Flickering footage at 120fps

[Miguel "Macgregor" De Olaso]

So, let me explain you a weird situation that i´ve seen a couple of times.

When shooting 120fps with a shutter speed of 1/250 and tungsten light sources in Europe (50 Hz) i get visible flickering in the image.
But when you change the settings to 100fps but you leave 1/250 of a sec, the flickering dissapears. What the???

Then in other situations using flicker free HMIs, the same thing happens.
120fps 1/250 flickering.
100 fps 1/250 no flickering.

Anyone experience this?

 

[Petter Brox]

Well, I suppose it's because you can't divide 120 (Fps) by 50 (Hz)... i.e. your framerate and your lightsource are out of sync.

 

[David Birdy]

The frame rate is seeing the "flicker" Or "Hertz cycle"... You can adjust your frame rate to aviod the flicker with a little trial & error testing...

As you adjust the shutter, the shutter limits the amount of light hitting the sensor. If you increase frame rates, (increasing scanning) you increase the need for light reaching the sensor.

Dave

 

[Hans von Sonntag]

Petter and David are right. I try to summerize the problem:

180 degree shutter at 25 pfs means a 1/50

180 degeree shutter at 100 fps leads to 1/200

180 degree shutter at 120pfs is a 1/240

You shortend the shutter from 1/240 to 1/250 because you can devide 250 by 50. On the surface this seems to be meaningful but obviously it's not.

I think follwing is happening, despite the fact that your light sources are flickerless (which they are obviously not):

The light switches on and off every 50th of a second. When shooting 100pfs 2 frames get exposed with one light circle. This won't change until one of the two gets out of sync, either the camera has a higher/lower frequency (frame rate) or the Hz of the lightsource changes. This is happening when the camera is overcranked to 120 fps. The lighing source is not exposing exactly 2 frames but 2 + 20%, the whole thing gets out of sync and the picture sequence will flicker.

What bothers me is the fact that you used flickerless sources.

Hans

 

[Miguel "Macgregor" De Olaso]

Ok, problem solved.

I did my math wrong between 50 and 60Hz.

120fps at 60Hz is fine.

http://www.cinematographyelectronics.com/index_flickerfree.html


BTW, is 150fps safe with flicker free HMIs and fluorescents at 50Hz?

 

[Petter Brox]

BTW, is 150fps safe with flicker free HMIs and fluorescents at 50Hz?

Yes, it should be (at least with the shutter set to 180 degrees), but then you'll have to choose a different camera (Or wait for EPIC...).

Petter.