Parallax is a displacement or difference in the apparent position of an object viewed along two different lines of sight and is measured by the angle of inclination between those two lines.
Non-parallax is therefore when there is no difference in apparent position when an object is viewed along two different lines of sight.
An optical pin fixed on a cork that is supported by a clamp is placed above a lens that is on the mirror as shown below. The cork is such that it slides up and down the glass rod.
Adjust height of the pin until it’s image is seen on the mirror.
The position of the pin is adjusted until the image from the mirror and the object pin seems to be moving together when you move your eyes.
The distance between the lens and the pin when there is no parallax between it’s image and the object is the focal length of the lens.