How are shutter speeds longer than a single frame's duration possible?

Started Jul 19, 2015 | Questions
Jothi
Jothi Regular Member • Posts: 176
How are shutter speeds longer than a single frame's duration possible?

I can understand how an 1/100 shutter speed at 50 fps works; the shutter stays open only a half of the time of a one frame, right?  But how can an 1/25 shutter speed be possible at 50 fps?

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CaliforniaDave Senior Member • Posts: 1,513
Re: How are shutter speeds longer than a single frame's duration possible?

Jothi wrote:

I can understand how an 1/100 shutter speed at 50 fps works; the shutter stays open only a half of the time of a one frame, right? But how can an 1/25 shutter speed be possible at 50 fps?

If you take your question as posed in the subject and google it, numerous responses come up and the very first one answers your question.

Sean Nelson
Sean Nelson Forum Pro • Posts: 13,181
Re: How are shutter speeds longer than a single frame's duration possible?
1

Jothi wrote:

But how can an 1/25 shutter speed be possible at 50 fps?

The camera makes a 1/25 second exposure and stores the same image on two consecutive frames of the 50fps video.  This process is repeated 25 times per second.   So because each frame is duplicated, you only get an effective frame rate of 25fps even though the video is, technically speaking, played back at 50 frames per second.

Pete Berry Veteran Member • Posts: 3,515
Re: How are shutter speeds longer than a single frame's duration possible?
1

Sean Nelson wrote:

Jothi wrote:

But how can an 1/25 shutter speed be possible at 50 fps?

The camera makes a 1/25 second exposure and stores the same image on two consecutive frames of the 50fps video. This process is repeated 25 times per second. So because each frame is duplicated, you only get an effective frame rate of 25fps even though the video is, technically speaking, played back at 50 frames per second.

That's it.

With a bit of a trick the GH4 will shoot video down to 1/2 sec. exp., and with my usual 30p 4K UHD, you get 15 identical frames - same exposure/framing - twice a second.

This is pretty amazing for tied down static night shots, with a 4EV light gain over the usual lowest 1/30 sec. exposure allowed - shooting ISO 800 = 12,800 @ 1/30 - and it plays back totally smoothly. Figures moving through the image barely register unless more brightly lit.

Also very good for capturing motion in dancers, etc, and pulling very printable 8.3mp stills

Pete

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