The Strobe Effect – How to match your Camera to your AC Mains

The other day, I took some shots with the Blackmagic Pocket Cinema Camera of our newly decorated Christmas Tree. I only used the light coming from the electric christmas tree LED lights, fed from the mains with 220 Volts, 50 Hz. The scene was shot on CinemaDNG RAW, with the camera set to 24 fps, 180° Shutter angle, 3200K, and ISO 800. I used the Zenit/LOMO Meteor 5-1 lens with fully opened f/1.9 aperture at various focal lengths between 17-69 mm. Camera was mounted to the Fancier FC-270A tripod and FC-02H fluid head. Looking at the footage in DaVinci Resolve I experienced a quite severe strobe effect in the form of pulsating waves. These waves are clearly discernable on the first short sample clip of ungraded footage rendered to a DNxHR HQ codex in a QuickTime wrapper, although the strobing is much more obvious when looking at the RAW image.

My first thoughts was that the strobe effect was due to the christmas tree lights and the frequency they created (50 Hz) which could create an issue with my 24 frame per second format; perhaps I should have set the camera to 25 fps instead? I did a search on YouTube and found a clip from Dombowerphoto (see below) which seems to confirm my suspicions; he recommends changing the frame speed of my camera to match the 50 Hz of my mains. However, the christmas tree shot is a part of a larger project which I shot on 24 fps. If the strobing comes from a faulty frames per second setting, what setting should I have used instead? And how to intercut it with the other footage shot on 24 fps in DaVinci Resolve? Or is there a way of working around the problem without changing the frame per seconds setting? These were the questions that I posed to the Blackmagic Forum and BMCuser.

There was a consensus in the Blackmagic forum and user community that the issue lays not in my frames speed but in my shutter angle setting, i.e. 180°, which creates a 1/48 seconds shutter speed. This setting is adapted for countries who use 60 Hz main power grids and NTSC colour television systems. For countries adapted to 50Hz and PAL colour televison systems, using 24 fps the safe Shutter Angles are 86.4° (corresponding to 1/100 seconds shutter speed), 172.8° (1/50 s), and 259.2° (1/33.3 s). If 172.8° wouldn’t work, I could try to change the shutter angle number one stop down (i.e. 86.4°) or up (259.2°) and check again. Alternatively, I could change the speed to 25 fps. As can be seen from the second clip of the test film above, changing to a 172.8° shutter degree angle, corresponding to 1/50 seconds shutter speed, was sufficient. I will use this camera setting henceforth when filming both outdoors and indoors. I prefer that of changing to 25 fps. My Sekonic L-398 light meter reads 1/50 seconds shutter speed on its Cine Scale anyway, which makes 172.8° even more optimal.

The Blackmagic Pocket Cinema Camera setting on 172.8 degrees Shutter Angle
The Blackmagic Pocket Cinema Camera setting on 172.8 degrees Shutter Angle

Another solution is to change the frequency setting on the LED lights using a dimmer. The thing with LEDs and other modern lighting solutions is that I might never find a perfect match that synchronizes with my shutter angle. And doing so would limit my freedom over the footage and exposure as well. It might work for me more optimal if I modulated the lights at a certain frequency other than 50 Hz. If the lights have several settings, I could check if there is a lower or higher intensity which would provide a more favourable strobe frequency. However, tungsten lights are the ones preferred for filming as the light created comes from a burning filament which stays lit withouth any blinking, even though the alternating current bounces, because the filament don’t have any time to cool down; LED lights actually shut off immediately each time the current bounces which creates visible flicker on camera (but not directly to the eye). The Austrailan Cinematographer John Brawley, who features in my favourite book Cinema Raw by Kurt Lancaster, considers consumer LED lights to be a disaster:

Even the same flicker speeds aren’t safe. A lot of them just don’t adhere to those old rules and use PWM to dim them. Seeing it a lot with exit signs too now. It’s a plague at any shutter or frame rate combination. So much for standards and convention. No one cares about this stuff.

The problem with this particular shot that I made with the Christmas Tree is that our lights don’t allow any dimming or modulation of the frequency, so I’m stuck with changing the shutter angle. Also, I won’t do any reshot of the scene as I have ample footage with marginal or almost no visible strobing in certain close-ups, and when rendering into a compressed format the strobing is mitigated anyway. I hope to have my new short entitled XmasTree Hunt, in which our Christmas Tree will feature as the main character, ready by New Years Eve.

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