Got a 7D, but can someone explain FPS vs Shutter Speed?

kcui

I'm playing around with Video mode on my 7D. Since I'm used to shooting stills, I'm not quite sure if I am understanding the differences and relationship of FPS and Shutter Speed.

I thought that at, say, 30 FPS, this would mean that the image sensor would capture data 30 times per second, resulting in 30 distinct frames (essentially capturing at a speed of 1/30 of a second).

Obviously this reasoning is wrong as I can set a 1/4000 second shutter speed. So the camera will operate at 30 FPS but exposing the sensor only 1/4000 of a second?

So, would this mean that at 30 FPS, those 30 frames in the span one 1 second would only capture 30/4000 of that second (I'm assuming evenly spaced intervals)? This would mean more "staggered" frames in the span and "choppier" video?

Perhaps I just answered my own question...or am I missing something?

JHarris1984

Shutter speed and frames per second don't really exist together with video since you're not actually using your shutter. 30 FPS for video basically is 30 separate pictures all combined to make a fluid picture. Similar to an animation done back in the day where you could flip the pages of the books and it would make Mickey Mouse move. Just a bit more complex.

Here is a link to a little more information on 1080p if you want to know a little more. http://en.wikipedia.org/wiki/1080p

kcui wrote: »

I'm playing around with Video mode on my 7D. Since I'm used to shooting stills, I'm not quite sure if I am understanding the differences and relationship of FPS and Shutter Speed.

I thought that at, say, 30 FPS, this would mean that the image sensor would capture data 30 times per second, resulting in 30 distinct frames (essentially capturing at a speed of 1/30 of a second).

Obviously this reasoning is wrong as I can set a 1/4000 second shutter speed. So the camera will operate at 30 FPS but exposing the sensor only 1/4000 of a second?

So, would this mean that at 30 FPS, those 30 frames in the span one 1 second would only capture 30/4000 of that second (I'm assuming evenly spaced intervals)? This would mean more "staggered" frames in the span and "choppier" video?

Perhaps I just answered my own question...or am I missing something?

kcui

JHarris1984 wrote: »

Shutter speed and frames per second don't really exist together with video since you're not actually using your shutter.

From everything I've read the two most certainly are separate and work in tandem.

The shutter speed on the 7D clearly has a effect on the exposure of the video image, and from watching demo videos online, higher shutter speeds at a given FPS increase the perception of staccato-like, stuttering motion. Lower shutter speeds mean more motion / image is captured in a given frame, thus smoother motion when viewed.

Scroll down to the bottom of this link to see a video of a waterfall parsed together with increasing shutter speed from left to right:
http://blog.planet5d.com/anh

studio1972

Shutter speed and fps do exist and do matter for video. Generally you should have a shutter speed of half the duration of the frame (so 1/60 in your example) as this will prevent the staccato look. You can control exposure using ISO and ND filters as required.

PilotBrad

Here's a good explanation of all of this...
http://blog.tylerginter.com/?p=385

Overfocused

The shutter speed is simulated while taking video. It's not actually triggering the shutter but it is still setting the video's exposure and F-stop

ziggy53

Overfocused wrote: »

The shutter speed is simulated while taking video. It's not actually triggering the shutter but it is still setting the video's exposure and F-stop

Close. The electronic shutter of the imager is used for video acquisition and shutter speed adjustment. The mechanical shutter does stay open for the duration of video mode.

The electronic shutter is also employed for the "quiet" modes of operation on Canon cameras. The electronic shutter can also reduce camera vibration for both photomicrography and very long lens work:

http://krebsmicro.com/Canon_EFSC/index.html

kcui

Ok, so I guess my question still hasn't been (directly) answered, and after looking at the material / links provided, I am still unclear if my understanding is correct.

To clarify, I made a (crude) representation in MS Paint (it is ACTUALLY useful for once!).

Despite the drawing, assume the red exposures are distributed equidistantly across the timeframe of 1 second. There are 30 exposures (frames) thus 30 FPS. Wider lines imply a slower shutter speed.

Is this representation accurate or am I missing something?

EDIT: Forgive the typos in the image. I was at work and trying to be discreet : )

ziggy53

Each individual frame has its own exposure, despite the frame rate. This is a valid statement up to the limit on shutter speed imposed by the frame rate itself. This is true whether you shoot video or stills at a rapid (continuous) frame rate.

Some digital video systems do allow slower shutter speeds than frame rates by digitally overlapping data, but I'm not sure of dSLRs with that capability.

kcui

ziggy53 wrote: »

Each individual frame has its own exposure, despite the frame rate. This is a valid statement up to the limit on shutter speed imposed by the frame rate itself. This is true whether you shoot video or stills at a rapid (continuous) frame rate.

Right, you so at 30FPS you are limited to 1/30 of a second, since it is essentially "continuous" video.

So were my deductions correct, in that in the case of say, 1/60 shutter speed, your total exposure time for all 30 frames combined is only 1/2 a second.

So at 30FPS and 1/60 shutter speed:

1/60 of exposure, then 1/60 of non-exposure .... repeat until you've capture 30 frames.

Likewise for 1/100 shutter speed @ 30FPS, you are capturing 30 / 100 ---> 3 /25 of exposure time over the 1 second, which means you are capturing 1/100 of a second in roughly 7/30ths (70/100) seconds apart....

ziggy53

kcui wrote: »

Right, you so at 30FPS you are limited to 1/30 of a second, since it is essentially "continuous" video.

So were my deductions correct, in that in the case of say, 1/60 shutter speed, your total exposure time for all 30 frames combined is only 1/2 a second.

....

Yes, as far as I know the current dSLRs are limited by the frame rate as to slowest shutter speed.

Accumulated exposure per second has no practical value that I'm aware of. The only things that matter are the frame rate and the exposure settings per frame and the sensitivity setting of the sensor.