The Quest for Parfocal and Power Zoom Lenses for Lightweight Filmmaking

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When it comes to single-operator filmmaking (videography), a one-size-fits-all solution is preferred, without too many accessories and controls. When shooting videos, zooming is a major component that controls visual expressiveness. One famous technique called Dolly Zoom creates a unique effect and is also fun to practice. Do note that these techniques are used not that frequently.

In the film industry, there is usually a camera operator who controls the zoom and focus of a parfocal cinema lens attached to external motors remotely. However, the question arises: which lens and body combination can achieve this without the need for such a setup, albeit with less reliability compared to the human-controlled massive setup?

Parfocal

We want the lens to maintain focus while zooming, a capability referred to as parfocality. This can be achieved in two ways:

• Optically: The optical design of professional-grade cine lenses already takes this requirement into account. The focus group glasses move along with the zoom group mechanically, ensuring that the focal plane does not change.
• Electronically: Photographic lenses are designed to be portable, and parfocality is not a priority in their optical design. The solution is to quickly react to the focal change by moving the glasses fast to simulate the optically parfocal ones. This may involve a sensor or encoder that reports the current focal length, and a control circuit that adjusts the focusing group location based on an algorithm or lookup table when the focal length changes.

Powerzoom (PZ)

Usually, the change of focus is achieved by turning the lens barrel. However, smoothly doing so without introducing camera shake while holding the camera is quite challenging. Hence, motor-controlled operation (powerzoom) is preferred.

Autofocus

The consensus is that Sony has the best autofocus, followed by Nikon, Canon, and Fujifilm.

Summary

Below is a table that lists some of the full-frame zoom lenses of interest grouped by their focal lengths.

Crop frame, APS-C, S35 lenses

Gimbals like RS3 Webill Crane have a focus motor available as an additional purchase, and they all offer a turning wheel to control that motor via wire. Some accompanying apps have a W/T button to turn the motor at a constant speed.

Most Sony zoom lenses achieve electronic parfocality, starting from their 28-70 kit lens over a decade ago. Sony has many old powerzoom lenses that do not meet optical standards, so they are not considered here.
Sigma/Tamron lenses usually come with some degree of focus compensation, but they are often not fast enough for most cases.
Nikon currently has only one crop-frame PZ lens, and there is not enough evidence for their standard zoom lenses tested for video. However, their marketing slogan “Nikon for Video” suggests that there may be more to expect in the future. Canon offers a unique 24-105 F2.8 PZ option with excellent quality, but the entire lens setup with PZ costs $4000 without a deal, which is equivalent to two GMII lenses with a discount. The R5C also has poor battery life, requiring an external power supply.
Therefore, choosing a Sony lens on a Sony body is usually the obvious choice currently. The Fujifilm X-H2s paired with its PZ lens is the second best option.