A parfocal lens is a lens that stays in focus when you change its magnification or zoom level. Whether you’re zooming in on a microscope slide or pushing a camera lens from wide angle to telephoto, a parfocal design keeps your subject sharp without requiring you to refocus. This property makes parfocal lenses essential in both scientific microscopy and professional video production.
How a Parfocal Lens Works
In any zoom lens, multiple glass elements move inside the barrel to change the focal length, which is what makes the image appear closer or farther away. In most consumer lenses, shifting those elements also shifts the point where light converges, meaning your focus drifts as you zoom. A parfocal lens solves this by keeping the focal plane stationary even as the focal length changes. The focus mechanism and zoom mechanism operate independently of each other, so adjusting one doesn’t disturb the other.
In a typical parfocal zoom design, groups of lens elements are arranged so that as some move to change magnification, others compensate to hold the image plane in the same position. The result is that you can set focus once and then freely zoom without the image going soft. In practice, even well-made parfocal lenses may need a tiny touch of fine focus adjustment, but the shift is negligible compared to a non-parfocal design where you’d essentially need to refocus from scratch.
Parfocal vs. Varifocal Lenses
The opposite of a parfocal lens is a varifocal lens. A varifocal lens changes focus as you change the focal length, so every time you zoom in or out, you have to manually refocus. Most consumer photography zoom lenses are varifocal, which is perfectly fine for still photography since you’re typically refocusing for each new composition anyway.
Parfocal lenses become critical when focus needs to hold during a continuous shot. Sports photography, wedding videography, and any live-action filming where you zoom while recording all benefit from parfocal behavior. Varifocal lenses also tend to produce more noticeable “focus breathing,” where the edges of the frame visibly shift inward or outward as you adjust focus. Parfocal lenses minimize this effect because the focus point isn’t changing during a zoom, keeping the framing stable and the image clean.
Parfocal Lenses in Microscopy
If you’ve ever used a microscope with a rotating turret of objective lenses, you’ve likely benefited from parfocal design. On a parfocal microscope, you can focus on a specimen at one magnification, rotate the turret to a higher or lower power objective, and find the specimen still in focus (or very nearly so). Without this feature, switching from 10x to 40x magnification would mean losing your subject entirely and having to hunt for it again with the focus knobs.
The way this works is straightforward: all the objective lenses on a parfocal turret are designed to the same “parfocal distance,” which is the fixed height from the turret to the slide. Even though the objectives have different magnifications and different physical lengths, their optics are engineered so the focal point lands at the same plane. On modern research-grade microscopes, the specimen stays in focus to within about a micron when switching between objectives. You may still need a small turn of the fine focus knob, but the adjustment is minor. This saves enormous amounts of time when examining samples across multiple magnification levels.
Why Filmmakers Rely on Parfocal Zooms
In cinematography, a parfocal zoom lens is what makes smooth, professional zoom shots possible. If you’ve seen a dolly zoom (the unsettling effect where the background seems to stretch or compress while the subject stays the same size), that technique requires simultaneously zooming the lens and moving the camera. The shot falls apart if focus drifts during the zoom, making a parfocal lens essential.
Crash zooms are another technique that depends on parfocal behavior. A crash zoom is a rapid snap from a long focal length to a wide angle (or the reverse), often used for dramatic or comedic effect. The operator focuses at the longest focal length and then quickly zooms out, and the subject needs to stay sharp the entire time. With a varifocal lens, the focus would be lost almost immediately, producing unusable footage.
Even for more routine shots, any time a filmmaker changes focal length during a take, a parfocal lens eliminates one major variable. A camera operator already managing framing, camera movement, and actor blocking doesn’t need the additional burden of chasing focus through a zoom. The result is cleaner footage with fewer ruined takes.
How to Calibrate a Parfocal Lens
Even a true parfocal lens needs to be properly calibrated through a process called “back focusing.” Many cinema lenses have a back focus ring specifically for this adjustment. The process is simple and takes about five minutes once you’re set up.
You place the camera on a tripod about ten feet from a detailed target like a focus chart with fine radial lines. Open the aperture as wide as possible to make the depth of field as shallow as you can, which makes any focus error obvious. Zoom all the way in to the longest focal length and use the front focus ring to get the sharpest possible image. Then, without touching the front focus ring, zoom all the way out to the widest focal length. If the image is no longer sharp, you adjust the back focus ring until it is, then repeat the process. After a few rounds, the lens should hold focus cleanly across the entire zoom range.
Cost and Availability
True parfocal zoom lenses cost more than their varifocal counterparts because the optical engineering required to hold focus across a zoom range is significantly more complex. Most consumer photography zoom lenses are varifocal. If you need genuine parfocal performance, you’re generally looking at cinema-grade glass.
At the more accessible end of the cinema lens market, brands like DZOFILM offer parfocal zoom lenses starting around $1,499 to $1,699 for individual lenses. Their Pictor line, for example, covers various focal ranges (20-55mm, 50-125mm, 14-30mm) with parfocal design and minimal focus breathing. A three-lens kit covering wide to telephoto runs about $4,800. These are designed for professional use with PL and Canon EF mounts. Higher-end options from manufacturers like ARRI, Cooke, and Angénieux can run well into five figures.
Some modern mirrorless camera lenses use electronic focus motors to simulate parfocal behavior, automatically adjusting focus as you zoom. This isn’t the same as a mechanically parfocal design, and the results vary depending on how fast and accurate the autofocus system is, but it can be a practical alternative for hybrid shooters who need decent zoom-hold performance without investing in dedicated cinema glass.