Everyone knows that primes are better than zooms. So why bother using zooms? There are some instances where a zoom might just be the better choice.
Primes are lenses that have a fixed focal length. They don't zoom. To make a zoom lens, additional elements need to be added to the design, increasing their overall size and weight. It also means that some compromises often have to be made in terms of image quality. Whereas a prime lens is fixed, all efforts can be made to maximise the lens' performance and minimise any unwanted effects such as aberrations and distortion. Zooms however cannot feasibly sustain this performance across the entire focal range, without adding even more correcting elements. As it happens however, there are a few zooms out there that do strive and even attain prime-level optical quality throughout their zoom range - though their size and weight is only equalled by their price tag. The Angenieux Optimo lenses are a favourite of Hollywood and more recently, Fujinon's HK Premier line of cinema zooms has been praised as sharper than Arri/Zeiss Master Primes by Claudio Miranda ASC.
So, primes are smaller, lighter, cheaper, faster and optically superior to zooms (exceptions aside!) They're better in every way - except they don't zoom. So you'd want a zoom only when you need to zoom in a shot, right? Not quite.
Whilst a prime lens is lighter than a zoom, a whole set of primes is not. A zoom might combine the focal lengths of three, four, five or more prime lenses - all in one package. Where weight might be a concern if equipment is needed to be carried long distances or up a mountain perhaps, a zoom can be invaluable.
The other evident advantage is you needn't keep swapping lenses to get different focal lengths and therefore perspectives. Not only is this clearly far quicker on a set - zooming takes one person and only a few seconds at most, compared to potentially a few minutes and two or three people. This even more advantageous if you're on top of aforementioned mountain the weather takes a turn for the worse! Exposing the sensor even to mild wind can result in dust and spots appearing in your footage, rendering it unusable.
Fujinon recently were kind enough to lend me one of their ZK series 19-90mm T2.9 'Cabrio' lenses for use on an expedition to Scotland. I'm currently in production shooting a wildlife documentary called The Tigers Of Scotland; last year I took a whole set of primes with me and whilst they perform wonderfully, it's a real hassle carry that case everywhere and changing lenses all the time. Having a decent cinema zoom this time around was simply invaluable. It lived on the front of the camera for most of the shoot; having it ready to go meant if we spotted anything whilst driving around we could simply stop and start shooting - without having to build the camera and choose a focal length.
There are a few considerations one should bear in mind when picking a zoom to use however; many of which separate photography zooms from cinema zooms. Along side all of the mechanical differences that are also the case when picking a prime lens - geared rings, long focus throw, de-clicked aperture etc, a cinema zoom really needs to be parfocal to be of much use.
Being parfocal means that when zooming in and out, the focus stays in the same place. This seems like an obvious feature, but photography lenses are notoriously far from parfocal. Focus up and try zooming in or out with a DSLR zoom and you'll see how wildly out of focus the image becomes, requiring you to refocus every time you change focal length. Being parfocal has two advantages, the first of which is clear - if you wish to zoom in or out during a shot, it needs to hold its focus. This simply can't be done with a photography zoom. And secondly, remaining parfocal affords even more time saving on set, in addition to the speed of just zooming in as opposed to changing lens. This is most evident in a fast paced shooting environment such as a documentary where you might be shooting a wide, only to crash zoom in to quickly grab a close up of the same subject. Not having to refocus might mean the difference between getting the shot and missing the opportunity.
As the shift from small chip cameras to large sensor cameras has become more widespread in many areas over the last decade, people having been wanting ENG (electronic news gathering) style broadcast lenses for their modern, Super 35 cameras. Some manufacturers have risen to the call and now produce Super 35 format, cinema zooms in a broadcast style package. The Fujinon 19-90mm for instance, is a great example. Many zooms now have detachable servo-units, enabling the zoom to be motorised and other features such as rec start/stop to be controlled from a handgrip on the lens, enabling operation from the shoulder. There are however, two brilliant additional features that have migrated over from ENG lenses and can now be found on some cinema zooms. Backfocus adjustment and a macro mode. Whilst not found on every zoom, I sometimes wish they were - they're both super useful features that you'll wonder how you ever managed without.
In a previous article I wrote about the recent wave of budget cinema zooms that have recently been announced. If you can't afford the 'prime beater' Fujinon HK Premier zooms, or even an ENG style zoom such as their ZK Cabrio line, there are definitely a growing number of true cinema zooms becoming available as viable alternatives to DSLR zooms. And to add even more excitement, Fujifilm have just announced a new line of even cheaper zooms - their MK line - that are, from first impressions, just as good optically as the Cabrios but for a third of the price.
Whilst a prime may still be superior to a zoom, there are still plenty of times where a zoom is simply the better choice - even if you never actually use it for a zoom shot. Modern cinema zooms are often so good anyway that the results are indiscernible from a prime. Any optical benefit you might have gained by using a prime is more than made up for by the speed and efficiency that only a zoom can provide. So unless you need the speed of a prime, perhaps you'll benefit from using a zoom more than you might think on your next shoot.