Video Shutter Speed Too High?

[GeHuaLi]

Notice in the reviews of camerars like Sony NEX 5, A55, etc., that the reviewer states that when the camera automatically chooses (without allowing user input) too high a video shutter speed in bright conditions, the videos look very bad.
This is counter-intuitive to those of us new to video, and wonder how a faster video shutter speed harms the video image.
For that matter, what is the difference between frame rate, MBPS, and shutter speed in a digicam?
Thanks.

[michaelnotar]

yes coming from still photographyi wondered the same.

video is traditionally shot at 24, 30 fps in the US for the NTSC broadcast system and 25 or 50 pfs for PAL. video cameras use a circular spinning shutter where half is opaque the other half clear, this is called a 180 degree shutter, each 180 part of it is either opaque or not. since 180 is half of 360 obv, shutter speed is traditionally 2x faster than the frame rate, ie 30 fps is 1/60th shutter speed. frame rate can be speed up and played back at 30 fps for slow mo effects (ie 300 fps played back at 30 will last 10 time longer than the actual event) or slowed down for a somewhat similiar blurred look of a still photo (with some smearing/sticky image look, kindof interesting).

a fast shutter speed in video can create a rolling shutter effect on certain video cameras with a cmos sensor. that is unsteady jerky pans. this is due to the fact that the majority of the fractional time inbetween frames is not recorded with a higher shutter speed. usually people use a nd filter in bright situations or use the internal nd switch on a camcorder. the whole frame rate and shutterspeed is a complicated discussion that is also highly subjective person to person for the exact look the film maker is going after.

i think mbps is some sort of a recording compression or quality, best to turn it to the highest, largest number. perhaps the instruction manual can help.

[michael]

There's so much to be considered here, that it woud take a book to explain.

In addition to what's already been mentioned, a shutter speed higher than normal (2X the frame rate) looks statico rather than smooth. You want some motion blurring in each frame for a more natural cinematic appearance. 1/125 sec and above just looks unpleasant.

Michael

[Christopher Sanderson]

I see that while I was typing this others have replied so much of this may be redundant!  

This is a very interesting subject and one worth spending some time experimenting with - that is if you have the manual control of video shutter speed available to you!

Frame rate (aka fps) is the number of complete image frames recorded in a second. More on that 'complete' emphasis in a moment. Normal frame rates are 24 fps for film/film-like/cinema; 25 fps for European video & television; 30 fps for NTSC/N.American video & TV; 60 fps for high end cinema. Note that these speeds assume that the frame rate used in recording will be the same as the frame rate used for presentation & screening. Naturally any difference in the recorded and presented frame rates will result in the perception of slow-motion or speeded up motion. The old technology of recording 'interlaced' images resulted in recording 2 fields per frame  or 60 fields per second. Old silent films often used 18 frames per second which when played back at 24 fps gave the characteristic Charlie Chaplin speeded up motion.

The adoption of 24 frames per second as a standard was really the result of experimentation with how the brain interpreted the series of still images shown as 'motion'. Much slower looked jerky and artificial and a faster frame rate used more expensive film which obviously the film studios did not like! It was the result of perception vs. Economy

Older mechanical cinema shutters basically used a rotating shutter. 180 degrees was shutter open and 180 degrees was shutter closed and also allowed a fresh unexposed frame of film to be moved into the 'gate'. So if the shutter had to rotate 24 times a second, that gave an effective shutter speed of 1/48. In the fifty plus years of film-making and watching, 1/48 of a second with it's accompanying blur of motion became the accepted way of seeing motion in a projected image.

Cameras came out with adjustable angle shutters to effectively give control of the shutter speed within a limited range but too small an angle and the resulting faster shutter speed made the film look jerky. A longer shutter speed and the motion blur was too blurry. So 1/48 became what is now termed the 'film-look. Only under unusual circumstances then was the shutter speed and frame rate changed.

One of those 'unusual circumstances' was and remains, motion in a high contrast scene - imagine a black car traveling across a white background. In this circumstance the brain is subject to an unpleasant latent image which can  result in the perception of strobing. Cinematographers have a few choices: speed the car up so that it is really just a blur in each frame; reduce the contrast or reduce the shutter speed so that there is more motion blur. Whatever choice is made to try and get rid of the problem, only if the effective contrast is lowered will the scene be acceptable to the viewer.

[GeHuaLi]

Thanks for the quick and very informative replys. Each filled in part of an admittedly comlex puzzle. Always wondered why the 24fps was such a Holy Grail.
Am I right, then, that a certain FPS implies a certain shutter speed, ie., 30 fps imlies a 1/60 sec. shutter speed. That would imply that my little P/S digicam (Kodak M1033) with 30 fps 720p video is using only a 1/60 shutter speed and varying ISO and or aperture (not shutter speed) to get a proper exposure on each frame, correct? No CMOS rolling shutter panning worries with that one!
On the other hand, with higher end cameras like the Sony NEX 5 and A55, is it true that they are potentially automatically changing shutter speed (as well as A and ISO) in varying video light conditions? So that in bright conditions, as the reviewer stated, a higher shutter speed than 1/60 may be chosen.
And it was implied in your explanation, NTSC HD TV sets operate at 30fps, so that the higher than 1/60 shutter speed above creates videos with higher frame rates than 30fps and are a poor match for playback on NTSC sets. As one of you mentioned, a 300fps video played at 30fps will be unnaturally spread out to 10 times its length. Have I followed so far?
I'm still a little puzzled why a higher end camera like the Sony NEX 5 would be programmed to potentially chose a video shutter speed in bright conditions which would create high FPS videos with unacceptable playback on HDTV NTSC sets here in the US, while my cheap little M1033 sticks only with the NTSC optimal 30fps. Is there another use for the high shutter speed high frame rate videos created on the high end cameras than watching on NTSC TV's?

[Christopher Sanderson]

The advent of inexpensive video cameras and (emphatically) the video DSLR has of course changed the 'rules' - mostly in the manner in which the manufacturers ignore accepted video & film 'rules' or norms and blithely allow Automatic modes to use shutter speed as a video exposure control. Fast shutter speeds at 'normal' frame rates are going to give badly 'strobing' or jerky motion perception - the brain just doesn't fill in the blanks very well and the perception of the actual successive still frames is more pronounced.

Actually IMO a small bit of interlacing jaggies or CMOS skew in motion actually helps the perception of motion - but this is contentious! Certainly if the jello-cam effect is shown as a still frame, the web audience will howl!

The way broadcast works, any frame rate recorded will come to your TV as 30 or 25 fps. Matching recorded frame rates to presentation frame rates is generally a good idea since the interpolation techniques used in changing recorded frame rates to presentation rates vary fairly widely in output quality. Some techniques are very good or easy, some are harder and need fairly intensive computation.

A high recorded frame rate (60 fps) presented at the same rate (60/60) will deliver smoother fast motion with higher shutter speeds. So a fast moving action movie or sports shot recorded and presented at 60 fps will look perceptibly better than a 30/30 fps version. The more frames the brain views in a second, the less 'in-betweening' it has to do. In actual fact of course, due to the higher shutter speed, each still frame is sharper and has more perceptible detail at this higher rate. But the higher frame rate in presentation is necessary for the images to feel 'smooth' at this higher recorded shutter speed.

[Peter McLennan]

Controlling exposure by varying the shutter speed and ISO is common with many consumer grade video recorders.  It's much easier to vary the shutter speed (which is an electronic function) than to vary the aperture (which is a mechanical function).  In fact, my $100 Creative Vado HD camcorder has a fixed aperture lens.  It shoots in very low light and in bright sunlight, solely by electronically varying  ISO and shutter speed.

The unfortunate result in bright conditions is "strobey" video since each frame is captured in a very short space of time.  This results in some pretty weird artifacting when shooting very fast moving subjects.  The blades on a turboprop aircraft for instance, strobe to such extent that they are frozen and bent when imaged with this camera.  The nearby vertical towers of the overhead electrical supply to the TGV trains in Europe were rendered razor sharp, but nearly horizontal as they ripped past the camera a few feet away at 300 kph. 

I estimated that my late-Jurassic Sony DV camcorder was using shutter speeds in the order of 1/2000 by shooting rapidly moving water.  These effects can be partially mitigated by using an ND filter to force the camera to use a longer shutter speed.

Short shutter duration motion imaging can be used creatively, too.  This was used to good effect in the beach landing scenes in "Saving Private Ryan"

[GeHuaLi]

   Lots of good help here. Now I think I've been confusing the duration of the shutter opening (shutter speed) and the frequency of shutter opening (frame rate) on digicams. As you can see, I'm starting from the beginning on understanding video mechanics.
   So you of you have a digicam taking video at 30fps but automatically choosing a 1/125 or higher shutter speed, you have a considerable portion of each frame where the shutter is closed. And this leads to some of the negative video effects with fast moving subjuects and rapid panning that you've all mentioned.
 Tell me if I'm finally getting warm on the difference between frame rate and shutter speed.
 We still have the problem then which the reviewer mentioned that a camera like the NEX 5 automatically choosing a very short video shutter speed has negative effects with apparently no upside in rapid motion situations. Peter McLennan seems to indicate that may be due to economics as it's easier to control S settings (and ISO) than A settings.  Do digicams which allow you to manually set the video shutter speed (I believe the Panasonic Lumix DMC-G2 was mentioned) have the capability to vary both ISO and aperture to achieve proper exposure?
 Chris Sanderson mentions the distinction between recorded frame rate (which we're discussing above) and presented frame rates. And explains that regardeless of recorded frame rate, the frame rate will be interpolated if necessary before being presented at 25-30 fps on my HDTV or computer screen. And that the inerpolation process varys in quality and that matching recorded and presentation frame rates is a good idea. So even a digicam that offers 60fps is going to be interpolated down to 30fps before being displayed on HDTV or computer. Does the 60fps recording rate really gain you much?
 

[Peter McLennan]

    Tell me if I'm finally getting warm on the difference between frame rate and shutter speed.

You're not only warm, you're hot. : )

 We still have the problem then which the reviewer mentioned that a camera like the NEX 5 automatically choosing a very short video shutter speed has negative effects with apparently no upside in rapid motion situations. Peter McLennan seems to indicate that may be due to economics as it's easier to control S settings (and ISO) than A settings. 

This is true for simple consumer video recorders like my Creative Vado and my Sony W350 and Sony TR 400.  How varying exposure is implemented on DSLRs, EVIL cameras or Micro Four Thirds cameras, I don't know.  I don't own any of those.


 Chris Sanderson mentions the distinction between recorded frame rate (which we're discussing above) and presented frame rates. And explains that regardeless of recorded frame rate, the frame rate will be interpolated if necessary before being presented at 25-30 fps on my HDTV or computer screen. And that the inerpolation process varys in quality and that matching recorded and presentation frame rates is a good idea. So even a digicam that offers 60fps is going to be interpolated down to 30fps before being displayed on HDTV or computer. Does the 60fps recording rate really gain you much?

It "gains you much" if you want to see your subject in "slow motion" (in your example at half realtime speed).  If you can display at 60 fps, then all bets are off.  This is very uncommon in any event.