What is a Parfocal Lens and is it Useful?

Prime lenses are often sharper and better corrected than zoom lenses because every element in the lens can contribute towards perfecting performance for a single focal length.

Zoom lenses, however, have to work across a range of focal lengths. This means compromises. Even the best zooms won’t match a prime, assuming they’re built with similar quality. That said, a high-end zoom can outperform a cheap or poorly made prime.

Zoom lenses that are not made for cinema use have one drawback when it comes to cinematography.

Let’s say you focus on a subject at a particular focal length, and then zoom in or out to reframe. Most zoom lenses will lose focus. Such lenses are called varifocal lenses.

But what if you had a lens thad didn’t lose focus as you zoomed in or out?

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What is a Parfocal Lens?

A parfocal lens stays in focus when you zoom in or out.

High-end cinema zoom lenses and some broadcast zoom lenses are parfocal. This feature saves time and frustration on set.

How to Identify a Parfocal Lens

Testing for a parfocal lens is simple:

1. Fix your aperture to one value. Wide open is a great start.
2. Zoom in and Frame your subject.
3. Focus on your subject. If you have a magnification function in the electronic viewfinder you can study the focus up close.
4. Zoom back out to another frame without changing anything else.
5. Compare the sharpness to the original frame. Again, using the magnification function will make it easier.

Do this across different focal lengths, distances, and apertures. Zoom in and out and in reverse. If the lens holds focus consistently, it’s parfocal.

Keep in mind, lenses marketed as parfocal might have minor focus shifts depending on the setup. Sometimes the lens is old an needs servicing. Always test a parfocal lens.

The Legacy of Parfocal Lenses

Pierre Angénieux developed the first widely recognized parfocal zoom in 1956. It was a 12-120mm lens that won an Academy Award for technical achievement. It set the standard for parfocal lenses in cinema and showed how precise and challenging they are to design.

Here are a few lenses that are parfocal:

• Angenieux Optimo zoom lenses
• Arri Signature zoom lenses
• DZOFILM Pictor zoom lenses

Why Are Parfocal Lenses Useful for Cinematographers?

For cinematographers, parfocal lenses offer practical benefits:

• Run-and-Gun Shooting: In low budget filmmaking work (plus documentaries and events), parfocal lenses save time when focus needs to stay sharp during quick zooms. Also, it reduces the workload for the focus puller, if there is one!
• Precision Focus in Unpredictable Scenes: Imagine being on set while the operator can quickly zoom into the talent, focus and zoom out – all in less than five seconds – for precise focus. If actors miss their marks, you can quickly zoom in, focus, and zoom out without delays.
• Simplified Setup: Smaller crews or solo shooters can work faster without relying on focus pullers or complex setups. If the camera moves during the shot, then this point is moot.

Mirrorless cameras with autofocus might seem to reduce the need for parfocal lenses. But autofocus systems aren’t perfect. They can fail in low light or struggle with fast-moving subjects.

In professional filmmaking, manual focus control with parfocal lenses is still king.

Should You Buy a Parfocal Lens?

Only you can decide the higher cost of parfocal lenses (rented or purchased) is worth the time saved on set. If the optics are top-notch and the lens fits your workflow, it’s a great investment.

Parfocal lenses are more expensive, but for professional cinematographers, the time saved on set make them invaluable.

While budget options exist, they often come with compromises in build quality or performance. If you’re considering one, make sure it fits your camera system and production needs. Make sure you’re not sacrificing important qualities like sharpness and so on for a parfocal design.

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