Need a bit of advice ref a video I did recently. I was a very sunny day and when I passed over the unit roofs I was shooting I got what I can only describe as a reflection off the metal roof.
My first thought was to use ND filters but I would appreciate the help as I’m unsure.
I have attached a downscaled video below, hopefully, you can see what I’m mean.
Thanks in advance.
Moiré patterns. The effect created by differential frequencies between the roof patterns and the camera pixels.
See them on TV news progs when they interview people that have narrow stripey shirts, etc - which is why you rarely see stripey clothing on TV people and in produced shows. And that’s because they’re a bugger to prevent.
Your camera has a rolling shutter. This is not a physical mechanism but rather the pixels on your cameras sensor are activated in descending rows, think of it a bit like a narrow slit moving down the sensor at a speed determined by whatever you set your FPS as which is the same as the sampling frequency.
Now the horizontal lines on the roof caused by the corrugated construction will appear to interact with the speed of the rolling shutter which results in the formation of the Moiré pattern. As you move further up the corrugated effect on the roof becomes narrower, meaning more horizontal lines will be sampled by the opening in the rolling shutter. This is a form of aliasing, or distortion.
Aliasing is a result of the device doing the sampling, in this case the rolling shutter, sampling at a frequency that is less than twice the frequency of the object being sampled, in this case the relative separation of the corrugated construction of the roof.
It’s much easier to demonstrate this effect with audio. Consider a commercial CD. The highest expected frequency is 22KHz so to reproduce this recording digitally it needs to be sampled at 44KHz or higher. If the sampling rate is less than this the result is that the higher frequencies, such as say snare drums, will sound distorted and unnatural. The same applies to video sampling but in this case patterns within the frame will appear distorted. A very basic example of this is when you see something like the wheel of a car on video appearing to rotate backwards when the car is actually moving forward. This is because the shutter speed of the camera is less than twice the rotational speed of the camera shutter, i.e. the wheel is being under-sampled.
A clever bloke called Nyquist, along with another Tefal Toaster Head called Shannon, produced a simple equation, known as the Nyquist-Shannon theorem whereby to truly reproduce a waveform (audio, video) it has to be sampled at a minimum of twice the highest frequency. The basic representation of this theorem is below.
N(number of samples) = 2 x hf(highest frequency to be sampled)
Regards
Nidge, whom is still at a loss as to why I don’t get invited to parties.
Wow, thank you for such a detailed response, im afraid this in the main has literally flown over my head but I trust in what you say.
So is this something I have to live with or is there a way to eliminate or minimise my exposure to this effect?
Hazco
Trial and error to minimise the problem.
The pixel frequency across the frame remains constant … the roof pattern frequency will depend on height. Certain heights will lessen/increase the effect.
Rotating so that the roof pattern doesn’t lie close to parallel to the frame edges may also lessen/increase the effect … or change how it appears completely.
This gives you some idea how the interference manifests itself during rotation.
You can always rotate back to the required orientation in post … but this can cause them to reappear - but probably less annoyingly.
Google “Moiré Patterns” for a better appreciation on how regular patters interact to create them when “superimposed” … and when recording you are superimposing the pattern of the pixels over the pattern of the roofing.
It is as the others have said down to trial and error but yes you’re right, in this case, ND filters won’t be of any use
Holy Moly very impressive. Just say that again:grimacing:
Yes, I used to work in TV and it’s as above! A bit of uncontrollable ‘strobing’ We sometimes gave the guest another shirt or whatever was ‘strobing’.
Thank you. I Always wondered what that was and how to avoid it. This sort of information makes being part of GA very worthwhile in my opinion.
Here’s an example with a short unedited video where you can see the vertical lines in the buildings and the boat masts flickering. This seems to be more around the centre of the frame. Can anything be done to prevent it?
Changing frame rate ?
I thought it was more a “noise” caused by a tight pattern and the frame rate?