Frame Rate/Shutter Speed Question.

Started Jan 6, 2013 | Discussions thread
Sean Nelson
Sean Nelson Forum Pro • Posts: 12,507
Re: Frame Rate/Shutter Speed Question.

Richard Frederick wrote:

Clearly, I do not understand this subject. I tried a search, but found nothing. Perhaps someone(s) here can enlighten me.

1. The Nikon D800 takes about 14ms to read the sensor, line by line. This amounts to about 1/74th of a second.

2. If I select a high shutter speed (say 1/2000th second, how does the camera manage this?

Well in stills mode it just uses the mechanical shutter, of course.   A "slit" (opening) between the first and second shutter curtains travels down the height of the sensor, exposing any particular photosite for 1/2000 of a second.  Once the second curtain has dropped the camera can take as long as it needs to read out the data because no more light is falling on the sensor.

In video mode, the key is that the exposure time is the time between clearing the photosites and then reading them out.   So during the 1/74th of a second that the camera takes to work its way from the topmost to the bottommost rows, it's clearing out some of the lower lines at the same time it's reading lines above that which it cleared out 1/2000th of a second ago.   So although there's no mechanical shutter involved, you can think of the sensor as having a "slit" similar to that exposed by the mechanical shutter, except that the "slit" is some number of rows which have been cleared but not yet read out by the camera.   As the camera moves down the rows, it keeps clearing out lower rows at the bottom of the "slit" one by one as it reads out the accumulated charge in the same number of upper rows at the top of the "slit".   It takes 1/74th of a second for it to move this electronic "slit" from the top to the bottom of the sensor.

If I understand this correctly, how does this produce more flicker than (say) a 1/60th shutter speed? I can visualize flicker being caused by a film camera using the above numbers, but I don't understand how the same applies to digital video.

I assume that by "flicker" you mean the discontinuity of movement in, for example, panning shots that you see in movie theaters that are projecting film at 24fps.   This is an issue at slower frame rates such as 24 or 30fps, and cinematographers try to mitigate it by using the "180 degree" shutter rule of setting the shutter speed to half of the frame rate (i.e., 1/48th of a second for 24fps film). This blurs the movement in the individual frames so that it doesn't seem as jarring, but at the cost of lost detail.

By the time you get to 60fps the images are being shown so rapidly that the eye doesn't have much time to discern any discontinuity of movement.  As a result the "flicker" effect of using fast shutter speeds is very much reduced.   It depends to some degree on the individual viewer, but in my testing I wasn't able to see much if any difference in rapidly moving traffic or fast pans at 1080p60 whether I used a 1/60, 1/120 or 1/1000 shutter speed.  This, IMHO, is one of the huge benefits of 60p - the ability to capture good sharp detail in moving images.  Purists will complain that it "looks like video" (as opposed to "film"), but it's really in the eye of the beholder.

The camera in question supports 720P and 60 fps, and I have used high shutter speeds with these parameters without noticeable flicker. 720P is really quite good video. Being able to use high shutter speed allows for large apertures (shallow depth of field) without resorting to neutral density filters, etc., so this will probably be my video mode.

If you're shooting videos for yourself, then shoot some test footage and choose whatever looks best to you.   Don't let someone else try to convince you that you don't like what you're seeing!

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