Step 1: Read the definition carefully.
We want the point where a perpendicular dropped through the centre of the camera lens meets the photo image.
Step 2: Recall the special points on a photo.
A photo has a principal point, a nadir (plumb) point and an isocentre, each tied to a different line through the lens.
Step 3: Check the nadir and isocentre.
The nadir is where the vertical plumb line meets the photo, and the isocentre lies on the bisector of the tilt angle, so these are not perpendicular to the photo plane.
Step 4: Focus on the perpendicular line.
The line that is perpendicular to the photo plane and passes through the lens centre is the optical axis.
Step 5: Name where it meets the photo.
The foot of this perpendicular (the optical axis) on the photo is the principal point.
Step 6: State the answer.
The point described is the principal point.
\[ \boxed{\text{Principal point}} \]