The circle of confusion is a measure of whether that region of the image is sharp enough to be considered in or out of focus. A soft lens (like the 75-300 quoted by JJ) will enter the OOF region early than a sharp prime when moving from the plane of correct focus.
Bob
No, that's wrong,
as are all of the other attempts to rewrite optical theory on this thread...
To quote from the Focal Encyclopaedia:
"
DEPTH OF FIELD - Strict geometrical theory of image formation by a lens..."
"
Circle of confusion can be defined as the patch of light a lens produces when it images a point source. This patch, which is circular on the lens axis, is at minimum size when the image of the object point is critically focused. The largest circle that is seen as a point rather than a circle at a specified viewing distance is identified as the permissible circle of confusion."
Now, the important bit...
"
Depth of field is the distance between the nearest object where points on the objects are imaged at the film plane as permissible circles of confusion"
The circle of confusion is dependent on the viewing distance for a standard (6x8 inch) print, for which the "standard" viewing distance is about 10 inches and, according to the Focal encyclopaedia is about 0.145mm.
It goes on to say that the circle of confusion on the negative will be proportiately larger/smaller depending on the negative size, as a smaller negative will need to be magnified more to get to the standard print size.
The scales you see on prime (and some zoom) lenses are a guide to what will be rendered as "acceptably sharp" at a given aperture, with "acceptably sharp" being defined using whatever circle of confusion the lens designer deems appropriate.
Finally...
Let's just think about this business of MTF being a factor in depth of field...
MTF, or Modulation Transfer Function is a methodolgy for measuring the resolving power of a lens using a standard target. The lens is critically focused on the target, typically at a distance of (IIRC) 300x focal length.
Typically, the sharpness of a lens is lowest at wide apertures, improves to it's best level two or three stops down from the widest aperture, then deteriorates as diffraction takes effect. So, to take the premise that MTF is a factor in DOF would mean that it would increase as the aperture is stopped down but then start to decrease as the resolution of the lens drops due to the factors already mentioned.
So...to conclude:
MTF or ANY other measure of lens resolution (which is measured when the lens is critically in focus), has
ABSOLUTELY NO BEARING WHATEVER on depth of field. What does change however, is the lens's ability to resolve at different apertures.