Removable lens mounted via bayonet — swap focal lengths without changing camera body. Foundation of professional cinematography.
Suddenly you need a 24mm instead of a 50mm? Remove the mount, insert new glass — this is the principle that modern film production has made possible. An interchangeable lens sits on a standardized mount and can be swapped out in seconds without changing the camera itself. This saves time, weight, and most importantly: nerves. Whether you're on a documentary set or a large production — the flexibility to switch between wide-angle and telephoto is non-negotiable.
Practically, it works like this on set: the camera stays on the tripod or in your hand, you unlock the current lens at the mount, take it off, and click the new one in. With professional systems — Red, Alexa, FS7 — the mounts are precise enough that you won't lose focus calibration afterwards. Your focus puller, of course, has to keep up: a new focal length means new marks on the focus ring, and a new aperture sometimes also means new ND glass. This, in contrast to a prime lens — a fixed focal length lens — is the compromise: flexibility instead of optical perfection. However, modern zoom lenses have become so incredibly good that this difference in the 4K era hardly matters for 90 percent of all jobs.
The world of interchangeable lenses is divided into two camps: zoom and prime. A 24–70mm zoom covers you while shooting, saving lens changes and thus dust on the sensor — a real argument, especially on location. A prime set (24, 35, 50, 85mm) gives you optically more precise images and usually better bokeh, but requires constant changing. Which way is right? That depends on your workflow. Documentarians tend to reach for zooms, while feature film DoPs build their prime lens cases. Important: Don't forget that every change is a risk factor — dust, slipped marks, lost moments. Some crews therefore have two cameras on set, one with a 24–70mm, one with a 70–200mm, to avoid having to change lenses at all.
Your system — whether PL-Mount (standard on Arri, Red), EF-Mount (Canon), or native Z/L-Mount — determines your lens arsenal. In the long run, this is a more important decision than the camera itself. Once invested, you'll need a whole new mount family if you switch. That's why large productions calculate hard: two identical cameras, each with a different zoom — cheaper and faster than constantly swapping lenses.