Written by Snehal Patel
Zoom lenses are an important tool for filmmaking in general, but even more important for multicamera cinematography. With the ability to quickly reframe, change focal lengths and maintain sharp focus, zooms become invaluable for achieving the shot under challenging conditions. Many cinematographers utilize zooms for various productions from films to television to broadcast.
So, what is the difference between a zoom and a prime? First and foremost, it’s the difference between a fixed focal length (primes) and a variable focal length (zoom). Focal lengths are magnifications, much like magnifications for a microscope. The longer the focal length, the closer you feel to a distant object. The wider the focal length, the lesser the magnification which results in objects in the distance feeling further away.
In these cases, the “object” is relative. A person’s eye can be a distant object if they are only seven feet away from the camera. A wider focal length lens would see a lot of the person’s body and face, along with the eye. With an appropriate telephoto focal length, we can get a tight shot of just the eye. Zoom lenses can render both extremes and all the in-between magnifications, with the turn of the zoom ring.
This ability comes at a cost, which is reflected in the fact that zoom lenses are typically larger, heavier and have slower T-stops. This is due to the complexity of the optical design and mechanics. Modern zoom lenses do a great job of covering a large focal range, while performing quite well compared to older lenses. Even so, primes typically offer a cleaner more precise look, especially when wide open. Since primes are generally faster than zooms, you can achieve a shallower depth of field with smaller T-stops (larger IRIS opening). These are the reasons why primes are preferred for creative purposes, while zooms are seen as being more utilitarian.
Optical Design. Zooms typically have many glass elements in their designs, where two or more groups of elements are variable and moving while zooming or focusing the lens. It’s not easy to design optics to cover large focal ranges like 70 to 200mm. Zoom designs are more complex than fixed focal length lenses (primes), and the designer must decide what compromises will be made and what is an acceptable level of distortion, chromatic aberration and exposure fall off from the center of the frame to the edge.
Mechanics. Since several glass elements in a zoom lens must shift when the focal length is changed, the mechanics have to be precise and repeatable. Well-designed cinema zooms will perform similarly even after a lot of time in the field. They are built with robust housings and well-lubricated moving parts, but the barrel designs are more complex than prime lenses, so servicing a zoom take more time and effort. Plus, there are more things that can go wrong with the mechanics, which means zooms are more likely to require servicing after heavy use.
Snehal Patel is a film and television professional with over two decades of experience creating content and adapting new technology. He started the first Canon Bootcamp in Los Angeles during the Canon 5D DSLR craze and has over twenty years of experience in cinema. Snehal has lived and worked in Chicago, Mumbai and Los Angeles as a freelance Producer & Director. He was a camera technical salesperson at ARRI, and currently is the Head of Cinema Sales at ZEISS. He represents the Americas for ZEISS and is proud to call Hollywood his home.