieces.
- Leica Microsystems, M822 — User Manual, document no. 10 743 504, version 04, English edition. Sections 7.3–7.4, 8.2.1 and 8.4.3, pp. 30–31, 43 and 46–47. Source for the procedure for adjusting the binocular tube and eyepieces, positioning the microscope and checking parfocality.
- Nikon MicroscopyU, Depth of Field and Depth of Focus. Explanation of the effect of eye accommodation on the sharpness assessed when looking through eyepieces and of the difference compared with recording an image on a stationary sensor.
- Peter Evennett, Depth of Field and depth of Focus Explained, Proceedings of the Royal Microscopical Society, 31/1, 1996, pp. 64–66. Explanation of how accommodation can mask an incorrectly positioned image plane; particularly p. 66.