Variable ND filter for film
The cmotion Cinefade filter is made up of two custom polarising filters which can be used as a motorised variable ND filter that is remotely controlled by a lens control system to allow for precise and fast exposure changes when the camera is inaccessible or when the creative brief calls for an exact T-stop to maintain a consistent depth-of-field.
The Cinefade variable ND filter can be used on any camera and cine-lens. It consists of two polarising filters which sit inside a matte box. One of the 4x5 filters remains static while the other is held inside a geared FilterTray. An ARRI CLM4, CLM5 or cmotion cforce motor attaches directly to the handle of the FilterTray without the need for any additional mounting equipment.
Mechanical end stops inside the FilterTray allow the system to easily and quickly calibrate itself at the push of a button, similar to how a lens is calibrated. The filter will therefore not spin freely and rotate through 360 degrees but is limited to a 90 degree rotation and is set to the positions between minimum and maximum light transmission.
The variable ND filter is able to attenuate up to 5 stops of light with an initial light loss of 1.5 stops at full transmission without any colour cast.
There are surprisingly few options available for professional filmmakers who want to use a variable ND filter on professional digital or analogue film camera systems, such as an ARRI alexa, Sony VENICE, RED or Panavision camera. Variable ND filters are commonplace in the photography market with screw-in filters that are rotated to vary light transmission. However, in professional motion picture film production, very few options exist and it is certainly not (yet) commonplace to mount a variable ND filter in front of the lens at all times. Some of the few available options are listed below.
PANAVISION LCND (LIQUID CRYSTAL ND)
Announced at Cinegear 2018 in Los Angeles, Panavision revealed plans to launch a Liquid Crystal Neutral Density (LCND) filter that allows filmmakers to dial in the light transmission in hard stops or gradually over time.
A prototype was displayed at the Panavision booth, which showed promise but was also greeted with a degree of pessimism. Historically liquid crystal technology has not met the expectations of professional cinematographers in terms of optical performance, reliability and longevity.
The technology is prone to degradation over time when exposed to light, very sensitive to small temperature changes and causes image artefacts.
Even though I am not an expert in the optical field, having spoken to experts in the relevant field, Panavision must have made some serious innovations in the area to feel comfortable with branding this technology with their highly regarded brand and I am looking forward to testing this technology when it becomes available in 2019.
SONY FS7 II & FS5 ELECTRONIC VARIABLE ND FILTER
Also using similar technology to the Panavision LCND filter, Sony introduced a electronically controlled variable ND filter in the prosumer camera Sony PXW X-180 and X-160 a few years ago and has since expanded the feature to the FS7 II and FS5 camera.
The problem with this solution is that the filter is built into the prosumer/semi-professional cameras so that you are only able to use them with specific Sony cameras.
If Sony feels like the optical performance of this filter will withstand the scrutiny of professional cinematographers, the electronic variable ND filter may soon be built-in to the Sony Venice cameras or be offered as a camera accessory, similar as to the Panavision LCND and our cmotion Cinefade Filter.
Interestingly, Sony has also realised the potential of using the variable ND filter for varying depth-of-field in one shot, similar to the Cinefade effect. The iris ring is rotated and the electronic ND filter automatically maintains the exposure, only affecting the sharpness of the background. Obviously, again you are limited to using specific Sony cameras and lenses, which do not expand to professional cine-lenses.
TIFFEN MULTI ROTA TRAY (MRT)
Tiffen markets their variable ND solution as the "world's lightest", which they achieved by fitting two very slim polarisers into one 4x5.65 sized geared filter tray. At first this may seem like a good solution, the two pieces of glass however are easily broken, especially when handled on set and during transport. Moreover, optical quality is lost by slimming the glass filters down and experts from Schneider and Arri agree that thicker glass results in better optical performance.
Also, It is not possible to easily mount a motor onto the tray to allow a LCS to remotely control the amount of ND, nor does it have mechanical end stops like our cmotion Cinefade Filter Tray to easily calibrate the filter, similar to how a lens is calibrated.
Similar products are RotaPola frames, which can be combined with a second polariser to form a variable ND filter. Besides our own cmotion Cinefade Filter, Arri released a Rota Pola frame last year, Vantage and Schneider have large Pola frames amongst other smaller competitors.
RED MOTION MOUNT (TESSIVE FILTER)
The RED Motion Mount started life as the Tessive Time Filter, which amongst other uses could be used as a variable ND filter. The company and its patents was transferred to RED, which released the much criticised Motion Mount which sports a variable ND filter as one of its features. Even though it sounds like a great solution that can be mounted between lens and sensor, thus keeping weight down to a minimum and freeing up space in the matte box, it is not a universally compatible product that can be used on any camera system.
Critics did not think the filter performed well in terms of optical performance, reliability and longevity and the product does not seem to have sold as well as expected.
With the close collaboration between RED and Panavision, it is possible to imagine that Panavision utilised the technology contained within the patents held by RED/Tessive and incorporated it into their new LCND filter announced at Cinegear 2018 but no clear word on that speculation yet.
CMOTION CINEFADE VARIABLE ND FILTER
OUR CMOTION CINEFADE VARIABLE ND FILTER
CAN BE USED ON ANY FILM OR DIGITAL CAMERA
CAN BE EASILY CALIBRATED USING THE BUILT-IN MECHANICAL END STOPS
HAS A GEARED RING FOR MOTORISED REMOTE CONTROL WITH A LENS CONTROL SYSTEM
HAS A MOTOR MOUNTING POINT FOR EASY RIGGING OF LENS MOTORS
ATTENUATES UP TO 5 STOPS OF LIGHT WITHOUT COLOR CAST
LOSES ONLY 1.5 STOPS OF LIGHT AT FULL TRANSMISSION
USES HIGH QUALITY CUSTOM SCHNEIDER POLARISING FILTERS
Motorised variable ND filters can make the AC's job easier and faster, as he/she does not have to constantly change full ND filters in ever-changing outdoor lighting environments. They also speed up the DP's exposure control while giving him more precision, as he/she is no longer limited to full-stop ND filters.
Shots where the camera is inaccessible such as on Russian Arms or even creative shots such as INT to EXT Steadicam transitions with a constant (shallow) depth of field without having to ride the iris are possible (read more).
Let us know your thoughts on variable ND filters and if you consider them to be a useful addition to your professional film set. Finally, please note that the author is not a filter expert and is only presenting his personal point of view in this article.
To find out more about Cinefade and our cmotion Cinefade filter visit www.cinefade.com.