What is the main point of the image in photogrammetry? How is it defined?

The answer given yesterday is not up to standard. I will correct it now.

The definition given in my book is this.

Let the aerial camera lens be point S, the plane where the photo is located is P, and the shooting place is approximate plane E..

Then let s be the vertical line of e, which is called the vertical line, and the intersection point a between the vertical line and p is called the image bottom point.

Let s be the vertical line of P, and intersect with P at O, and O is called the principal point of the image.

The bisector of ASo with angle S intersects with P and E at C and C, and C and C are called isocenter.