Parfocalling is a procedure by which a microscope is adjusted to remain in focus throughout the magnification range. If the microscope is not parfocalled, an object in clear focus at one magnification will quickly go out of focus as you zoom to a higher or lower magnification, requiring you to zoom and focus at the same time. Not only is this inconvenient, it also increases strain on your eyes and wear and tear on the microscope. Fortunately, parfocal adjustment on a zoom stereo microscope is quick and easy !
Set the correct interpupillary distance by moving the eyetubes closer together or farther apart to match your individual setting. You can tell when the setting is right when you are able to look into the eyetubes and see the image framed on ONE circle.
When done properly, your eyes will quickly and easily register the circle. Your eyes are very adaptable so if the interpupillary distance is off a little bit, your eyes will adjust to it. However, this increases strain on your eyes. To avoid eye strain, look up from the microscope at a distant object for a moment and then look back into the eyepieces. The circle should appear quickly and easily. If not, re-adjust the interpupillary distance until it does.
Line up the marks as shown in the photo.
We are going to choose a coin but you can use any flat specimen that does not have a lot of depth.
Samples with easily identifiable features will allow us to quickly determine focusing.
(Not the zoom knob with the numbers on it.)
Depending on your model, you may have two focus knobs, one for course adjustment and one for fine adjustment.
The image may now be slightly out-of-focus but DO NOT re-adjust the large focus knob.
Instead, while looking into the microscope, close your right eye and while adjusting the right eyepiece, bring the specimen into focus with your right eye ONLY while adjusting the diopter as shown in the photo.
When the image of your specimen is clearly in focus in the right eyepiece, close your right eye and repeat the procedure until the left eyetube is focused clearly.
Just as we did for adjusting the interpupillary distance, use the same "look-away" technique to ensure that focusing through each tube is quick and easy.
The microscope is now "parfocalled" and should hold focus throughout the entire zoom range. You may notice that a microscope parfocalled by someone else is NOT exactly parfocalled for you. This is because each person has different focusing abilities: i.e. different interpupillary distance, some people wear glasses, etc. In this sense, parfocalling means adjusting the microscope to fit you and it is well worth the effort.