Help me with the math: Shutter speed & Frames per second...

[floyd]

Profuse apologies if this topic has been discussed on another thread. Please shut it down immediately if this is the case.

First, I am a newbie when it comes to drone video and photography. Please pardon my ignorance. Secondly, I fully understand not everything read or viewed on the internet is truthful and can be trusted. All that said, after watching a bunch of youtube videos authored by what appear to be knowledgable drone pilots/videographers/photographers, it seems the optimal setting for filming video that will eventually become slow motion footage is frames per second doubled to arrive at shutter speed over one.

Example: If my fps is 30 the optimal (eventual slow motion) shutter speed would be 1/60. Therefore, in one second I shoot 30 frames of 60 images, or one frame with two images, right?

But then folks say to buy a neutral density filter so on sunny/bright days it is possible to film with a shutter speed of 1/60 of a second without over exposure. Wouldn't increasing the number images per frame improve slow motion? Rather than throwing on an ND filter, why not increase shutter speed in such a way that puts full images on one frame?

Example: (Filming at 30 fps) If 1/60th of a second over exposes my video and halving the light at 1/120th of a second is a proper exposure, wouldn't that provide smooth slow-motion? Now I'm putting four images on one frame.

Why is the one frame to two images the best ratio for smooth slow-mo? What am I getting wrong?

 

[Kilrah]

Firstly that "rule" is about motion blur, not "slow motion".

Therefore, in one second I shoot 30 frames of 60 images, or one frame with two images, right?

No, you shoot 30 images that are each exposed during 1/60th of a second. If you shot at 1/120 the shutter is opened for a shorter amount of time and will thus capture less motion blur.

 

[floyd]

Firstly that "rule" is about motion blur, not "slow motion".

No, you shoot 30 images that are each exposed during 1/60th of a second. If you shot at 1/120 the shutter is opened for a shorter amount of time and will thus capture less motion blur.

I'm already learning something. So if 1/120th captures less motion blur why is 1/60th always recommended? And only fps relates to slow motion, not shutter speed, correct?

Sorry I'm sounding so dense about this. I just want to understand why ND filters are so often recommended when it seems I higher shutter speed would be equally effective.

 

[Kilrah]

Becasue people want the motion blur as it looks smoother to the eye, higher shutter speed gives an ugly "strobe" effect on fast moving things.

 

[ekto]

I'm already learning something. So if 1/120th captures less motion blur why is 1/60th always recommended? And only fps relates to slow motion, not shutter speed, correct?

Sorry I'm sounding so dense about this. I just want to understand why ND filters are so often recommended when it seems I higher shutter speed would be equally effective.

You can try and add motion blur in your video editing software. Sometimes the result is good.

 

[JacksonML]

Sometimes is the key word. If you're filming on a sunny day and you have to set your shutter speed to 1/4000 you're going to get a very crisp and jarring video. Added motion blur in post can work but isn't going to look the same and will introduce artifacts. Just to be clear lets clear up some terminology.

Framerate (30fps, 60fps): This is how many pictures/frames/images the camera is capturing every second. 30fps is a fairly typical framerate. 60fps is smoother than 30fps, but isn't the best when being used for slow motion. 120fps or higher is ideal for slow motion.
Shutter speed (1/60, 1/120): This is how quickly each frame is exposed, or how long light is exposed to the camera for each frame. If you've taken high speed sport photos before you know how important it is to have a fast shutter speed so that you have minimal motion blur. It is somewhat the opposite in most video situations, as a high framerate causes for some choppy footage. See the video I've included below for a comparison between fast and slower shutter speeds in video and the difference they make (skip to 1:07 for the test I think highlights the major differences):

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One thing that I would like to stress is that a slow shutter speed is not always the correct way to record video. Sometimes you want these sharp movements for stylistic effect. High shutter speed video is used in many Hollywood films. You just have to learn what you think looks the best and gives the best effect.

 

[floyd]

Sometimes is the key word. If you're filming on a sunny day and you have to set your shutter speed to 1/4000 you're going to get a very crisp and jarring video. Added motion blur in post can work but isn't going to look the same and will introduce artifacts. Just to be clear lets clear up some terminology.

Framerate (30fps, 60fps): This is how many pictures/frames/images the camera is capturing every second. 30fps is a fairly typical framerate. 60fps is smoother than 30fps, but isn't the best when being used for slow motion. 120fps or higher is ideal for slow motion.
Shutter speed (1/60, 1/120): This is how quickly each frame is exposed, or how long light is exposed to the camera for each frame. If you've taken high speed sport photos before you know how important it is to have a fast shutter speed so that you have minimal motion blur. It is somewhat the opposite in most video situations, as a high framerate causes for some choppy footage. See the video I've included below for a comparison between fast and slower shutter speeds in video and the difference they make (skip to 1:07 for the test I think highlights the major differences):

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One thing that I would like to stress is that a slow shutter speed is not always the correct way to record video. Sometimes you want these sharp movements for stylistic effect. High shutter speed video is used in many Hollywood films. You just have to learn what you think looks the best and gives the best effect.

Very informative! Thank you for the explanation and video. I'm going to search youtube to see if I can find a companion of slow-motion shot at 24/30/60/120/120+ frame rates, by drone or DSLR.