In photogrammetry, what is the focal length?

In photogrammetry, the focal length is defined as the distance from the lens of the camera to the image sensor (or film) when the subject is in focus. This critical measurement allows the camera to project the real-world scene onto the image plane accurately, producing clear and well-defined images.

The concept of focal length plays a vital role in determining the perspective and scale of the photographs taken. A longer focal length captures narrower fields of view with higher detail, while a shorter focal length allows for wider views, capturing more of the scene in the entire frame. Understanding focal length is essential for geodetic engineers and photogrammetrists when designing surveys, as it directly influences how images are scaled and interpreted in spatial measurements and analyses.

Options that refer to the total length of the camera, the distance between the photo and ground distance, or the distance from the camera to a survey point do not accurately describe the focal length in the context of photogrammetry and would not provide the necessary clarity and precision needed for effective image capture and analysis.