Par•fo•cal [pahr-foh-kuhl] adj.
of or relating to different eyepieces (of telescopes or microscopes) that all focus their images in the same plane, so that they can be interchanged without readjusting the instrument.¹
To simplify, if a compound microscope is parfocal, it means that when you change magnification sequentially (ex. 4x to 10x to 40x to 100x), it will only require a very slight turn of the fine focus knob with each increase or decrease to get the image in focus. Note, this is only true in sequence. For example, if you go from 4x directly to 100x, the image will no longer be in focus and will require greater adjustment.
It means something slightly different for a stereo microscope. If you get a sample in focus at the highest magnification on a parfocal instrument, you will then be able to zoom out to any lower magnification and the sample will remain in focus.
Microscope cameras are parfocal if both the camera image and eye piece image are in focus simultaneously. Some c-mounts actually have a focus adjustment to achieve this. (Tip: If your camera image is not in focus, check to make sure the eye pieces are set to zero)
Why This Matters
When a microscope is parfocal, it is easier to use. The user can work more efficiently without wasting time constantly refocusing. Just imagine trying to refocus at high magnification using the coarse adjustment knobs – trust me, it is a difficult task.
Indeed, most microscopes today are parfocal, however you may come across some lower quality scopes that were not manufactured parfocal in order save cost. If you own a non-parfocal scope, you can make it parfocal by adjusting the objectives with shims (spacers). Unfortunately, with stereo scopes it is more complicated and may require a professional technician to achieve parfocality.