Frame Rate / Shutter Speed Relationship...

J

Johnny5

Member
Hello all, I have recently been experimenting with differing frame rates / shutter speeds to get some faster / slower effects with the DVX100 (no A or B :( ) but I have found myself stuck with some finer points of the relationship between the two.

I can't seem to wrap my mind around the concept of how one can increase the shutter speed but still keep the frame rate the same. I have always understood shutter speed to effect the light entering the lens and thus hittting the CCD - the higher the shutter speed, the higher exposure required to compensate, and also the less motion blur. Frame rate I understood to be the number of images flashed on the screen per second. Pretty basic understanding...but after looking at the DVX Book and thinking about shutter speed and pairing it with Frame rate my brain started to have trouble...

I guess my question comes to this: If a camera is filming at 24fps, and the shutter speed is set to 1/48, the shutter "eye" is open 24 times a second, and closed 24 times a second which is easily reconcilable with 24 frames per second. For the sake of ease, lets say I shoot at 24fps and set the shutter speed to 1/96, meaning the "eye" is open 48 frames per second. How can the "eye" be open 48 times per second yet still display only 24 discreet images.

My previous experience tells me that setting the shutter speed to higher would cut light down and severely reduce motion blur (and vice versa)...but why would I want to take it into consideration when overcranking or undercranking the camera in any other capacity but to reduce / increase light?

I'm not sure if this is even clear...but any insight would be appreciated!
Thanks!

-John
 
Good question.

But would that 96 mean it would be open twice per frame, or still only once but for less time?

I think the latter. Maybe that's why you need more light for the xposure then, and you get that very non-motion-blurred effect.
 
Not quite how it works. In film or in a video camera, the shutter only opens once per frame. What the shutter speed controls is how long the duration of that opening is.

So when shooting 24fps, the most wide-open the shutter can get is 1/24, but it can open and shut as quickly as 1/1000. There's still only one opening per frame, but you can control how long that opening lasts for.

When shooting 24fps at 1/48, it opens once per frame, and stays open for half as long as the frame is held in place. Imagine a film camera; the film sprocket moves the film into place, then the shutter pops open and then closed, exposing the frame; then the sprockets move that frame of film out of the way and move the next one into position.

So you can make the shutter speed as short as you want; each frame of film gets held in place for 1/24 of a second. But the longest duration you can get for the shutter speed would be 1/24; after that the sprockets will have moved the film out of position and it can't expose for any longer than that. But short exposure is no problem.

(okay, sidebar: yes you can use slow shutter speeds on the 100A and 100B, and what that means is that the frame actually gets held in place longer than 1/24. If you use a shutter of 1/6, the frame will be held for 1/6 of a second).
 
From my understanding this is how it works...

FPS is only how often it captures whats hitting the CCD's. So its only doing it 24 times in a second, thus producing 24 captured frames. When running at 24 fps it will ONLY ever capture 24. Shutter speed is how long the light is allowed to hit the CCD's or the film. So if you run your camera at 24 fps and 1/24, the shutter will technically never close.

If you run it at 24 fps and 1/48 then techinically, light would be falling on the CCD's for only half the time of if it was at 1/24. Even more, if you went to 24 fps and 1/96 then it would be 1/4 of the light of 1/24 and half the light of 1/48. This is assuming that the light reaching the CCD is constant.

Basically the higher the number the more light you will need. You will also have less blur. Higher shudder speeds and a ton of light allow you to capture a car moving at 120 mps and make a single frame look like the car is standing still.

I hope that helps.

Ya what Barry said lol.
 
When the camera is capturing images at 24fps, it means that it takes 24 'pictures', if you will, in 1 second.

Shutter speed is measured in seconds. It has to do with how long the shutter is open, exposing the image to the CCD's. A shutter speed of 1/48, means that the shutter is open for 1/48 of second. If you set the shutter speed to 1/1000, then the shutter opens and closes very quickly, for 1/1000 of a second.

The faster shutter speed (open and close) doesn't allow as much light in as a slow shutter speed.

I hope that helps.
 
Ok, I think I'm understanding this...is that why when filming at 30P you want to set the shutter to 1/120? To allow the same ratio of light to hit the frame that would hit it at 24 fps (namely half the time the frame is held in place?)

So whether overcranking or undercranking, you adjust the shutter speed to always shine light on the frame for half the time it is held in place if you are trying to emulate fast or slow footage being played back at 24fps?

What would filming at 30P and setting the shutter speed to 1/48 do to playback? Would it look too blurry?

-John
 
I don't think you want to set the shutter speed to 1/120 when shooting 30P? I think the default shutter speed is 1/50 or 1/60 (not sure). Anyone?

It really depends on WHY you want to set the shutter speed. If you're wanting to adjust the exposure, I prefer starting with the ND filter, then IRIS, and then maybe shutter.

Keep in mind that as you use faster shutter speeds, a moving image will appear sharper, but without the motion blur, you can get a strobe effect.
 
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