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I'm about to buy my first DSLR and I'm trying to understand every single aspect of it.

If I've got a prime lenses and I'm shooting a video with f/2.8 aperture, I'm going to have a fixed depth of field (1. nearer points where sharpness is acceptable, 2. actual focus point and 3. points further away in which sharpness is acceptable). So when I turn the focus ring what is happening to these three factors 1,2,3 mentioned above, technically? Is this fixed depth of field moving forward and backward linearly for the actor/object to be kept in focus while in movement?

I've been trying exhaustively to find this specific information here, on google, videos on youtube but I can't find the straight answer for my question.

Thanks

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    I would strongly suggest changing the question to be about changing the focus ring(and not about video specifically). Videography is off topic here, and stack exchange actually has a video site avp.stackexchange.com . To keep this on topic it has to be about photography or at least relevant to still photography: meta.photo.stackexchange.com/questions/28/… – dpollitt Dec 31 '14 at 16:04
  • possible duplicate of How does a focus ring bring a subject in or out of focus? – dpollitt Dec 31 '14 at 16:19
  • @dpollitt - technically, video is only off topic if it is in a context that isn't also still relevant to photography. In this case, it would still be relevant. That said, people do often close at the first site of the word video even if it still fits the site guidelines, so it is still probably advisable to avoid the trigger word. – AJ Henderson Dec 31 '14 at 16:41
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First, a word about what depth-of-field is and is not:

In a way, depth-of-field is an illusion. There is only one plane of focus. Everything in front of or behind the point of focus is out of focus to one degree or another. What we call DoF is the area where things look, to our eyes, like they are in focus. This is based on the ability of the human eye to resolve certain minute differences at a particular distance. If the slightly out-of-focus blur is smaller than our eye's capability to resolve the detail then it appears to be in focus. When you magnify a portion of an image by making it larger or moving closer to it you allow your eye to see details that before were too close together to be seen by your eyes as separate pieces of the image.

In the case of your prime lens, you have a fixed focal length (other than any focus breathing that might be occurring as you change the point of focus) but that does not mean your lens also has a fixed Depth of Field at a particular aperture. Depth of Field (DoF) is determined by the combined result of several variables: focal length, subject distance, aperture, image magnification (related to sensor size)/display size, viewing distance, and visual acuity of the viewer. Often all but focal length, aperture, and subject distance are assumed to remain constant (8x10 print viewed at 10 inches by a person with 20/20 vision) when discussing DoF. Remember, focal length is not how far it is from the image plane or the front of the lens to a subject in focus. Focal length is defined as the distance behind the front of the lens where colimated light striking the front of the lens is focused as a point.

As you turn the focus ring you change the subject distance. That is, you change the distance at which a subject would be in focus. Assuming the same focal length and aperture, as you reduce the subject distance the DoF becomes narrower. If you increase the subject distance the DoF becomes wider. As you focus nearer the ratio between the part of the DoF in front of the subject distance and the part of the DoF behind the subject distance also changes and approaches 1:1 (the subject distance is halfway between the nearest and farthest distances within the DoF). As you focus farther the ratio moves the other way until you reach the hyperfocal distance at which everything past the point of focus is within the DoF and the ratio between the part of the DOF in front of the subject distance and the part of the DoF beyond the subject distance is 1:∞.

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Independently of what you shoot, video or still, what lens you use, changing focal point change Depth of Field. You can play for example with this online DoF calculator to see the effect: http://www.dofmaster.com/dofjs.html

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You do not have a fixed DoF. The DoF depends, apart from focal length and aperture, also on the subject distance (=what you are changing with the focus ring). So, with eg. a 50mm lens and f/2.8, if you focus at 46.6 meters, your DoF will even extend to infinity (hyperfocal distance)! While at 1 meter, the DoF is only 4cm.

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