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Variable shutter speed

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Ty Ford
Variable shutter speed
on Jan 18, 2014 at 5:16:27 pm

Hi,

I recently bought a digital camcorder and have a question about shutter speed. This camcorder supports manual shutter speed adjustment from 1/30 sec up to 1/1022.7.

Are there links you can point me to that would help me better understand why such a wide speed range is made available, which speeds to use and when? I understand the slower speeds will allow blur and I've learned over the years that for my simple work, 1/60 gets it done. Is that due to syncing with 60Hz AC lighting? Or was it arrived at for another reason?

And, while I'm thinking about it, since much of what I shoot today is 30P, is it so bad to have the shutter at 30, or will that prove problematic?

Thanks,

Ty Ford

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Todd Terry
Re: Variable shutter speed
on Jan 18, 2014 at 5:43:23 pm
Last Edited By Todd Terry on Jan 18, 2014 at 5:52:07 pm

Hi Ty...

Ok, first a little optical/mechanical/photographic history...

Cine film cameras typically had (and still have) a moon-shaped spinning shutter... basically a mirrored disc that's cut in half and and spins around the center point (or what would be the center point if it was still a full circle). When it's at one position it is exposing the film, when it is at its opposite position it is reflecting what the lens sees to the viewfinder.

If it is spinning at 24 revolutions per second and is exactly "cut in half" (a 180° shutter), then each exposure is 1/48th of a second.

Ergo, the 1/48th shutter speed is considered "normal." Now, note that all shutters aren't exactly 180°... my 35mm camera for example has a 170° shutter... but for simplicity's sake we'll just talk about 180.

SO... a "normal" shutter is "one over twice the frame rate." In the video world, if you are shooting 24p, then it's 1/48th. For the 30fps you are shooting, "normal" would be 1/60th.

Now, of course you don't have to shoot at "normal"... but it tends to give the most pleasing look to motion. As you'd imagine, at 1/48th fast motion will give a fair amount of blur in each frame. Some people might think it'd be better to shoot higher, so each frame is good and crisp. But your brain needs that blur to interpret motion as smooth. Too high a shutter speed gives you the "narrow shutter look," which is very choppy and staccato. It's used for effect often in features to make action movies look more "actiony." See "Saving Private Ryan" to see it used very very well. See "Gladiator" (and a zillion other movies) to see it used poorly. We call it "narrow shutter" because in a film camera that mirrored disc has a movable section that literally physically makes the exposure hole more narrow.

On the opposite end, too low a shutter speed gives too much motion blur, and moving images can look sort of, well, "mushy" for lack of a better term. If you are shooting 30p and a shutter speed at 1/30th, that might be ok for subjects that aren't moving very much... landscapes, still lifes, very very slow tilts or pans, talking heads, etc. Or if you are shooting in dim conditions and really need the slower shutter for exposure. But if you are shooting anything with much movement you'd probably get more pleasing results at 1/60th.

You can (and would want to) also shoot at a higher shutter speed if you plan to slow-mo the footage using some kind of frame interpolation software (Twixtor, etc.) to recreate the missing frames... in that case you'd shoot at an appropriate shutter speed for however much you intend to slow mo, as if your camera could overcrank like a film camera. But that's a hole 'nother can of worms.

So... yeah, you can tweak your shutter speed, a little up or a little down to compensate for some exposure, but I wouldn't do it much unless you purposely want to start affecting the motion look of your images.

T2

__________________________________
Todd Terry
Creative Director
Fantastic Plastic Entertainment, Inc.
fantasticplastic.com



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john sharaf
Re: Variable shutter speed
on Jan 18, 2014 at 5:57:21 pm

Hi Ty,

There is both practical and creative benefits from the adjustable shutter speed settings in modern video cameras,

The first is to imitate either the temporal characteristics of film or video, depending on your creative intention. For example, at 24p the shutter in off position would be 1/24th of a second which creates a very blurry pictures.

Film cameras nominally set their shutters at 180 degrees (allowing half the time for exposure ans half the time for viewing as the mirror.shutter spins). This effectively makes the shutter speed 1/48th.

The same is true for 30p, which is a very common frame rate for the internet and other compressed playback. Here it's important to remember to turn the shutter on again to 180 degrees (effectively 1/60th of a second) for proper temporal motion.

Only at 60i should you leave the shutter off; as this is the nominal setting from video cameras before variable shutters. Because the camera is making 60 "half frames" per second the effective shutter speed is 1/60.

In addition one can "skinny up" the shutter for what's often referred to as the "Private Ryan" effect. This creates a "staccato" motion as was used to great effect on the Spielberg movie during the battle scenes. If this is your intention you should test to determine exactly how much skinny you want; 1/125, 1/150 ???

Another use of the "variable" shutter is to eliminate rolling when photographing a computer CRT display. Adjust the speed slowly while watching through the VF or on your own display through the camera until the rolling stops. It not always possible, especially if shooting at 720/60p because the slowest shutter speed is 1/60.

Hope this helps!

JS



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Ty Ford
Re: Variable shutter speed
on Jan 18, 2014 at 10:57:55 pm

John & Todd Thanks so much for the gentle tutorial.

I get every word and point!

I'm just wondering what they had in mind to have what seems to be such high and variable shutter rates possible on this camera. Could it simply be a "because we can" thing. Or are there uses for 1/1000 other than the Private Ryan effect? BTW, how far up do you suppose they goosed it up to to get that effect?

Regards,

Ty Ford

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john sharaf
Re: Variable shutter speed
on Jan 18, 2014 at 11:10:57 pm

Hi Ty,

Probably not more than 1/250 or 1/500th.

Can't imagine a use for higher speeds except to further increase the strobe effect or for instrumentation purposes where an especially sharp or "frozen" image would be helpful for interpretation (think surveillance or forensics).

JS



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Todd Terry
Re: Variable shutter speed
on Jan 18, 2014 at 11:45:03 pm

John is right, as always.

You can get the "Private Ryan" look easily at 1/250th.

Those ultra high shutter speeds are rarely going to prove very useful. My primary camera will go down to an 11.25° shutter at its narrowest, which at 24fps is 1/2000th of a second. I cannot even conceive of when I'd ever need to use that.

You'd need a boatload of light for that, too... dim interiors, no.

Something else occurred to me... you asked in your original post if the 1/60th "normal" shutter for video was tied to the 60Hz power. My immediate thought was "No, no, not at all. It's because at 30fps a 180° shutter gives you 1/60th, they have nothing to do with each other." But... in a way you are right. The 1/60th shutter doesn't directly have anything to do with the 60Hz power... but the 30fps (60i) that your camera shoots does, following in the footsteps of the early TV cameras that were pretty much slaves of the AC power cycles. So in a very indirect sense one could say that a normal shutter speed of 1/60th for video is a result of our 60Hz power.

T2

__________________________________
Todd Terry
Creative Director
Fantastic Plastic Entertainment, Inc.
fantasticplastic.com



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Rick Wise
Re: Variable shutter speed
on Jan 19, 2014 at 12:12:01 am

My perhaps incomplete memory of the techniques used for the "Private Ryan" look was that the shutters of some of the hand-held cameras during the landing were purposefully set slightly "wrong" so that the claw started to pull down the negative before he shutter was closed. This produced a slight smearing.

Rick Wise
Cinematographer
San Francisco Bay Area
http://www.RickWiseDP.com


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