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:∞.
