Focal length can be defined as which distance?

• A. The distance from the camera to the subject.
• B. The distance from the sensor to the lens.
• C. The distance between the optical center of the lens and the image plane when the lens is focused at infinity.
• D. The length of the zoom ring travel.

Answer: (C)

Focal length is a property of the lens that describes the distance from the lens’s optical center to the image plane where the image forms when the lens is focused at infinity. In simpler terms, for distant objects whose light rays arrive as parallel, they are brought to focus at a plane located a fixed distance—the focal length—from the lens. This distance determines how much of the scene you capture and how large subjects appear on the sensor.

The distance from the camera to the subject changes composition and perspective, not the lens’s inherent focal length. The distance from the sensor to the lens relates to camera design and mount, not the defining optical distance for focusing at infinity. The zoom ring travel is a mechanical way to change focal length on a zoom lens, but it isn’t the focal length itself.