There’s really no trick to understanding lenses for filmmaking. If you’re transitioning from stills photography to videography, you probably already know a lot about conventional cameras lenses, and while they do share certain obvious similarities with their cine lens counterparts, there are some the key differences you should be aware of.
T-stops
No doubt, you’re already well acquainted with the familiar f-stop camera settings that determine the size of an aperture in your lens. Ranging roughly from f/1.2 to f/22, the figure is a ratio obtained by dividing the focal length by the diameter of the entrance pupil. Cine lenses use a more accurate unit, called transmission stops, aka T-stops, to measure the actual amount of light passing through the iris.
T-stops allow the shooter to reliably ensure consistent lighting from shot to shot to shot, especially helpful when dealing with varied angles and other shifting conditions on a film set over a span of time. The T-stop number is calculated by dividing the f-stop of the lens by the square root of light transmittance, which can be impacted by factors such as the number of lens elements and lens coatings.
It sounds complicated, but thankfully lens manufacturers do the legwork for you upfront, and cine lenses are just as easy to read and adjust. Whether its focal length is 40mm, 50mm or 85mm, each of the Sigma T1.5 full-frame prime cine lenses, for instance, maintain a consistent T-stop range of 1.5 to 16 in this series. The Canon CN-E 20-50mm EF T2.4 EF and CN-E 45-135mm EF T2.4 EF, on the other hand, both maintain a constant T2.4 throughout their respective zoom ranges.
Focus rings
Most contemporary still-photography lenses adjust focus electronically, although you usually do have the option to shift to manual focus mode. Most professional cine lenses, on the other hand, typically omit an autofocus feature and rely instead on a manual focus ring that’s built into the barrel and marked with visible stops, allowing you to execute focus pulls and offering more control over focus throw (the focus rotation angle on a cine lens is more than twice that of most still lenses). You’ll also find a zoom ring on cine zoom lenses to keep the image in focus while zooming, as well as iris rings etched with those T-stop markings mentioned above.
Specialized features
Anamorphic
Known for their ability to produce a wide field of view with little distortion, anamorphic lenses compress an image while the camera is recording, which then needs to be stretched out in post-production to achieve expansive cinematic 2:39:1 aspect ratios. The more common spherical lenses, on the other hand, project images onto a sensor without affecting the aspect ratio.
Parfocal
Cameras used in still photography employ what’s called varifocal lenses – the moment you zoom the lens, you vary the focus. By contrast, a parfocal lens holds its focus whenever you zoom in and out, which is crucial in filmmaking where zooming can become a decisive factor in the creative effects you want to achieve.
Entry-level alternatives
Known for their impeccable optical performance and superb build quality, high-end cine lenses are bigger and heavier than many still-camera lenses, as well as far more expensive and usually beyond the reach of most newcomers to filmmaking. The good news is that brands like Canon, Panasonic and Sony now offer budget-friendly digital film camera models – including the EOS R5, Lumix GH6 and FX30 – that deliver pro-level results with a wide range of more affordable lenses to choose from as well. Third-party brands like Rokinon also offer economical alternatives including the Xeen line of cine lenses available in Sony E-mount and Canon EF-mount.
Focus breathing
Whether you’re shooting with a prime or zoom lens with an internal focus design, focus breathing can happen when focusing and certain lens elements move while others don’t, resulting in slight but jarring changes in the appearance of image size. This can even even occur with some of those more expensive professional cine lenses. Some lenses, however, like the Sony SEL FE 20-70mm f/4.0 G E-mount lens and Rokinon’s Xeen lineup are designed to minimize focus breathing, and some cameras, like the Sony Alpha A7 IV, now come equipped with a built-in focus-breathing compensation feature to help deal with this issue.