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I have a question about the video mode of my DSLR.

When i record i can set the frame rage (e.g. 25 fps). In that setting my DSLR should take 25 images within 1 second.

In manual (or "s" mode) mode i can set the shutter speed of the taken video. Technically the maximum should be 1/25 because of the frame rate. But i can also choose values above.

Question: How is it technically possible to record a video with 25 fps having set the shutter speed more than 1/25s. ( e.g. 1/4s)

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    it would probably help if you told us your camera model. it's possible that it just ignores the setting.
    – ths
    Commented Jul 12, 2016 at 8:07
  • @ths Nikon D3200 - when i change the shutter speed, the image changes aswell (lighter / darker)
    – fubo
    Commented Jul 12, 2016 at 8:08
  • I think you need to read the manual carefully. It's not possible to expose for longer than the frame time, but it is theoretically possible to add frames together (tho' I doubt your camera is doing this). You may find that the apparent shutter time is different when in video modes.
    – Carl Witthoft
    Commented Jul 12, 2016 at 11:32
  • I once had the same question and I asked on DPReview. Have a look at the answers.. dpreview.com/forums/post/56171573
    – user152435
    Commented Jul 12, 2016 at 14:07
  • @CarlWitthoft It IS possible to exposure longer that frame time. Think about a) two frames per exposure, or b) time lapse in the night ( when exposure time is far longer than 1 second and there are 25 or 30 or 50 frames in one second of the final video).
    – Alex.S
    Commented Jul 12, 2016 at 15:21

1 Answer 1

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Whether this happens inside your specific camera is not clear. However, it is possible to have a shutter speed slower than the FPS would naively allow. In practice, it's very unlikely that a camera would do this if uses a mechanical shutter for video, but it can be practical in electronic shutter cameras.

Imagine a shutter speed of 0.5 seconds, and a framerate of 4fps. Each 'exposure' lasts 0.5 seconds, but each frame's exposure overlaps with neighbouring frames - the frames are not the same, but they overlap.

Imagine:

  • A is the light value that arrives between 0 and 0.25 s
  • B is the light value that arrives between 0.25s and 0.5s
  • C is the light value that arrives between 0.5s and 0.75s
  • D is the light value that arrives between 0.75s and 1s
  • etc...

Then:

  • Frame 1 contains the light A+B
  • Frame 2 contains the light B+C
  • Frame 3 contains the light C+D
  • etc....
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  • This only works if you have multiple sensors. There is no physical way to "share" photoelectrons in the analog regime. I don't think anyone's suggesting a post-processing partial sum of readout frames.
    – Carl Witthoft
    Commented Jul 12, 2016 at 18:52
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    @CarlWitthoft the physical shutter speed would have to be faster than the frame rate, but simulating overlapping exposures with a single sensor is certainly possible.
    – Harry Harrison
    Commented Jul 12, 2016 at 18:57

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