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Why does my arm dissappear? Wierd

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chris pike
Why does my arm dissappear? Wierd
on Feb 16, 2018 at 12:47:48 am

These are screen shots from video. Shot this with a Canon 5D mark iii. The cameraman said, hey your arm is dissappearing, during the shoot. So it wasn't a postproduction issue.




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Mark Suszko
Re: Why does my arm dissappear? Wierd
on Feb 16, 2018 at 3:07:23 pm

Glitch in the Matrix?

Or are you shooting 24 frames per second? Maybe with additional shutter effects? I'm going to guess this only happens standing against the bright window background?


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Blaise Douros
Re: Why does my arm dissappear? Wierd
on Feb 16, 2018 at 6:39:07 pm

It's just motion blur, and the window behind is so bright that it's just overpowering the brief exposure of your arm waving around. I bet the shutter speed was slower than normal.


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chris pike
Re: Why does my arm dissappear? Wierd
on Feb 16, 2018 at 7:08:34 pm

...the window behind is so bright that it's just overpowering the brief exposure of your arm waving around...

Someone once explained to me that light will "wrap around" you from behind, it is too bright. I guess that is what you mean. However, I'm not sataisfied with the physics of this explanation. Doesn't light travel in a straight line?


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Blaise Douros
Re: Why does my arm dissappear? Wierd
on Feb 16, 2018 at 8:26:21 pm

Yes, light travels in a straight line, but "light wrap" (caused by an off-axis light illuminating the subject from behind) is not the cause of this (though you'll see its effects at the edges of your body as a highlight reflecting off you from the window behind). This is a shutter speed problem coupled with overexposure.

A slow shutter causes motion blur--basically, the exposure of the frame captures movement from point A to point B, causing a blurred image. So regardless of what's behind you, a slow shutter will let you see "through" moving objects because they're only in one location for part of the frame exposure.

The second factor is the brightness of the light behind you. Digital cameras have a set of values for brightness--on a regular DSLR shooting 8-bit 4:2:0 footage, those values go from 0 to 127. The sunlight behind you is bright enough that it's maxing out the brightness--those windows are registering a 127, meaning the whitest white the camera can perceive. It's actually far brighter than that--you'd probably have to have values up in the 300s or 400s to see detail in those windows and still see anything in the foreground. But cameras don't work like that, so anything brighter than 127 just gets lumped into 127. This is called a "blown highlight."

Now, normally, you'd see your blurred arm in front of a darker background, but because the sunlight is so bright, the area behind your arm is bright enough that it's ALSO registering a 127. The motion blur from your arm might be darkening it down to a 240 from an actual value of 400, but your camera can't see that--all it can see is "number bigger than 127," so all you see is white, meaning most of your arm disappears.

The solution: use a faster shutter (shutter speed and framerate are different--look it up if you don't understand it), gel the windows, or push more light into your foreground and drop the exposure. Or do all three!


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chris pike
Re: Why does my arm dissappear? Wierd
on Feb 16, 2018 at 7:02:16 pm

Yes, it isn't shutter speed alone. I'm doing 24 frames per second. Somehow, the bright window is combining with shutter speed to do this.


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john sharaf
Re: Why does my arm dissappear? Wierd
on Feb 16, 2018 at 8:06:02 pm

HI Chris,

A common error in shooting 24 FPS is to forget to turn the shutter "on"

When shutter is off, the exposure is 1/24th of a second not the 1/48th

This is what it looks like.

I've made this mistake myself, if it's just a talking head you won't notice it, but with the type of action in your sceen it becomes very apparent.

JS



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Robert Olding
Re: Why does my arm dissappear? Wierd
on Feb 16, 2018 at 8:56:20 pm
Last Edited By Robert Olding on Feb 16, 2018 at 9:05:17 pm

What your seeing is motion blur.

If your shooting at 24 FPS, set your shutter to at least 1/50 of a second. This still may not help so you'll have increase the shutter speed to something higher. To capture a human in motion sharp as a tack, a shutter should be set to 1/250 of a second or higher. Be careful though, if you go too high, the motion (although sharp) will look unnatural.

Increasing the shutter speed will let less light into the camera so you'll have to do one or more of the following to compensate: increase the light falling on the front of the subject, increase the camera ISO, open the aperture on the lens, drop your frame rate. Dropping your frame rate will help, but in your case I wouldn't recommend it.

My recommendation would be to open the aperture as wide as you can. Set the frame rate to 24, set the shutter to 1/50 of a second. Then, set the ISO to something no higher than you need to avoid noise (visual grain and splotchiness).

At this point, if your overexposed, then close the aperture until the exposure is correct.

If needed, add light to the front of the subject. In your case, you've got a lot of light coming from the background windows that you can put to work. Just hang up a couple of white sheets on both sides of the camera just out of the frame. These will bounce that light right back on to the subject and help balance out the light coming from the windows.

Robert Olding
http://www.8streetstudio.com
Minneapolis, MN


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chris pike
Re: Why does my arm dissappear? Wierd
on Feb 18, 2018 at 7:22:57 pm

Thanks for the help everyone. I understand now how the disappearance of my arm is really a form of motion blur. I won't attempt to shoot in this location again with that bright background. It was too much trouble.


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