Camera geometry — name for the distance from the projection centre to the photograph In a frame camera, the straight-line distance measured from the perspective (projection) centre to the image (photograph) plane along the principal axis is called:

Introduction / Context:
Understanding basic camera constants is essential in photogrammetry. The distance from the lens’s perspective centre to the photograph governs image scale and enters every orientation equation used for mapping from aerial photos.

Given Data / Assumptions:

• Frame (central projection) camera.
• Photograph plane perpendicular to the principal axis for a vertical exposure.
• Calibration provides the camera constant (f).

Concept / Approach:

The line from the perspective centre perpendicular to the image plane intersects the photo at the principal point. The length of this line segment is the focal length f. In photogrammetry, the calibrated principal distance (distance from the perspective centre to the calibrated principal point) is effectively the focal length for mapping purposes; small differences can arise from lens principal point definitions but are accounted for in calibration.

Step-by-Step Solution:

Verification / Alternative check:

Camera calibration certificates list f (camera constant) and principal point coordinates; these are used directly in interior orientation.

Why Other Options Are Wrong:

• (a) Principal distance is often numerically equal to f in calibrated systems, but the standard physical term for the distance from projection centre to image plane is the focal length.
• (b) Principal line refers to the line of collinearity through the perspective centre and principal point within the photo plane.
• (c) Isocentric distance is unrelated here.
• (e) Nadir offset concerns tilt and is not a camera constant.

Common Pitfalls:

• Assuming principal distance and focal length always differ; for most calibrated mapping cameras they are equivalent within small tolerances.

Final Answer:

Focal length.