How to parfocal microscope objectives

Think how you’d feel if you had to adjust your TV every time you changed channels.

Yet some microscope users have to refocus every time they switch magnifications. And they’re probably not too happy about it either – it’s disruptive and can lead to eye strain. 

However, it’s a problem simply solved with parfocal lenses.

What are parfocal objectives?

Parfocal means that when one objective lens is in focus, then the other objectives will also be in focus – or at least will require only minor adjustments, which saves you time and reduces wear and tear on the microscope. 

Fortunately, the objective lenses in pretty much all new microscopes are already parfocal, so you’re probably in luck.

But if you happen to own a non-parfocal scope, there’s a procedure that will allow you to parfocal your lenses. 

Parfocal procedure

But first a ‘health’ warning. If you’re not a skilled ‘techie’ or if you don’t want to risk damaging your lenses, consider asking a professional technician to do the job for you.

Otherwise, first check out your objectives. If you have a 4x or a 10x objective in your microscope, leave those in place – they aren’t adjustable. Remove the other objectives that you wish to parfocal from the turret.

The following steps will parfocal the higher power objectives using the 10x objective as a base line. The other objectives will be adjusted in order moving up from the 10x microscope objective.

Step 1

Start by removing the outer cover of your objectives, keeping track of which cover goes with each objective, so you don’t mix them up! They should unthread easily without being forced.

Step 2

Locate the ring that allows you to adjust parfocality. See if you can turn the ring. Some objectives have a dab or drop of optical cement holding them in place.

Step 3
If it is fixed in position, remove the cemented spot.

Slightly dampen a Q-tip with acetone and carefully remove the cement. Work the ring back and forth until the ring spins freely.

Step 4

After each ring is free, reinstall the objectives (without the covers on) on your microscope in the usual ascending order (place the 40x, then the 100x. etc.).

Step 5

Put a stage micrometer (or anything flat and lined) under the microscope and rotate the 10x lens into position. Bring your lined sample into focus.

Step 6

Rotate the next objective up into position. This time, instead of using the focus knob to focus on the sample, carefully rotate the parfocality adjustment ring on the objective itself. This movement is somewhat sensitive so move the ring very slowly. 

Once the sample is in focus, repeat the procedure with the remaining objectives.


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