The biggest and most noticeable difference between still and cine lenses is the price. Some of the top-end lenses used in Hollywood cost a fortune.
What’s so special about them and why cannot we use still lenses to shoot motion picture?
Sure we can. And many filmmakers operating on a tight budget do. But once we discover all the small (and big) details about cine lenses it becomes clear why professional cinematographers use heavy and extremely expensive glass and why it makes such a difference.
There are quite a few differences. Let’s have a look at the lens qualities that are not welcome in the cinematography world.
Most of the still lenses suffer from what is called lens breathing. What it means is that when we change the focus on the lens the field of view changes (the lens zooms in or out a bit). This can be clearly noticed in the animations below:
In the above animation, I used my photo lens and I just rotated the focus ring from the minimum to the maximum distance. Just have a look at the edge of the frame and you will notice the obvious movement/zoom. In the cine world, we want to avoid this effect as much as possible (in order not to distract the viewer when doing subtle rack focus).
Most of the still zoom lenses are vari-focal, which means that whenever we zoom in and out, the focus point changes. Let’s say we use 70-200mm zoom lens. We are at 70mm and we focus on a person 2 meters from us. When we zoom in to 200mm the focus is gone and we have to adjust it again (which in most cases will also exhibit a breathing mentioned in the previous paragraph).
While the above effect isn’t a concern for a photographer, in the video world it’s common to zoom in or out during a single take. Using vari-focal lens to keep a subject in focus while zooming would be very difficult. That’s why all the cine zoom lenses are parfocal.
Also, most Cine lenses have an internal adjustment to calibrate tracking. Zoomed all the way in, the object in the center of the image should stay in that exact position, when zoomed out.
Color/contrast and size weight matching
Cine lenses come in sets. For example,w Cooke S4 or Zeiss Master Primes are sets of prime lenses from the wide-angle to the telephoto focal lengths. What’s important, they are also matched to yield results that are as close as possible in terms of color and contrast.
All the lenses from the same set are also matched to be the same size and weight so the camera doesn’t need to be rebalanced and all the accessories such as follow focus or matte box can remain in the same position and don’t need to be readjusted when swapping to a different focal length.
Notice that all the lenses in the picture above have the same length and diameter.
In cinematography, focus pullers rely on the focus marks on the lens. They have to be calibrated properly and be very precise. Otherwise, the 1st AC’s job would be a lot harder. In photography world, especially when using auto-focus, focus marks on the lens are not very useful.
Professional cine lenses feature focus ring teeth and are designed to work with follow focus system. In the picture above we can see that the focus throw is long, compared to still lenses where it is usually very short to allow the AF to focus quickly.
Cine lenses also feature hard stops for the focus ring, which means that once we reach minimum focusing distance or infinity the focus ring stops. In still lenses, this is not the case, and we can rotate the focus ring over and over again. This is extremely important feature when a follow focus is used – when we draw marks on the follow focus ring they become useless when we go out of scale with a lens where no hard stops exist.
Still lenses are designed to be lightweight. All the internal parts of the AF system are built from a very delicate materials in order to be able to use as small and silent motors as possible. This makes them more fragile and prone to damages.
Cine lenses are made of high-quality metals and alloys. They are durable and weatherproof. Professional film glass is meant to be rented and used constantly in various conditions and it must perform reliably. Lens malfunction causes delays for the whole crew on set.
This point is strictly connected with a previous one. Cine lenses are workhorses and it’s important to make the service process as easy and as fast as possible.
Film glass is designed to be easily disassembled and calibrated.
Photo lenses are more difficult to service due to AF & VR electronics and tiny and delicate parts make the whole process much more tedious.
Vignetting, sharpness in corners
Professional photography lenses tend to vignette very little, but it’s much easier to correct it in post, than in the video. It’s also much more noticeable when there’s a movement in the picture. Professional cine glass doesn’t vignette almost at all.
Uniform and calibrated T-stop performance
We use F-stops to describe still lenses and T-stops to describe cine lenses. What’s the difference and why does it matter?
F-stop describes the opening of the lens. It means that two different lenses with the same F-stop would have the same opening that we could measure. But it doesn’t mean that the same amount of light hits the sensor. If the lens is built differently there’s a pretty big chance that there will be a different amount of light hitting the sensor.
It is a very critical aspect in cinematography to have precisely the same amount of light in the same scene using different lenses. Shifts in exposure would make it difficult to match the footage later and would slow down the post-production process, or even cause some damage to the footage.
That’s why there is a T-stop in cinematography which tells us exactly how much light is transmitted through the lens to the sensor/film. This parameter alone makes the still lenses less practical.