Parfocal means a lens stays in focus when you change its magnification or zoom level. In a microscope, switching from a low-power to a high-power objective on a parfocal system requires little or no refocusing. In a cinema zoom lens, parfocal design lets a camera operator zoom from wide to tight without losing sharpness on the subject. The principle is the same in both cases: the optics are engineered so that changing magnification doesn’t shift the focus point.
How Parfocal Works in Microscopes
A compound microscope typically has several objective lenses mounted on a rotating nosepiece, each offering a different magnification. On a parfocal microscope, you can focus sharply at 10x, rotate to 40x, and find the image still essentially in focus. You might need a tiny tweak of the fine-focus knob, but you won’t have to hunt for the specimen all over again.
This works because the entire lens cluster inside each objective can be translated up or down within its metal sleeve using locking nuts, aligning the focal plane of every objective to the same reference point. The critical measurement is called the parfocal distance: the span from the mounting flange on the nosepiece down to the specimen plane. For decades, most biological microscope manufacturers standardized this at 45 millimeters, making objectives from different brands largely interchangeable. Leica, Olympus, and Zeiss all use a 45 mm parfocal distance. Nikon is the notable exception at 60 mm, which means Nikon objectives generally can’t be swapped onto a Zeiss or Olympus microscope and remain parfocal.
If your microscope’s objectives aren’t quite parfocal out of the box, many objectives have a small adjustment ring on the barrel. The standard procedure is to focus carefully at 10x, switch to the next higher power, and then rotate that adjustment ring (not the microscope’s focus knobs) until the image sharpens. You repeat the process for each objective, always using the 10x as your baseline.
Parfocal Zoom Lenses in Film and Video
In cinematography, parfocal takes on a slightly different meaning but the same core idea. A parfocal zoom lens holds focus on a subject as the camera operator changes focal length, zooming in or out during a continuous shot. The focus mechanism and zoom mechanism move independently inside the lens, so adjusting one doesn’t disturb the other.
This is why all professional cinema zoom lenses are parfocal. Techniques like the dolly zoom (where the camera physically moves while zooming in the opposite direction) depend entirely on the lens holding focus throughout the shot. Crash zooms, a signature of directors like Quentin Tarantino, require the operator to snap from maximum focal length to a wide angle in an instant. If focus shifted during that movement, the shot would be unusable.
Parfocal zoom lenses are also common in sports and wedding photography, where subjects move unpredictably and there’s no time to refocus after every zoom adjustment. The tradeoff is complexity and cost: engineering the zoom and focus mechanisms to operate independently requires more glass elements and tighter manufacturing tolerances, making these lenses significantly more expensive than their non-parfocal counterparts.
Parfocal vs. Varifocal Lenses
The opposite of a parfocal lens is a varifocal lens. On a varifocal design, changing the zoom or focal length also shifts the focus point. Every time you zoom in or out, you have to manually refocus.
Most consumer zoom lenses for DSLR and mirrorless cameras are varifocal. They were originally designed for still photography, where the photographer refocuses for each individual shot anyway. For single frames, this isn’t a problem. For video, it creates two issues. First, the operator has to zoom and refocus simultaneously, which is difficult to do smoothly. Second, varifocal lenses tend to produce visible “focus breathing,” where the edges of the frame shift slightly as focus changes. Audiences perceive this as the image subtly expanding and contracting, which looks amateurish in professional work.
Some modern cameras try to compensate for varifocal lenses with software features like facial recognition and focus tracking. These help, but they introduce a margin of error and a slight delay that can still produce soft frames during a zoom. For any shot where the focal length changes while the camera is rolling, a parfocal lens remains the more reliable choice.
Parfocal Eyepieces in Telescopes
Telescope eyepieces can also be sold in parfocal sets. The benefit is identical to microscopy: when you swap from a low-magnification eyepiece to a high-magnification one, the object you’re viewing stays in focus. For astronomers tracking planets, satellites, or transient events, the seconds saved by not refocusing can mean the difference between catching a moment and missing it entirely.
How to Test if a Lens Is Parfocal
For a zoom camera lens, the test is straightforward. Focus on a subject at one end of the zoom range, then slowly zoom to the other end. If the subject stays sharp throughout, the lens is parfocal. If it drifts out of focus, it’s varifocal. Run this test at the lens’s widest aperture, where depth of field is thinnest and any focus shift will be most obvious.
For microscope objectives, focus on a specimen at your lowest magnification, then rotate to the next objective without touching the focus knobs. A properly parfocaled microscope will show the specimen still in focus, or close enough that a small turn of the fine-focus knob brings it back. If you lose the specimen entirely, the objectives need adjustment or aren’t matched in parfocal distance.