Flickering image and the 24p myth

Some of the recent major projects encountered a small technical problem with a flickering light level in the recorded image.

The problem results from a combination of frequencies that interfere with each other. The lights operate at a particular frequency (measured in hertz), the shutter speed on the camera is another frequency, if using the RedRock or P+S then the spinning glass inside the mount is oscillating at another frequency and of course there is the frame rate itself.

The common way to solve these problems is generally by altering the shutter speed on the camera. The SonyEX1 also has a particular function to electronically shift the frequency to avoid the problem. The P+S can adjust the speed of the spinning glass in the mount which can also be used to circumvent the light phasing.

Generally however the main culprit we’ve found is the HMI lighting and the 24p mode on the JVC201/251 cameras. The frequency of the light from the HMI’s oscillates in cycles (hertz) that line up with the 24 frames per second speed of the camera resulting in a flicker.

Fortunately the problem is very easily solved – shoot 25p instead of 24p. The frequencies no longer line up and the flicker is gone.

There are good reasons to shoot 25p on top of circumventing this issue. 25fps is commensurate with PAL broadcast and delivery standards – TV and DVD and all electronic media, where 24p is not. 24p is Only  good or useful for making a celluloid print for theatrical release projection. Just about all the short film festivals around the world now very rarely project short films from celluloid so you’re unlikely to ever need to make a film print of a short film.

25p and 24p look exactly the same to the human eye, its impossible to tell the difference so you arent sacrificing anything in terms of the aesthetic of the image. And its no problem at all to produce 24p version from a 25p source if you do need to make a theatrical print; 1 frame per second is simply removed which is undetectable to the human eye.

24p became very popular for video in the USA and North America because the NTSC frame rate is 30fps. 30 and 24 look very very different and so 24p became highly desirable to get away from the plastic smooth TV-look of NTSC. But in PAL countries (Europe and much of Asia) there is no visual benefit in shooting 24p over 25p, no human eye can tell the difference. 25 is more broadly compatible and looks exactly the same.

So within the school the recommended frame rate to use is 25p rather than 24p. Both the JVC cameras and the Sony EX1 can shoot native progressive 25p and this will avoid any issues with flicker from the HMI lights as well as give the most compatible format. You can of course always export to 24p if need be and create a 24p master that will be visually identical.