Bad 7D Aliasing - LED lights?

[kenwilx1016]

So I recently shot a music video. One of the scenes was shot on a stage using the house lighting which are LED lights.

This is a freeze frame from one of the clips. as you can see the motion of the talents head is deplorable.

The settings were as follows:
ISO 800
Shutter: 50
Aperature: 2.8
1080p 24fps

My question is, what caused this?? Could it have been the LED lights? Also, can it be fixed in post?

 

[shyguyfilms]

It doesn't look like aliasing to me, though I may be wrong (I'm new to the whole DSLR video thing myself). My guess would be that it IS caused by the lights. Some LED lighting is achieved by flashing alternating LEDs rapidly so they appear to be on all the time. A lot of electronics people do this so they can control a large number of LEDs with a smaller number of control outputs, technically this is either charlieplexing or multiplexing, or so they can control more lights w/ less power.
In reality, not all the LEDs are on at the same time and when the singer/guitar player-guy moved, you captured the same effect you see if you wave your hand in front of a fast strobe light (the kind used for halloween/in clubs).

 

[Rick Burnett]

I'd guess the lights as well, that's why you are seeing distinct copies of the singer as he moves, when the lights are the brightest. There are a few other threads I read in the past on here about the same issue. I don't think you are going to fix that easily in post.

 

[TimurCivan]

The LED is flickering and cause multiple exposures per frame. Nothing you can do about it.

 

[Samuel H]

lights

and if that's right then it would be there too if you had shot on film, RED, alexa, EX3, consumer camcorder, iphone, anything; did anybody else shoot at that gig with a different camera?

 

[Implicit]

I had this effect when i was shooting at night under a lampost, they must flicker as well. At least i know it was the lighting not the camera that was the problem!

 

[Rick Burnett]

Easiest way to see if the lights you are under are blinking is increase your shutter speed for a moment very high. You'll be amazed at how many LED lights are actually flickering at high speed. There is no way to film with those LEDs because they turn on and off so fast that they are basically creating a strobe effect if they are lighting a moving object.

 

[M. Gilden]

+1
It is absolutely the LED lights, nothing at ALL to do with aliasing. One of my photographer buddies (Marc Kushner) took an interesting picture in the NYC subway that helps illustrate what we're talking about:

You can clearly see the LED sign on the side of the train updating multiple times during the exposure because it is not a smooth blur like the rest of the train.

I knew what the problem was as soon as you said "LED lighting". That's a pretty big no-no. Anything with a noticeable refresh rate will be a problem.

 

[Alex H.]

Shutter: 50

Where are you located? If you are in NTSC territory, you're working with a 60Hz electrical system and any shutter speed other than 1/60 is going to cause problems under artificial lights. You have to match your shutter to the electrical system. (PAL territory = 50Hz electrical = 1/50 shutter.)

That said, apparently, dimmable LED lights cause the problem no matter what and there's no way around it.

 

[M. Gilden]

Where are you located? If you are in NTSC territory, you're working with a 60Hz electrical system and any shutter speed other than 1/60 is going to cause problems under artificial lights. You have to match your shutter to the electrical system. (PAL territory = 50Hz electrical = 1/50 shutter.)

While that is true, I'm under the impression it wouldn't cause this. What you mention causes a completely different problem that is more related to a fluctuation in brightness over time. For a music video, that's honestly not necessarily a bad thing, and for short clips it is often not noticeable. I've shot at 50 before (and actually at 1/48 using ML to get the exact speed) indoors in NTSC territory. But this is multiple exposures in the same frame.

 

[Cosimo Bullo]

+1
It is absolutely the LED lights, nothing at ALL to do with aliasing. One of my photographer buddies (Marc Kushner) took an interesting picture in the NYC subway that helps illustrate what we're talking about:

You can clearly see the LED sign on the side of the train updating multiple times during the exposure because it is not a smooth blur like the rest of the train.

I knew what the problem was as soon as you said "LED lighting". That's a pretty big no-no. Anything with a noticeable refresh rate will be a problem.

GREAT example!

 

[Alex H.]

While that is true, I'm under the impression it wouldn't cause this. What you mention causes a completely different problem that is more related to a fluctuation in brightness over time. For a music video, that's honestly not necessarily a bad thing, and for short clips it is often not noticeable. I've shot at 50 before (and actually at 1/48 using ML to get the exact speed) indoors in NTSC territory. But this is multiple exposures in the same frame.

Tungsten isn't the biggest culprit in this. Florescent and HMI are bad. And dimmable LED is apparently the worst of all. But why would it not be the cause of this? If the flickering of the light source is pulsating at a faster rate than the shutter speed, the exposure of the frame is going to capture multiple pulses of the light, yes? When those frames are put into motion, the problem is noticeably not in the exact same spot for each frame, and thus the rolling bands of nastiness.

 

[natums]

Try a variety of shutter speeds, you might find a 1/30 gives enough blur-per-flicker to give a nicer overall effect despite having more blur.


Also, you can change shutter speed in ML?! I keep finding more crap you can do with that thing every time I go online.

 

[M. Gilden]

Tungsten isn't the biggest culprit in this. Florescent and HMI are bad. And dimmable LED is apparently the worst of all. But why would it not be the cause of this? If the flickering of the light source is pulsating at a faster rate than the shutter speed, the exposure of the frame is going to capture multiple pulses of the light, yes? When those frames are put into motion, the problem is noticeably not in the exact same spot for each frame, and thus the rolling bands of nastiness.

It sounds reasonable, but I've never seen it cause anything like this before. Rather, most of the problems (in my experience at least) are more like light shifting- getting darker and lighter over time, sort of like the way the phase shift of a CRT monitor results in a horizontal line that scrolls upwards (or downwards, depending on shutter speed).

I think these are worse because there is actually a noticeable moment of them being OFF, not just getting darker. I'm not 100% sure though. Have you see this sort of effect caused by the shutter in the past?

Also, you can change shutter speed in ML?! I keep finding more crap you can do with that thing every time I go online.

Oh, yes. Magic Lantern is incredible.

 

[Rick Burnett]

Tungsten has a slower on/off time so it continues to glow while the power is changing. The problem is lights with really fast response times because they don't remain on for very long at all after power is switched.

As to the effect seen here, the math is simple. There are almost (roughly counting) 7 steps in the image scene. This means the light has to flash 7 times in 1/50 to make that effect (assuming the artist was moving the whole time the frame was exposed and never stopped through the whole 1/50). Therefore that gives us a rough cycle time of at least 2.857ms or 350Hz. And that is probably not even consistent because it depends on the dimming setting of the LED fixture.

Standard tungsten lights turn on and off 2 times a second, so they really happen at 120Hz, however, because of the response of the bulb is so slow, it's not a problem.

Lights like HMI and Fluorescent turn on and off a lot faster (there is more range in their brightness over time). This causes TWO issues:

1. If your frame rate is not a multiple of 1/60 in the US, each frame can have a different brightness depending on which portion of the cycle was captured in each frame. This is flickering like artifacts.

2. Given the cameras use a rolling shutter, as the shutter moves down the scan lines, the light is still changing constantly. This can cause banding on one frame. Nikon D90 had this really bad. The slower your rolling shutter, the more susceptible to this affect you are (and the D90 read/reset is horrible).

 

[David G. Smith]

I ran into a rolling shutter issue with LED stage lighting and it was addressed in another thread. I have some video examples of what I ran into. I got some great responses about it. Check this link:

http://www.dvxuser.com/V6/showthread.php?243016-Horrible-flicker-noted-under-LED-stage-lighting-example-video.

 

[M. Gilden]

Tungsten has a slower on/off time so it continues to glow while the power is changing. The problem is lights with really fast response times because they don't remain on for very long at all after power is switched.

As to the effect seen here, the math is simple. There are almost (roughly counting) 7 steps in the image scene. This means the light has to flash 7 times in 1/50 to make that effect (assuming the artist was moving the whole time the frame was exposed and never stopped through the whole 1/50). Therefore that gives us a rough cycle time of at least 2.857ms or 350Hz. And that is probably not even consistent because it depends on the dimming setting of the LED fixture.

Standard tungsten lights turn on and off 2 times a second, so they really happen at 120Hz, however, because of the response of the bulb is so slow, it's not a problem.

Lights like HMI and Fluorescent turn on and off a lot faster (there is more range in their brightness over time). This causes TWO issues:

1. If your frame rate is not a multiple of 1/60 in the US, each frame can have a different brightness depending on which portion of the cycle was captured in each frame. This is flickering like artifacts.

2. Given the cameras use a rolling shutter, as the shutter moves down the scan lines, the light is still changing constantly. This can cause banding on one frame. Nikon D90 had this really bad. The slower your rolling shutter, the more susceptible to this affect you are (and the D90 read/reset is horrible).

Exactly, that's how I thought it worked.
So, like my example earlier with the CRT monitor, you're getting the average exposure of light at that moment, not multiple exposures. Thats why I've never gotten anything like this shooting at 1/48 exactly. Its the LED, even at 1/60 you'd likely have it.

 

[Rick Burnett]

With the LED you are always going to have it on moving objects that are illuminated by the LED. The only way to avoid it would be to shoot at the exact frame rate of the blinking LED and stop recording before the blink starts again. Which of course is impossible!

 

[PDR]

I ran into a rolling shutter issue with LED stage lighting and it was addressed in another thread. I have some video examples of what I ran into. I got some great responses about it. Check this link:

http://www.dvxuser.com/V6/showthrea...noted-under-LED-stage-lighting-example-video.

I believe this is slightly different than the OP's problem. This type of LED flicker can be partially treated in post by temporal smoothing and/or deflickering algorithms.

Download this A/B compare video and have a peek (If you're on a mac, use something like VLC to play it).
http://www.mediafire.com/?5s13xjf59j91x23

It's not ideal , but sometimes it can turn unusable footage into something usable

The "negatives" include weird temporal artifacts if you use strong settings, loss of details (smoothing), increased processing times. Often you would use a mild sharpener after to give the perception of details (I didn't in this example)

 

[David G. Smith]

I believe this is slightly different than the OP's problem. This type of LED flicker can be partially treated in post by temporal smoothing and/or deflickering algorithms.

Download this A/B compare video and have a peek (If you're on a mac, use something like VLC to play it).
http://www.mediafire.com/?5s13xjf59j91x23

It's not ideal , but sometimes it can turn unusable footage into something usable

The "negatives" include weird temporal artifacts if you use strong settings, loss of details (smoothing), increased processing times. Often you would use a mild sharpener after to give the perception of details (I didn't in this example)

Wow, that is very good. So what temporal smoothing and/or deflickering algorithms did you use? I am on a Mac, if you are not, are their Mac equivalents? Thanks.