You could be forgiven for not knowing the benefits of cinema lenses over ordinary lenses for videographers, but they're hugely useful. Richard Sibley lays out what you need to know
Although optically many cinema lenses are exactly the same as their photographic counterparts, they can physically look very different. Cinema lenses are often larger, with more pronounced focus rings, as well as extra larger focus distance markers.
The reason for the differences are all to do with how the lenses are designed to be used for shooting moving, rather than still, images.
One of the first things you will find yourself asking is ‘why is the aperture setting in T-Stops rather than F-stops?’. The answer is quite straightforward and makes very logical sense. The F-stop is an assumed value based on the diameter of the lens and the focal length. It assumes that all of the light that enters the lens will be used to expose the image, where as in reality some of the light that enters the lens is lost.
A T-Stop is an actual measurement of the amount of transmitted light. As this is an actual, rather than theoretical, value, of the light that is transmitted through the lens, it means that you can quickly switch lenses without the exposure changing. So you can swap between a 35mm lens and a 50mm that are both set to T1.5 and know that the exposure will be the same.
With most cameras offering fully electronic aperture control, finding a contemporary camera lens that has a manual aperture ring can be tricky. Obviously if you are shooting with a matched camera and lens combination it isn’t a problem, with more and more videographers using third-party cameras, such as the Black Magic Cinema cameras, or smaller compact system cameras, such as the Panasonic Lumix GH4 or Sony A7 series, being able to manual control the aperture is essential. When you turn the aperture ring on a camera lens it will click into place at each stop, or each 1/3EV of a stop. Turn a cinema lens and you will notice that there are no hard clicks into place. Once again, the reason for this is to maintain fluidity when shooting moving footage. If you are filming a shot panning from a dark space to a lighter space, you can carefully change the aperture setting as you pan, so that the exposure is correct from start to finish. A good example of this is when a car is driving through a tunnel. By changing the aperture smoothly, but quickly, you can go from light to dark and back very quickly.
The catch is that you will also change the depth of field when you change the aperture. To counter this you can use an variable ND filter instead. And if you are fairly very confident, and have a spare pair of hands, you can try adjusting the aperture and a variable ND filter at the same to change the depth of field, without changing the exposure.
One very obvious difference is in the construction of the lenses. On a photographic lens there may be a knurled rubber or plastic grip around the focus and aperture rings, but on a cine lens, the ring is geared; it has teeth around it. This is so that they can be used with a follow focus device. A follow focus has a gear whose teeth lock with those of the focus ring. This allows the focus of the lens to be quickly, and smoothly, focused with precision. The same goes for aperture rings, which can also be used with follow focus type devices so that the aperture can be precisely changed.
Very expensive cine lenses are often purchased as kits, featuring a whole selection of prime lenses. Often these lenses will be matched for size and weight. This means that the lenses can be quickly swapped, without adjustments having to be made to compensate for the lenses balancing differently on a shoulder rig or stabilisation device.
It also means that any accessories, such as follow focus pullers or ND filters can be swapped easily between lenses.
More affordable cine lenses, such as those by Samyang, are often optically very similar to their photographic counterparts. One thing you may notice with these lenses are the way that they ‘breathe’ as you change the focus distance. Focus at the minimum and maximum focus distance and can see the frame changing at the edges, moving in or out. The very expensive cine lenses have complex designs that mean that this ‘breathing’ effect isn’t an issue, or is at very least kept to a minimum. This allow the focus of a scene to change, without affecting the composition.
For the most part breathing isn’t an issue for enthusiast videographers. The effects are often very slight, and will only be noticeable if you are refocusing within the same scene regularly.