AR15, thanks a bunch for the following:
For example, a target placed at 300yds may show sharp image focus from 290 to 310 on the parallax knob.
You can reliably say that target is at 300yds after racking though the focus and splitting the focus range in the middle. Now, the scope will not always be parallax error free at the 300yd marking through. You still have to bob your eye around behind the scope and remove the parallax error that way.
Now I know why there's parallax after you've first focused the eyepiece to see the reticule clear and sharp then focused the scope sharply on that 300 yard target.
The mechanics of the knob moving lens mounts inside the scope are exactly the same from scope to scope of that model. A given change from infinity to some shorter range will move those mounts exactly the same for each of them. And each ones movement for a given range change is based on each lens group moving based on the optical design specs for the focal length of each group. But lens making machines don't make each one of a given spec exactly the same; their focal lengths vary by a few percentage points.
For a 15X scope, the combined focal length of the lenses in front of a second plane reticule will be 22.50 inches for an eyepiece focal length of 1.5 inches (typical for scopes with a 3 inch eye relief). If the actual focal length of those front lenses is actually 22.40 inches, that'll change the focus point for a 300 yard target about 1/10th of an inch. That image will be focused about 1/10th inch in front of the reticule and parallax will easily be seen. If the actual focal length was 22.60 inch, the target image would be focused about 1/10th inch behind the reticule.
(Note that the physical length of a telephoto lens is less than what a long lens of the same focal length will be, Rifle scopes use telephoto lens systems so one with a 22.5 inch focal length may be only 8 inches long physically. Such is the magic of optics.)
With these normal variables for a given zoom range in a scope, there'll be less error at the lower magnifications, more error at the higher ones. This is why cameras had autofocus stuff put in them. The fixed mechanics of split image rangefinders in them were not exact; long telephoto lenses were often out of focus 'cause their focal lengths were a few percentage points off from what the rangefinder's mechanics were made for. Some rangefinder cameras had interchangeable cams cut for exact focal lengths (marked on the lenses as accurately measured) so long lenses would focus correctly on film.