Strange moire patterns on the flare of a headlamp

[Domen Martincic]

Hi everyone!

This is my first post and I hope I have chosen the right subforum. I have been a lurker for a long time and I have to say it's a great privilege for me to be able to read insightful discussions and be able to learn from experienced people in the community. Internet, in that respect, is amazing. That being said, I have an issue that's puzzling me and I am not sure why it's happening.

I've shot a few tests of scenes where the main (and only) light source would be a headlamp. We tried only LED headlamps as they are most common these days. Some were flickering on different dimmer levels, but I was expecting that. I was not expecting this. 

There is a moire pattern in the flare. The air was very humid and there was slight drizzle so the light flares are really pronounced. The pattern appears and disappears as the head moves. It's at its most pronounced when the head is turned directly at the camera and the flares are strongest.

Light source is a cheap Decathlon LED headlamp:

https://www.decathlon.com/collections/camp-hike-headlamps/products/hl900-usb-v3-600-lumen-rechargeable-head-torch-327493?variant=40155598815294  

I figure it could be a poor LED problem? Frequency, maybe plastic lens, something? Camera is Alexa Mini LF, 3.8K ProRes, lens is Ultra Prime 32 mm.

The DI grader that I'm working with said that he's seen similar patterns in footage with LED lights, but never in the flare.

Please help!

 

[David Mullen ASC]

My guess is that it’s a scrolling roll bar effect from the frequency rate of the LED.

[Domen Martincic]

Thank you, Mr. Mullen. 

I've posted this question on the cinematography subreddit and got this comment from user JonahFlechette:

Quote

This is not because of the headlamp, but either the lens or the filter you have on your camera. Some lenses will cause flare aberrations like this due to the way the glass elements are manufactured and designed. Otherwise a doublet (two glass elements sandwiched with optical glue) in your lens might have separated, causing this effect. Some filters (ie ones made of plastic or resin) can cause this effect too.

Source: I’m a lens technician

Edit: saw the comment that said this is an Ultra Prime. Yes, Ultra Prime does this.

What do you guys think? Has anybody had this experience? 

[Nicolas POISSON]

So it would be some kind of flare ? Since the headlamp is a tiny but powerful source of light pointed right to the lens, this makes sense. The overall black background would also help to make this flare visible.

[Domen Martincic]

It appears to be some kind of flare. I stopped the lens down a bit - from 1.9 to 2.0 and it has then completely disappeared.