In parallax method for the determination of focal length of a...

In the parallax method for determining the focal length of a concave mirror, we use an object pin and its real image formed by the mirror.
The method of 'no parallax' involves aligning the tip of the object pin with the tip of its real image. This is only possible if the image formed is real.
For a concave mirror, a real image is formed only when the object is placed at any position beyond the focus ($F$), i.e., $u > f$.
If the object is placed between the pole ($P$) and the focus ($F$), the image formed is virtual, and it cannot be caught on a screen or used for the standard parallax alignment with a physical pin in front of the mirror.
Therefore, the object should be placed at any point beyond the focus ($F$) of the mirror to ensure a real image is formed.