Why would video have different DOF than stills, ceteris parabus?

[bigbrother]

So this was odd. I shot some portraiture with my M50, 50mm at f/2.2, both in still and video. My subject was standing near some flowers. In the stills, only her face and the flower branch near it were in focus. But in the video, a much wider range was sharp- her entire body and ... wait I think I just got it. The FOV was somewhat similar in both, but I know the cropping is different in video mode (I don’t mean 4K, I know the difference in that cropping is enormous.) I wonder if I was actually further away when taking the video and it was cropped more narrowly, thus the range of sharpness would be higher.

[rs-eos]

DOF is driven by many factors: focal length, distance of the focal plane to your subject, aperture and the circle of confusion.  Where the circle of confusion is related to the size of the sensor.

And as you've found, since video uses a cropped portion of the sensor, to get the same field of view to that of a still that uses the entire sensor, your distance to the subject would have changed and thus affect DOF.

[MikeSowsun]

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@rs-eos wrote:

DOF is driven by many factors: focal length, distance of the focal plane to your subject, aperture and the circle of confusion.  Where the circle of confusion is related to the size of the sensor.

And as you've found, since video uses a cropped portion of the sensor, to get the same field of view to that of a still that uses the entire sensor, your distance to the subject would have changed and thus affect DOF.

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Are you talking about the cropping for 16x9 video vs 3x2 stills? I can't see that being much of a factor in Depth of Field.

[rs-eos]

No, not the aspect ratio.  The actual area of the sensor being used.  DSLRs often will not use the full sensor area for video.  And thus apply yet-another crop factor to the equation.

e.g. with the 5D IV, you get a 1.7 crop factor applied when capturing 4K video.

[MikeSowsun]

The OP has said they are not using 4K video. There should be minimal crop factor with 1080p video.

For the OP to notice a difference in DOF between video and stills there must have been some other factor involved.  Maybe they were shooting 4K and did not know it.